Author Topic: Piston clearance for 2.75hp  (Read 5737 times)

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Offline Ian

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Piston clearance for 2.75hp
« on: 13 Jul 2010 at 06:47 »
Folks, I was wondering what piston clearance people use for the alloy club pistons. The doco I believe says .06 top and .04 bottom. In the past I have had issues with pistons for other larger bikes from this manufacturer needing slightly larger than the nominated clearance - any thoughts ?

Offline Dads24

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Re: Piston clearance for 2.75hp
« Reply #1 on: 18 Jul 2010 at 11:51 »
Hi Ian, Modern pistons are usually cam ground or slightly oval. This is so that as the mass of metal expands around the gudgeon, when heated, the piston is circular in the bore. Similarly, when the rings lands are cut the head part is normally undercut slightly as the head of the piston is the hottest under combustion and expands a little more. Thus, when measuring piston to bore clearance it is measured between the gudgeons (not on the gudgeon) on the full skirt although the feeler guage is normally longer that the piston anyway. Make sure you check it at both ends of the bore. JP Pistons in Sth Aus recommend 0.004" for their Dougy pistons. This is a normal figure for pistons this size with no oil ring. Just up to this job myself. Regards John

Offline Ian

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Re: Piston clearance for 2.75hp
« Reply #2 on: 19 Jul 2010 at 01:18 »
John, so you have had no issues with needing extra clearance ? I have on other types of bikes - JP clearances seem to assume cooler running motors than we have !

Offline ian scott

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Re: Piston clearance for 2.75hp
« Reply #3 on: 19 Jul 2010 at 14:52 »
Ian. I think I recall giving mine an extra thou. This last pair haven't seized and I've had a few "hot" moments.

Offline Ian

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Re: Piston clearance for 2.75hp
« Reply #4 on: 19 Jul 2010 at 21:59 »
Thanks for that - think I may do the same !

One other question - did you use the circlips or set it up with the brass buttons to hold in the pin ? We just had an issue with a circlip fitted one where they disintegrated and caused heaps of damage to the bore.
« Last Edit: 19 Jul 2010 at 22:24 by Ian »

Offline graeme

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Re: Piston clearance for 2.75hp
« Reply #5 on: 20 Jul 2010 at 02:14 »
Hi Ian
I fit buttons to all pistons now, after bad experiences with circlips in the past - plus all the stories I've heard from others as well. Good insurance!
Cheers, Graeme

Offline eddie

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Re: Piston clearance for 2.75hp
« Reply #6 on: 20 Jul 2010 at 06:40 »
Yes, beware when fitting circlips - there are circlips and 'circlips'. Flat circlips need a square bottomed groove to sit in, whereas some pistons that are intended for use with wire circlips only have a round bottomed groove which may not give enough location for a 'proper circlip'.
                      Regards,
                                    Eddie.

Offline Dads24

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Re: Piston clearance for 2.75hp
« Reply #7 on: 20 Jul 2010 at 11:47 »
Hi folks, a lot of experiences with pistons, heat and circlips. Lucky we humans learn from and share experiences. I also think that modern lubricants have a part to play along with the lubricant system. What oils are you using? I prefer older basic oils more suited to the period of bike SAE 30. Regards John

Offline ian scott

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Re: Piston clearance for 2.75hp
« Reply #8 on: 20 Jul 2010 at 12:23 »
Bronze or aluminium buttons.

Offline Ian

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Re: Piston clearance for 2.75hp
« Reply #9 on: 20 Jul 2010 at 12:45 »
John - we use the older style oils - Penrite Shelsey - the friends bike that had the issues I think was running the equivalent of 30 grade - I run the heavier one in mine but have original style pistons and pins. I am just about to do up the motor on my sons restoration project.
« Last Edit: 20 Jul 2010 at 23:28 by Ian »

Offline Chris

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Re: Piston clearance for 2.75hp
« Reply #10 on: 20 Jul 2010 at 13:15 »
Hi all
     I have had problems with JP pistons when following their recommended clearances. On one occasion I had to remove the barrels and hone quite extensively three times before I could get the motor to run without tightening up. I am fairly sure that this subject was discussed quite extensively on this forum a few years back when from memory I think the final conclusion was that 6 thou. clearance was recommended instead of the 4 thou. prescribed by JP for sidevalve 2.3/4hp engines.
    In respect of end pads instead of circlips; the original pads used by Douglas in their cast iron pistons were made from copper. In the late 20s and 30s, the models that used pistons with end pads instead of circlips used pads made of brass. The conclusion from this is that it makes little difference what material is used copper, brass or bronze.
Chris.   

Offline Ian

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Re: Piston clearance for 2.75hp
« Reply #11 on: 20 Jul 2010 at 23:27 »
Thanks Chris