Two stroke v 4 stroke piston

Hi

Can anyone tell me the difference of 2 stroke and 4 stroke pistons

Can 2 stroke piston be used in a 4 stroke engine

Thanks

Dougie

A piston doesn't know if it is fitted in 2 stroke or 4 stroke, provided the bores are the same diameter and the piston top to centre of gudgeon pin are the same, then it should work.

It is an odd question, can you say what prompted it?

Andrew.

I'm documenting an engine build on tiktok

It's a 4 cylinder holt

I found some pistons on ebay, the correct size with 2 rings and pin I posted them on tiktok and I have had lots of questions asking why I'm using 2 stroke piston

Like you I thought a piston is a piston

Dougie 

Edited By Dougie Swan on 24/11/2022 14:39:02

4 stoke pistons will typically have an oil control ring in addition to the compression rings.

Most (not all) 2 stroke pistons won't have the oil control ring.

But will it make a great deal of difference

Skirt length might matter between engines with different strokes.

Some two-stroke engines have holes through or cut-outs in the skirt of the piston to let the compressed petrol/air mixture in the crankcase access the transfer passages and go up into the cylinder head.

4 strokes often have separate valves, 2 strokes often use the piston as a valve - via the ports in the cylinder wall.

The piston top on a 2 stroke might have features to help retain the mixture and swirl it - like a 'wedge' shape. I dont think 4 strokes do.

Dave

And, of course, two strokes will need fixed rings - so they cannot rotate around the piston.

Height above the piston pin will need to match the engine. Things are not as simple as just choosing the correct diameter.

Ok

So the rings are pinned, what does that do?

This is an engine from a home made set of castings so the rods and cylinders were made to suit

I've built lots of engines and never had this dilemma before

Bottom line is will they work or not

Dougie

Edited By Dougie Swan on 24/11/2022 15:58:33

A two-stroke piston will have pegged rings, no oil control rings or piston oil slots, and quite likely, a top ring which is L shaped in secton, fitted at the very top. It will also be likely to have cut-aways in the skirt below the gudgeon pin holes, if post ww2, and if pre ww2 or an outboard engine, the top of the piston will be an odd shape, not at all flat or lightly domed.

A four-stroke piston will have a land above the top ring, and a third (or fourth) ring slotted so that oil can drain back inside. The sides of the piston beween the thrust faces (ie around the gudgeon holes) may well be well back from the bore, if it was for a high performance engine, and the top of the piston is likely to be raised in the middle, and with cut-aways for valve clearance.

If what you need is a quirky ashtray, the two-stroke is better as it has no oil slots. If you need a source of a sensible alloy to melt down, light alloy pistons can be a good choice. In this case, avoid forged pistons as they won't melt down so nicely. Before ww2 many engines had cast iron pistons, though. If for a different purpose, helpful comments are going to be in short supply as we have not been told the purpose for which the piston is needed.

Regards, Tim

Edited By Tim Stevens on 24/11/2022 16:00:30

Because two-stroke engines rely on ports (holes) in the cylinder bore to get the working fluid in and out of the combustion chamber, there is a risk that the end of a loose piston ring will spring out into the port, snagging, and destroying lots of expensive parts. So, two-strokes have pegs in the ring grooves to prevent rotation of the rings.

Cheers, Tim