beware piston height on rebuilt motors on sidevalve engines

[craigb]

At the risk of looking like an idiot, I thought i would share this experience in case it helps someone else. This is on a very early sidevalve engine that I 'inherited' with the words 'its all machined, just needs to be assembled'. The motor had been bored to take 0.080" oversize JP pistons - not that this doesn't make me a little nervous. The block and head both look like they had been faced. On assembling, I did notice the pistons sat a little proud and if memory serves me correctly, not dissimilar to those on my Amilcar. I think also, despite it being very clear, the penny didn't drop that the Morris has a completely flat head with combustion chamber above the valves, the Amilcar has it above the piston. So while the engine did turn over ok, with the pistons sticking out about 0.030" and head gasket about .040", when it fired up there was a distinct knock. It ran and drove for a few minutes but I knew it wasn't right and pulled the head and sump. You could see the mark on the head of the markings that were on the top of the piston for two cylinders.

I asked around and while i could get a thicker head gasket made locally, I figured that will always be something to deal with. I spoke to a very experienced engine builder and he said there is only one way to do it right, so I removed the pistons and took the top off them with my lathe. Pistons have to come out through the bottom and that's a bit tricky getting them down while carefully placing and withdrawing them on the side opposite to the cam (the manual describes this but not how difficult and close it all is). Getting them back and rings compressed with no room to move is another order of difficulty!

Moral of the story I guess - never buy a car already in pieces and know what you are working with and particularly with old stuff, don't assume a part of the shelf will fit without modification. I guess if i had ordered the engine work I would imagine the machinist would say "hey, you realise i had to take 1mm or whatever it was off the block, so you will have to match the height of the pistons". The guy who had the work done, by the time it was handed over to me he was suffering from advanced brain cancer and could easily forget something like that. Of course if I had more experience but by sad coincidence my cousin who was very experienced in engine building who assisted me in the rebuild you would think would normally pick that up too, but he was very ill and died within weeks of completing the build. Anyway, with a bit of luck all sorted now!

[philthehill]

Thank you for posting reference the problem encountered relating to the piston to head clearance.

My own 1380cc engine block which had been decked required the piston tops to be shaved otherwise they protruded above the top of the block.

In my experience - when building an engine nothing should be presumed - each element should be thoroughly thought through and checked out for clearance and fit.

You can never be too careful or too cautious.

Phil