Author Topic: 2 3/4 Cast iron piston clearance and dimension.  (Read 18809 times)

0 Members and 1 Guest are viewing this topic.

Offline Dave@NZ

  • Full Member
  • **
  • Join Date: Feb 2013
  • Posts: 35
  • Location: New Zealand
2 3/4 Cast iron piston clearance and dimension.
« on: 05 Apr 2014 at 08:03 »
Hi,
For the engine I and restoring we are are about to have new cast iron pistons blanks machined up to replicate the original pistons, I supplied the old ones to the machinist and they have measured them up found some discrepancies and asked question which at this stage don't have a good answer.

1, The original front piston was found to have 2mm offset in Gudgeon pin to skirt,  i.e. piston pin not drilled through dead centre of the piston , offset is to non-thrust side. Is this correct. The two old cast ones I have at home the pin is dead centre but I am not sure if they are both front or rear or front and rear. Possibly one of them was not machined the same originally by accident.

Q, Are the gudgeon pins machined in to the piston offset or in the centre?

 

2, Can you also recommend a piston to bore clearance i.e. front cylinder vs rear cylinder. I suspected 0.0015" would not be far off and did read model T ford has 0.0025".

Q, recommended piston to bore clearance for cast-iron pistons, front cylinder/rear cylinder.
 

3, The top ring land is smaller on the rear piston than same land on the front piston, possibly due ring groves being machined out for new rings from time to time.

Q, Does any one have a match  set of pistons who can measure up the dimensions from the top of the piston down to the lower ring so we can make ours the same?


Thanks in advance.
David.

Offline cardan

  • Master Member
  • ****
  • Join Date: Jul 2007
  • Posts: 1231
  • Location: Adelaide, South Australia
    • Leon's Vintage Motorcycle Page
Re: 2 3/4 Cast iron piston clearance and dimension.
« Reply #1 on: 05 Apr 2014 at 11:08 »
Hi Dave,

I hope someone can come up with an original drawing, as I'd be most interested to know.

For clearance, I've been using 0.001 + 0.001 per inch bore i.e. about 0.003" for a 61mm bore. I'm sure 0.0025" would be fine if you're a careful rider, but I'd worry about 0.0015" in an air-cooled engine. If you do go this close, be gentle with the bike, particularly when it's cold.

Also I'd relieve the lands just a little: perhaps 0.0005" above the lower ring and 0.001" above the top ring.

[Edit: these clearances are suggestions for cast iron pistons in air cooled motors. If you're using alloy JP pistons, you'll need maybe 2 or 3x these figures.]

Re your other questions: I doubt it matters much. The bike will certainly run OK with the gudgeon pins central in the pistons, and I doubt that the exact position of the ring grooves is too critical. But if someone knows, it may as well be done to original.

Cheers

Leon


Offline Doug

  • Administrator
  • ****
  • Join Date: Mar 2004
  • Posts: 4322
  • Location: Glen Mills, PA, USA
Re: 2 3/4 Cast iron piston clearance and dimension.
« Reply #2 on: 05 Apr 2014 at 15:39 »
David,

I have not seen any specifications for the 2-3/4hp, but I have seen numbers for a cast iron piston Douglas used in their 1925 OB (an OHV model). On a 68mm standard bore they used 0.005" at the bottom of the skirt, 0,008" at the top of the skirt, 0.010" for the land between the two rings, and 0.012" for the top land. You would need to proportion this for your smaller bore. There is no indication that they were eccentrically grinding the skirt for extra clearance on the 'sides' (but well worth considering).

This is a bit more clearance than what Leon suggested, but perhaps Mr. Douglas wanted to make sure there were no warranty claims for seizure and was not so concerned with the longevity (i.e., by then the machine would be out of warranty!) Also this was for an OB, a sporting model that would be expected to be run harder than a 2-3/4hp. But for any machine a careful run in period is the key to a long cylinder life.

The gudgeon pins were central in the bore (in the plane of the crankshaft) and I am not aware of any Douglas models where they either offset the gudgeon pin or the cylinder axis to create a Désaxé effect.

-Doug

Offline Dave@NZ

  • Full Member
  • **
  • Join Date: Feb 2013
  • Posts: 35
  • Location: New Zealand
Re: 2 3/4 Cast iron piston clearance and dimension.
« Reply #3 on: 06 Apr 2014 at 02:27 »
Hi Leon and Doug,
Thanks for your replays

We will advise for the pistons to be made with:
Gudgeon pins centred in the piston diameter.
Rings in same position on each piston some where close to original pistons.
0.0025"-0.003" piston to bore clearance, possibly making the front 0.0025" and the rear 0.003".

One thing I haven't quite understood is the taper.
Say the piston is 2.405" diameter at the skirt and the skirt is parallel, the lower ring land 0.005" less at 2.400 and top ring land is 0.001" and the diameter is 2.399". Is this correct with what you were meaning Leon?


Doug, were you referring to piston to bore clearances with all your measurements or piston diameter with the later.
ie 0.005 " lower piston skirt to bore clearance , 0.008" upper skirt to bore and so on.

If yes would that mean:starting at the bottom 0.003" taper in the piston skirt from bottom to top, 0.005" first ring land and 0.006 at the top land?

Just for interest I measured up two high performance used two stroke pistons interestedly the skirt is parallel non cam ground with 0.011" difference from the lower skirt to the top ring land on both pistons.

Regards,
David   

Offline Doug

  • Administrator
  • ****
  • Join Date: Mar 2004
  • Posts: 4322
  • Location: Glen Mills, PA, USA
Re: 2 3/4 Cast iron piston clearance and dimension.
« Reply #4 on: 06 Apr 2014 at 03:32 »
David,

The values given in my reply were diametric; total clearance. For example the bore was given as 2.677" diameter and the bottom of the skirt was dimensioned 2.672" diameter for 0.005" clearance. The top of the skirt runs hotter than the bottom so it requires more clearance. Once engine designers realized this, most piston skirts thereafter were tapered. Albeit a very slight amount.

-Doug

Offline cardan

  • Master Member
  • ****
  • Join Date: Jul 2007
  • Posts: 1231
  • Location: Adelaide, South Australia
    • Leon's Vintage Motorcycle Page
Re: 2 3/4 Cast iron piston clearance and dimension.
« Reply #5 on: 06 Apr 2014 at 06:58 »

Yes it's standard to use diametric clearance - i.e. the size of the feeler gauge that will fit between the piston and the skirt.

I searched quite hard (in old books and magazines - the web didn't yield much) for info re cast iron piston clearances, and for the veteran years all iron pistons I could find reference for were cylindrical (no cam grind). I found nothing re tapering the piston, although I know Harley Davidson used both tapered bores with cylindrical pistons and cylindrical bores with tapered pistons at various times around WW1-1920s.

My suggestions for the lands were half a thou  to one thou smaller diameter than the diameter of the body of the piston. I measured various new-old-stock pistons and never saw anything even approaching eleven thou in terms of difference between the bottom of the skirt and the top land.

Of course, most reasonable dimensions will work OK. Two years ago I acquired a single cylinder FN that had been "restored" by an "expert". When I remarked it didn't have any compression, the "expert" told me about the brand new cast iron piston he had made as part of the restoration. Clearance? Yes. Twenty Eight Thou. The piston was a better job than the rest of the engine!!!

Cheers

Leon

Offline Dave@NZ

  • Full Member
  • **
  • Join Date: Feb 2013
  • Posts: 35
  • Location: New Zealand
Re: 2 3/4 Cast iron piston clearance and dimension.
« Reply #6 on: 07 Apr 2014 at 08:33 »
Hi
I will measure the old pistons I have here and post the dimensions and then see if we can work through the figures to see what should be done to the new pistons.
Thanks.
David.

Offline Bob M

  • Full Member
  • **
  • Join Date: Apr 2014
  • Posts: 28
  • Location: Melbourne, Australia
Re: 2 3/4 Cast iron piston clearance and dimension.
« Reply #7 on: 13 Apr 2014 at 01:03 »
Sorry to be so slow to respond to this discussion but I caught up with it rather late.
I have a 1938/39 Polson piston catalogue to hand and you will be unhappy to find out there is no such thing as a single 2 3/4 piston and single set of dimensions. Polson list four different types for the various 350 sv models between 1914 and the mid 30's. All have 60.8mm bore but differ elsewhere.
This is just a hasty message to let you know before you give your machinist the go ahead.
As obviously the catalogue lists all Douglas pistons of the period with dimensions I thought I'd draw up a table of them all for inclusion on this site if the moderator wishes. The catalogue also contains info on clearances and cam grinding which owners may also want to know about.
This may take a day or so as I have to fit it in around other matters. I'll do it as soon as I can.
Cheers, Bob McGrath

 

Offline cardan

  • Master Member
  • ****
  • Join Date: Jul 2007
  • Posts: 1231
  • Location: Adelaide, South Australia
    • Leon's Vintage Motorcycle Page
Re: 2 3/4 Cast iron piston clearance and dimension.
« Reply #8 on: 13 Apr 2014 at 02:53 »

Bob! You know most of these Douglas things are four strokes? Perhaps you have a (Douglas) Bantam in the shed?

I wonder if the Polson pistons are alloy, even for the early bikes that had cast iron pistons originally, like the 1914 model. I have a NOS pair of Polson pistons for a c1924 Isle of Man Model 2 3/4 (RW or TW, both OHV), and I'd be most interested if they are listed in your book. Somehow I doubt it.

Cheers

Leon

Offline Bob M

  • Full Member
  • **
  • Join Date: Apr 2014
  • Posts: 28
  • Location: Melbourne, Australia
Re: 2 3/4 Cast iron piston clearance and dimension.
« Reply #9 on: 14 Apr 2014 at 00:25 »
Hi Leon,
Yes, you picked why I don't visit often. Not enough two-stroke content for me but I'm happy to share whatever info on other things that I have.
Polson catalogue both cast iron and alloy pistons as you will see and as 1938 was early days of cam grinding pistons they have something to say about that as well.
Remember this is a 1938 catalogue plus the 1939 supplement. It isn't a list of everything Polson ever made. They remained in business for another 40-50 years and extended their range well beyond what this catalogue lists.
This catalogue has two virtues. Firstly it gives important piston dimensions and secondly it gives stock numbers so you can identify any pre-war Polson piston you have.
Anyway, I'm laboriously copying the Douglas entries out. I wish I could just scan them but they don't scan well enough. Yellowed paper, tiny type face, etc, etc, don't make for a good scan.
To change the subject slightly, I also have a 1929 Timken taper roller bearing catalogue which includes a Douglas entry. Would it be useful if I copied that for the site also?
Cheers, Bob

Offline Bob M

  • Full Member
  • **
  • Join Date: Apr 2014
  • Posts: 28
  • Location: Melbourne, Australia
Re: 2 3/4 Cast iron piston clearance and dimension.
« Reply #10 on: 15 Apr 2014 at 11:31 »
I have a problem. I cannot upload the promised piston dimensions document. Apparently it is a .docx file not which is not acceptable. Only a plain .doc file will upload. I've just upgraded to Windows 8.1 which includes the latest version of Word. It's driving me nuts as all the icons have been shuffled around. Is it the upgrade or something else?
Are there any computer boffins out there who can tell me what I've done wrong.
Cheers, Bob

Offline Doug

  • Administrator
  • ****
  • Join Date: Mar 2004
  • Posts: 4322
  • Location: Glen Mills, PA, USA
Re: 2 3/4 Cast iron piston clearance and dimension.
« Reply #11 on: 15 Apr 2014 at 11:48 »
Bob,

You can still do a 'save as', and just below where you enter the file name there is a pull-down menu listing older and alternate types of file extensions such as *doc. Or send the file to me and I will convert it to a PDF format and attach it to your post (or convert it to an older MS Word format for you.)

-Doug, site moderator

Offline Dave

  • Administrator
  • ****
  • Join Date: Mar 2004
  • Posts: 3956
  • Location: Australia
Re: 2 3/4 Cast iron piston clearance and dimension.
« Reply #12 on: 16 Apr 2014 at 07:53 »
Bob,

Thanks for letting us know about this. It was the settings on the Forum server that would not allow attachment of .docx files.
 
The settings have now been changed so you can attach both .docx and .xlsx files.

Dave

Offline Bob M

  • Full Member
  • **
  • Join Date: Apr 2014
  • Posts: 28
  • Location: Melbourne, Australia
Re: 2 3/4 Cast iron piston clearance and dimension.
« Reply #13 on: 16 Apr 2014 at 09:28 »
Terrific Dave, I'm now having another go at uploading the document. It seems to have gone okay.
Cheers, Bob


To download a copy of the document, please click on one of the links below in the format that best suits you.

Douglas Piston Dimensions.docx - Microsoft Word 2010 document

Douglas Piston Dimensions.doc  - Microsoft Word 97 - 2003 document

Douglas Piston Dimensions.pdf - Adobe Reader PDF document










Additional formats added - Dave, 17th April 2014
« Last Edit: 29 Apr 2014 at 18:25 by Dave »

Offline cardan

  • Master Member
  • ****
  • Join Date: Jul 2007
  • Posts: 1231
  • Location: Adelaide, South Australia
    • Leon's Vintage Motorcycle Page
Re: 2 3/4 Cast iron piston clearance and dimension.
« Reply #14 on: 16 Apr 2014 at 10:44 »

Brilliant! Thanks Bob.

A couple of comments.

Polson pistons have a bit of a reputation for "growing". Interesting given that their recommended piston skirt to cylinder clearances are on the "snug" side, particularly for their aluminium alloy pistons. I'd guess that a JP piston run with these clearance would likely seize on a hot Australian day. Maybe it's the close fit recommended for the Polson pistons that gives rise to their reputation for nipping up?

The skirt clearances recommended for the cast iron pistons are similar to numbers I've found elsewhere - maybe half a thou less than the number I use. However the reductions for the ring lands is more than I've seen elsewhere. Interesting, and I might reduce my lands more than I have been doing for my next cast iron piston. Wherever possible I've been using cast iron pistons in my veteran bikes with excellent results. I use cast iron pistons turned cylindrical (not cam ground, or tapered) in air-cooled bike engines without drama. I'm not a performance guy.

I assume there is a tiny misprint in the section of the table relating to the TS and CW pistons. I assume the second piston listed has two 4mm rings? If so, the wider rings are usually found in earlier engines, although I'm sure the 3/32" (approx 2.5mm) rings in the other piston would be fine in a veteran engine. Some modern rings are very narrow and deep, and put a lot of pressure on the cylinder wall - if you have trouble compressing the rings to go into the bore using just your fingers the rings might be too springy for an early motor.

Thanks again for the effort of transcribing this valuable info, Bob.

Cheers

Leon

Offline Bob M

  • Full Member
  • **
  • Join Date: Apr 2014
  • Posts: 28
  • Location: Melbourne, Australia
Re: 2 3/4 Cast iron piston clearance and dimension.
« Reply #15 on: 16 Apr 2014 at 23:46 »
Hi Leon,
That's curious. I've never heard of any concern about Polson pistons growing before. They were always regarded with respect here in their home town. The old Die Cut Y alloy brand was another story entirely but Polsons never gave bother. In fact I'm hoping to come across a Polson CY209 for a Villiers Sport engine I want to use.

I checked your remark about the tiny misprint with the TS-CW piston rings but no, 2x4mm rings is as listed in the catalogue. My reading is that it is as you say, two 4mm rings not rings of 2x4mm section. That would fit in with how all the other ring dimensions are laid out.

Concurrently with this I have been going through a little exercise with David Beanham at Modaks. He had some mystery two-stroke cast iron pistons so I lent him some of mine for identification purposes. In the ensuing gossip we compared Polson catalogues. I have the 1938 & 39. He has a 1942 example. The major point of interest was the changes to the piston finishing information. By 1942 they didn't just suggest cam grinding could be beneficial, it was now standard practice and there was a lot of information on it. Unfortunately it was very pictorial with diagrams and nomographs. Reproduction presented the same problems as my catalogue. Yellowing paper, small typeface and general grubbiness and wear and tear. Polson should have known people like us would be poring over their catalogues 75 years later and planned accordingly. What a pity they didn't. (Tongue firmly in cheek.)

Finally, I'm not sure what has happened to pages 2 and 3 of my document. Only page 1 appears on my computer. Have I done something wrong?
Cheers, Bob



Offline Hutch

  • Master Member
  • ****
  • Join Date: Nov 2012
  • Posts: 377
  • Location: Queensland, Australia
Re: 2 3/4 Cast iron piston clearance and dimension.
« Reply #16 on: 17 Apr 2014 at 02:35 »
Hi All,
Thanks to everyone who has posted information on this topic as I am in the same boat as Dave and have a pair of 2 3/4 cast iron piston blanks and was wondering how I was going to machine them?!. I found this information in "Service Station and Motor Mechanics Manual", by George George, 2nd edition 1947 (Angus and Robertson). The first edition was in 1940, so I guess the information would be similar to that in your Polson catalogue Bob? (and I think George George was in Australia at this time). The information on piston clearances would be for water cooled engines, not air cooled ones, I would think.

Cheers
Ian





« Last Edit: 17 Apr 2014 at 08:10 by Dave »

Offline Dave

  • Administrator
  • ****
  • Join Date: Mar 2004
  • Posts: 3956
  • Location: Australia
Re: 2 3/4 Cast iron piston clearance and dimension.
« Reply #17 on: 17 Apr 2014 at 08:00 »
Quote from: Bob
I'm not sure what has happened to pages 2 and 3 of my document. Only page 1 appears on my computer. Have I done something wrong?

No Bob, your attachment worked perfectly.  I'm the one who got it wrong. I only put an image of page 1 on display as a sample of what the document contained. But I should have put all three pages so it can read in full on the screen and then downloaded if needed.

So have just added the other two pages.

Dave

Offline cardan

  • Master Member
  • ****
  • Join Date: Jul 2007
  • Posts: 1231
  • Location: Adelaide, South Australia
    • Leon's Vintage Motorcycle Page
Re: 2 3/4 Cast iron piston clearance and dimension.
« Reply #18 on: 17 Apr 2014 at 09:52 »

Sorry Bob - by "small misprint" I just meant that document says "4m" rather than "4mm". Maybe the stories I've heard about Polson pistons are not true! I checked my 350 RA pistons and they are Hepolite, from the home country, and not Polson.

Ian your stuff is fascinating too, but I agree that the rather snug recommendations for piston fit are likely for water-cooled, and not air cooled, motors. Water-cooled motors warm up more slowly, and evenly, and on a hot day run at lower temps than an air-cooled bike motor. For a cast iron piston I'd be reluctant to go below 1 thou per inch of bore, and 1 thou + 1 thou per inch of bore is a good margin if worried about seizing.

I'd be interested to hear if anyone has fitted iron pistons with clearance less than 1 thou per inch of bore and what the results were. Also if anyone has other info on iron piston fits, please share it with us.

Cheers

Leon


Offline graeme

  • Master Member
  • ****
  • Join Date: Oct 2004
  • Posts: 613
  • Location: Hobart, Australia
Re: 2 3/4 Cast iron piston clearance and dimension.
« Reply #19 on: 18 Apr 2014 at 12:12 »
Hi Leon
I have no experience in fitting cast iron pistons to an engine (apart from the Zenith that my brother rides, but that was the original piston in the original bore), but I do know for sure that the veteran Douglas that Jack Nelson sold to Ian Sargent has cast iron pistons - well so Rob Miller told me at the first Douglas rally, and I also think Neville Hunter uses iron pistons in his bikes. Also Bert Pol makes cast iron pistons to suit 2 3/4 Douggies, he must have a recommended clearance.
Cheers, Graeme

Offline Bob M

  • Full Member
  • **
  • Join Date: Apr 2014
  • Posts: 28
  • Location: Melbourne, Australia
Re: 2 3/4 Cast iron piston clearance and dimension.
« Reply #20 on: 19 Apr 2014 at 02:13 »
Hi again Leon,
Just for your interest I am attaching another table relating to clearances for Villiers cast iron pistons. I thought you would be interested as two-strokes run markedly hotter than four strokes.
The 196cc Villiers and the 2 3/4 hp Douglas having much the same bore makes the clearances in this table of even greater interest.
You'll also see at this stage (1945)  Villiers were not cam grinding two-stroke pistons but apparently taper grinding them.
Have fun, Bob

To download a copy of the document, please click on one of the links below in the format that best suits you.

Villiers Cast Iron Pistons.docx - Microsoft Word 2010 document

Villiers Cast Iron Pistons.doc  - Microsoft Word 97 - 2003 document

Villiers Cast Iron Pistons.pdf - Adobe Reader PDF document






Additional formats added - Dave,22nd April 2014
« Last Edit: 29 Apr 2014 at 18:25 by Dave »

Offline Dave@NZ

  • Full Member
  • **
  • Join Date: Feb 2013
  • Posts: 35
  • Location: New Zealand
Re: 2 3/4 Cast iron piston clearance and dimension.
« Reply #21 on: 25 Apr 2014 at 08:45 »
Hi,
And thanks for the information on this topic and very interesting reading page 533 of the manual from Ian with respect to piston clearances.

Lindsay contacted Bert for his recommendations on how to machine the cast iron pistons.
 Bert's recommendations were:
The clearance we used is 0,10 mm to 0,12 mm on the skirt , the top has 0,1 mm more . Front and rear the same . My opinion on tin coating is simple ; NO . We made the pistons in cast iron for the correct expansion , same as the cilinders , and for the lubrication of cast iron on cast iron .A coating on the pistons may disturb proper lubrication.


looking at the old parts we had at home and made the following observations,
The collection of used pistons we have and from what we could measure the skirts were basically parallel and the top ring land 0.008” to 0.0015” less than the skirt diameter and 0.003' to 0.008” less than the skirt diameter between the rings, just remembering that we are measuring used pistons.

WE passed all this information along to the machine shop:
 to use it as a guide but ultimately they will have to decide what is best to do and wondered if the figures below would be not far out or should we stick to .1-.12mm as the piston manufacture used.
Not less than 0.003" piston to bore clearance, possibly making the rear more than 0.003” ?

So long story short they are underway as far as we know no news yet so all must be good, it may be a month or two before we here how they have got on.

Dave.


Offline cardan

  • Master Member
  • ****
  • Join Date: Jul 2007
  • Posts: 1231
  • Location: Adelaide, South Australia
    • Leon's Vintage Motorcycle Page
Re: 2 3/4 Cast iron piston clearance and dimension.
« Reply #22 on: 26 Apr 2014 at 02:01 »

Maybe just specify clearances 0f 0.003" -0" +0.001" (i.e. between 3 and 4 thou). As for reductions for the ring lands, I'll leave that to you. Good luck.

Leon

Offline Bob M

  • Full Member
  • **
  • Join Date: Apr 2014
  • Posts: 28
  • Location: Melbourne, Australia
Re: 2 3/4 Cast iron piston clearance and dimension.
« Reply #23 on: 29 Apr 2014 at 01:29 »
Leon,
I am embarrassed. I know what table I intended to put up giving Villiers cast iron piston clearances but I fluffed somewhere and sent something entirely different but with a similar title. If you are still interested in Villiers cast iron piston clearances for a 61mm bore piston I'll have another go.
Cheers, Bob

Offline cardan

  • Master Member
  • ****
  • Join Date: Jul 2007
  • Posts: 1231
  • Location: Adelaide, South Australia
    • Leon's Vintage Motorcycle Page
Re: 2 3/4 Cast iron piston clearance and dimension.
« Reply #24 on: 29 Apr 2014 at 03:30 »

Yes, please. I was too polite to ask!

Leon

Offline Dave@NZ

  • Full Member
  • **
  • Join Date: Feb 2013
  • Posts: 35
  • Location: New Zealand
Re: 2 3/4 Cast iron piston clearance and dimension.
« Reply #25 on: 29 Apr 2014 at 05:00 »
Thanks leon for your input and information.
I have to be careful on this one to offer the best available information for the engineers guide them not direct them otherwise if any problems arise they could say we built it as you said now it doesn't work what would you like us to do$$ especially since I don't own it and are not paying for the repairs.

Hopefully after reading the email the reconditions they are able to come to the same figures you mentioned hopefully my guidance has worked.  ''Not less than 0.003" piston to bore clearance, possibly making the rear more than 0.003” ?

David.

Offline Bob M

  • Full Member
  • **
  • Join Date: Apr 2014
  • Posts: 28
  • Location: Melbourne, Australia
Re: 2 3/4 Cast iron piston clearance and dimension.
« Reply #26 on: 29 Apr 2014 at 06:38 »
Here we are then.

An immediate pre-war Villiers table showing recommended piston clearances for their pistons made in various materials. At this stage Villiers made a 61mm bore 196cc cooking engine with c/i piston and fixed c/i head, and a 61mm bore 196cc Super Sport with alloy head and alloy piston. It's remarkable how much more lively the cooking engine is when fitted with the otherwise identical alloy piston from the Super Sport.
 
It is unfortunate the table was drawn up at the time before standardisation when firms named their alloys with no regard to any international standard. Luckily 'Y' alloy is so historically significant that it always gets a mention but LAC No 10 is a mystery to me.

Cheers, Bob


PS Saw no Douglas at the All British Rally at Newstead last weekend. Did I not just look hard enough?


To download a copy of the document, please click on one of the links below in the format that best suits you.

Villiers Piston Clearances.docx - Microsoft Word 2010 document

Villiers Piston Clearances.doc - Microsoft Word 97 - 2003 document

Villiers Piston Clearances.pdf - Adobe Reader PDF document





Additional formats added - Dave, 29th April 2014

 
« Last Edit: 29 Apr 2014 at 18:26 by Dave »

Offline cardan

  • Master Member
  • ****
  • Join Date: Jul 2007
  • Posts: 1231
  • Location: Adelaide, South Australia
    • Leon's Vintage Motorcycle Page
Re: 2 3/4 Cast iron piston clearance and dimension.
« Reply #27 on: 30 Apr 2014 at 01:32 »
Bob - Thanks for that! The detail of the tapering is interesting, and again the 10-thou-ish reduction on the lands is more that I have found elsewhere for veteran pistons, but consistent with the Douglas clearances. The bottom-of-skirt clearances for cast iron pistons are bang-on the numbers I've been using lately (found elsewhere): either 1 thou per inch of bore, or 1 thou + 1 thou per inch of bore.

David - The only problem with specifying "no less than 0.003" for clearance is that it puts no limit on the upper end of the clearance range. An engineer may decide "we don't want the bloody thing to nip up" and give it 5 or 6 thou clearance - as you would for an aluminium alloy piston. I prefer to specify exactly what I want, that way no-one else decides for me and it's all my fault if it doesn't work! Not sure if I mentioned in this thread [edit: Yes I have! Sorry] that an "expert" sold me a 1910 FN with a new cast iron piston he had made with 0.028" (28 thou) clearance, ten times what I think it should have been. He'd also arranged the little end so that the gudgeon pin could be literally dropped though the piston and out the other side; clearance was 3 thou. End gaps in the rings were 70 thou - yep nearly 3mm. When I took him to task, his response was "you have to give these things some clearance". Moral? Some "experts" (mine advertised in the club magazine that he would restore your veteran/vintage motor for you) have unusual ideas: unless your trust is very deep, specify exactly what you want.

Cheers

Leon

Offline Hutch

  • Master Member
  • ****
  • Join Date: Nov 2012
  • Posts: 377
  • Location: Queensland, Australia
Re: 2 3/4 Cast iron piston clearance and dimension.
« Reply #28 on: 02 May 2014 at 00:06 »
Hi Bob,
A quick internet search revealed LAC 10 as an aluminium copper alloy made by British Aluminium Co. Ltd.
it was for general castings, age hardened. Made up of Cu 9.8%, Fe 0.7%, Mg 0.25% balance Al.

Cheers

Ian

Offline Bob M

  • Full Member
  • **
  • Join Date: Apr 2014
  • Posts: 28
  • Location: Melbourne, Australia
Re: 2 3/4 Cast iron piston clearance and dimension.
« Reply #29 on: 02 May 2014 at 04:55 »
Thanks for that Ian. It's nice to solve a mystery even if there is little chance of ever coming across a piston made of LAC No 10 and needing the info.
Cheers, Bob

 

motorcycle