600 OHV new Pistons wanted

[alexd912]

Hi there,

I am just in the process of stripping the engine down on my G31 and the piston skirts seem to have some bad scoring in them. The rear piston is by far the worst and that appears to have quite a bit of play when in the cylinder. Can you get new pistons and rings for the 600 OHV models?
Are the any typical measurements or tolerances I can measure to determine if the pistons or piston rings are worn?

Also the top of the rear piston was covered in carbon compared to the front piston which was very clean.

ALso how do I find the model number on the Amal carb as I would like to get a rebuild kit and some new fixings as the float bowl hex nut is quite worn.

Best regards

Alex

[alexd912]

Here are some pics of the piston.

Some post mention JP, is this some company that supply pistons or refers to a bike?

Also in one post it was mentioned Himan pistons?

Any help here would be very appreciated.

Also is any one aware if you can get new head gaskets rings? One helpful member suggested that they can be made, but just wondering if anyone would have NOS or some alternative suggestions?

On the piston is mentioned 020 from what I can make out, is the oversize of the piston?

Thank you very much in advance for your help.

Alex

[graeme]

Hi Alex
I am almost certain that JP don't have pistons in their range that suit the 600 OHV machines. They could possibly make them for you if you wanted to provide a piston as a pattern , but it is a pretty expensive way to go.
There is a prominent member of this forum who from memory fitted pistons from a K2 Honda 4 in his 600 - I'll let him comment on what this entailed.
Cheers, Graeme

[Ian]

Graeme, I think you are referring to mine ? I am not sure which model Honda mine came from - certainly the era you mentioned. We had to more out to about +80 for them to fit but thats not an issue with these barrels. We made up a dummy ring for the oil ring groove and used the standard Honda compression rings. Seems to work fine.

[Alan Cun]

Hello All,

In the distant past I have used pistons from 4 Cyl Kawasaki around 1000 cc. I have used these in both EW and E29 600 but a 1/8 inch alloy spacer is needed under the barrels. I discard the little end bush and find a firm fitting gudgeon directly into the rod. The gudgeon is then allowed to rotate in the piston which is enlarged to suit the larger diameter pin.

I also have used  the same pistons in my 600 OHV models that was back in the late  80's and I think it was a much easier conversion  than the SV models.

From memory the pistons are around 70mm and my rough calculations brings the cc up to just short of 630 cc.

regards Alan

[alexd912]

Hi there Alan,

Thanks for this information. I have to take some measurements but I get the impression that the rear piston is worse then the front, but need to check.

Once again thank you for the invaluble information.

Best regards

Alex

[alexd912]

Hi there,

does anyone know if these pistons are suitable for a 600 OHV engine like my engine?

http://cgi.ebay.co.uk/Douglas-motorcyle-pair-pistons-600cc-1930s-bike-/290552701385?pt=UK_Motorcycle_Parts#ht_500wt_1156

These pistons appear to have 4 rings where mine have 3.

Best regards

Alex

[Doug]

Alex,

The pistons indicated are for the S6 side valve 600cc (Hepolite #9890.) An appropriate Hepolite number for the 1931-35 600cc OHV models would be #3639. I think you would find the SV pistons too tall in the dimension from the wristpin to the top ring to fit in the OHV engine.
 
-Doug



[Fix typo in part number. 09Apr11, -Doug]

[oily bloke]

Hi Alex,
I am in a similar situation with a Vintage Raleigh (I am re building a Dougie as well so I cant be all bad!!) and I have found this site which lists the Japanese and British pistons they stock with all the dimensions. You need to know the bore size required (not necessarily the pistons you have because they may be worn) the crown height, the crown type IE flat, dome, semi dome etc, and the gudgeon pin size. I am not that familiar with your model and don't know what sizes are standard. Once you know this you are up and running. Maybe someone on the forum can furnish you with the standard piston dimensions and go from there. It is possible that the pistons you have fitted are not original Douglas and, therefore, may not be ideal sizewise. You may have to compromise on the pin and the bore and some machining work may be required to match the barrels to the available pistons and visa verse. I have recently got a pair of Honda CB750 pistons into my EW350. It ups the compression slightly and the small end needed a larger bush to accommodate the Honda pin and I had sleeves/ barrel liners fitted to the barrels as the were on max oversize and light machining was required to the piston crown. If the work is done well and with plenty of thought you can end up with an engine that is virtually smoke free and a significant power increase can be gained to boot.

http://www.classic-motorcycles.co.uk/index.shtml

Best of luck
Andy

[oily bloke]

Hi again Alex,
A mate of mine has a pre war hepolite book and the info refers to a 600cc G32 OHV 1932-35. G31 is not listed and I assume the dimentions are the same. It would be worthwhile measuring yours. Info is as follows;
Bore 68mm. Crown height 29/32", Overall length 2 - 3/32", Flat crown with beveled edge. Gudgeon pin 5/8".
Hope this helps.
Cheers
Andy.

[MRD]

Hi All
I have attached a part of the Hepolite book. In the middle it refers to the G32, I don't know how much this differs from the G31. But I hope it helps.
Regards       Martin



Larger version

[eddie]

Hi Guys,
              I have also been looking for suitable pistons for the 600 OHV engines and stumbled across Suzuki GS850 pistons. The standard Douglas piston is 68mm dia - the GS850 is 69mm (40 thou larger), the height to the rings looks to be OK and the gudgeon pin is 16mm (4 thou larger than the Douglas). The only obvious modification is the need for new valve pockets to suit the different valve orientation.
                     Regards,
                           Eddie.

[Doug]

Oops,

Had a type in my previous post. Part number corrected to 3639.

-Doug

[alexd912]

Hi there everyone,

Sorry I have been a bit quiet recently just this project has been on hold for a few weeks. My Petrol tank is still being chromed so it was obvious I was going to miss the vintage bike run here. Also things seemed to slow down as the requirements got bigger than anticipated. So I went out and bought a modern shaft driven Yamaha so I could enjoy the short summer we have here in Finland.
So I will post my measurements here of the pistons and barrel bores later but I think I will have to get new pistons as these are just so scored.
Has anyone looked in to having some made? If so and they would like to combine an order with me then I'm open to that.

Best regards

Alex

[eddie]

Alex,
        From the pictures you posted of your pistons, the scoring doesn't look to be too bad or on the thrust faces. If they were mine, I would try carefully filing away the areas that have 'picked up' to remove any high/hard spots. When pistons pick up like this, the surface gets contaminated with material from the cast iron cylinder - likewise, the barrel will be contaminated with alloy from the piston. It is imperative that this is removed otherwise it will only pick up again. Any alloy adhering to the bore can usually be removed by lightly honing the bore or by using 400 grade wet and dry paper. Whilst dealing with the pistons, make sure the rings are still free in their grooves ( the alloy may have been drawn over the rings - trapping them in the grooves).
                          Regards,
                                      Eddie.

[alexd912]

BTW for those that havent seen this there are some NOS Dougie pistons for sale here:

http://www.vintage-motorcycle.com/index.php?language=en&site=9&suchtext=douglas

Alex