Piston Reworking

[2robert4]

With reference to my recent post on balancing thanks to everyone for your input it has helped a lot.

I now happen to have another issue which I did not anticipate, in order to balance the reciprocating masses ie the conrods and piston assemblies a problem has arisen with matching the weight of all four pistons.
The problem I have is three of the pistons are within 2 grams of each other which is good but the fourth and lightest piston is some 15 grams lighter than the lightest of the other pistons even with the heaviest wrist pin fitted. Now 15 grams does not sound much but in terms of removing material it is a lot on a thin walled alloy piston, not to mention extremely time consuming with the small power tools required for this type of job. I would not mind so much but these pistons are from a quality manufacturer ie Hepolite and I would not expect the discrepancy that I have even though these are NOS items I have held for some time.

What does the team think? Should I try to source another piston of the same type and within the same weight as the three pistons from a specialist or just buy a new set and cut my losses. The only ones I can see are some that are made in India under the name of "County" has anyone any experience of using these pistons? The only reason for my taking such a forensic approach to this build is I am trying to build longevity into the bottom end with it being a long stroke unit. 

[jonnyb]

In Peter Burgess's book he refers to balancing pistons "within 0.5g is accurate enough"! So as you say 15g is a lot. However, he does suggest removing metal from inside the gudgeon pin as these are heavier and don't affect piston strength. What do the gudgeon pins weigh? Steel has a density of 7.86g/cc so you would be removing about 2cc of metal, I think, which still seems a lot.
As it happens I was speaking to the guys who are working on my engine today so threw your issue into the discussion. They also thought 15g was a lot to remove from a gudgeon pin to get balance. They are using County brand pistons from Moss and consider them to be good quality. They said the pistons came fairly close in weights as a set but one of the rods was quite light so the others have all been lightened to match.
Hope this helps.

[jonnyb]

Sorry wrong link try post below

[jonnyb]

www.ebay.co.uk/itm/232647230994?hash=item362adb0612:g:i8YAAMXQVT9Szbld&amdata=enc%3AAQAIAAAAwJQafIxj60fh6fNpDbPnnnc2Q%2F%2BGmjFIr2FPoyDnXX0XUPotyg1zALyPTVzjqGLWZPXDyHj59KdHiF0bHhXIuz5MQnDyPZazWuRw3e6mcSFVnZzN4OQami1K%2BbJh6A%2FG%2BtOs46%2FpVOQs%2FrY4fQzepfHvG%2FMFBp8GwFc%2FUpgA65GTpSinH9e%2BfwG0q46TmteJIx7gUCZ1B5wnKOu53rZbsEZbOOmvXyfou55EJ2a0F21Rw1aUAn%2FpQdQrg96406YAkA%3D%3D%7Ctkp%3ABk9SR9rR_LXWYg

[2robert4]

Thanks for your prompt reply it definitely gives me "food for thought" and as my rods are all matched with little material removed it would appear that another set of pistons may be the solution.

[vue from the north]

Sept 20, 2023 16:01:41 GMT jonnyb said:

In Peter Burgess's book he refers to balancing pistons "within 0.5g is accurate enough"! So as you say 15g is a lot. However, he does suggest removing metal from inside the gudgeon pin as these are heavier and don't affect piston strength. What do the gudgeon pins weigh? Steel has a density of 7.86g/cc so you would be removing about 2cc of metal, I think, which still seems a lot.
As it happens I was speaking to the guys who are working on my engine today so threw your issue into the discussion. They also thought 15g was a lot to remove from a gudgeon pin to get balance. They are using County brand pistons from Moss and consider them to be good quality. They said the pistons came fairly close in weights as a set but one of the rods was quite light so the others have all been lightened to match.
Hope this helps.

    here is the good book, alas i suspect for copyright reasons we couldnt upload it to the forum, like we have the 1.5 workshop manual in the download section

[canuck]

Hello Nigel,
Yes 15 grams is quite a throw, and odd. In old times measurement this equals half an ounce.  

When I balanced my pistons/rods I  combined the piston and rod as a whole, after all this is the total reciprocating mass. FWIW this helped and I should think you may find success with this approach. Remember that these 3 main engines have offset of the connecting rods to consider. This does limit the ability to 'mix and match' so make sure you get this right. 

All the best going forward. 

B