Pistons

Hi All

I recently decided to start looking at my ABC 3 HP engine, which hasn't run for at least 7 years. My understanding was that the engine had been fitted with a pair of Hillman Imp pistons without using any baseplates to reduce the compression. So the plan was (is) to replace the pistons with something more suitable such as Honda GT160 pistons.

However, once the heads were off I was surprised to find that when the engine was rotated to TDC the piston crowns were around 7/16" down from the top of the cylinder tops.




Does this mean that these piston crown centres when at TDC are in a similar position to the original pistons and that they may be OK to use?

Any thoughts anyone?

If your cylinders haven't been packed out at their bases, I think you'd better have a look at your conrods! I'm not familiar with Imp pistons, but the consensus is that the compression height is significantly more than the Sopwith originals. Can it be that your rods have been shortened? If so, this would increase their cyclical angularity with the bores, meaning increased side-loads, and potentially, bore-wear.
Certainly, the compression space appears adequate: you'd have to measure it. 7 or 8 to 1 would be acceptable.
After that, try it out!
(Of course, your pistons aren't bound to be Imp at all! Does anyone recognise Honda GT 160s?)
ATB, Paul.

hi phil
I think the measurement is about 1/4 diff. but I will check for you,I know someone who has used Honda piston but not run his bike yet, The writing on the piston tops does not look like imp but it is difficult to see.The main thing I think you have to look out for and would be expensive to replace is the crank,WILL it stand it I run my abc with imp piston with a 1/4 plate on the bottom of the barrel with no problem and I don't intend taking it out Also it will solve the problem paul mentioned.i will phone you later when I have all the facts

hi again the diff. between the abc and Hillman gudgeon pin is 3/16".
I have spoken to the man who used the Honda pistons but he decided to get some cast as the others were 10 th. under.
you can buy Honda piston oversize but it is stil an path to be trod.
if the Hillman imp piston are ok I would leave them.
They could be another make of piston hence the numbering,the ones I have are hepolite ref 1811 plus 30 th.
original bore 67.786 mm bh25030
you can buy rings and piston from s l thorntons shrewsbury 01952 252892
also check the base of the cylinders to see if they are the same that is where they skimmed them down to raise the compression they can vary between 295 th. to 245 th. base thickness.
I also skim the bottom of the imp piston down to match the abc it makes them lighter and also helps with the balance of the engine

hi again after all that technical stuff I realised the reason he did not put compression plates under the barrel.
you have to alter the inlet and exhaust manifolds to account for the extra distance between the inlet and exhaust manifold studs.
if you have twin exhaust then only the inlet manifold

Hi All

I have some other photos which I will post this evening that showing the gudgeon pin, piston skirt and the con rod.

Thank you for the advice so far.