shorter pistons?

lingeringstin... · Trackday Trickster Joined: 01 May 2014 Karma :

I've been looking at some oversize pistons to put in a four stroke barrel I'm looking to experiment with and the only real difference I can see is that they're shorter than the standard pistons, but the crown height is the same so I assume there would be no problem with valves and compression. They just have a shorter piston skirt. To my mind that should work fine, but am I missing something obvious?

A100man · World Chat Champion Joined: 19 Aug 2013 Karma :

Comparable (crown) height must be relative to the gudgeon pin..
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stinkwheel · Bovine Proctologist Joined: 12 Jul 2004 Karma :

Valves and compression no.

My limited understanding of such things is that having a longer stroke length than the engine they're designed for increases the chances of the piston canting over at TDC and BDC, causing the skirt to broach the oil layer and rub directly on the barrel.

This is why longer stroke engines tend to have longer piston skirts.

I could of course be entirely wrong in this understanding as it's tentative at best.
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lingeringstin... · Trackday Trickster Joined: 01 May 2014 Karma :

To my way of thinking there shouldn't be any real disadvantage to having a shorter piston so long as the crown height stays the same. The gudgeon pin is the pivot point and as far as I can see if it's in the same location then there really shouldn't be THAT much difference if the piston skirt was shorter by a little bit. I doubt the lateral forces of a few mm difference in rod length would be enough to cause any problems.

The whole point of this exercise is to fit the biggest bore pistons I can safely get into the bored out liner. It looks to me like the liner can be safely bored out to about 60mm or a bit more and there aren't any standard oversized pistons that big, so I've been looking at pistons for other things and have found a few that look like they might work.

The standard pistons for this barrel have slightly recessed crowns and are about 53mm diameter and 47.2mm top to bottom at the longest part. The other pistons seem to be flat top and come is sizes ranging from 57mm on up to too big for the liner so there's plenty of choice.

Standard piston, recessed top, about 47mm tall
https://s14.postimg.org/7go661bj5/piston.jpg

Shorter piston, flat top, about 43mm
https://s16.postimg.org/3utk6n9xh/exhibit_A.jpg

Longer piston, flat top, about 49mm
https://s17.postimg.org/65lvkgc27/exhibit_B.jpg

Using a longer piston might mean shortening the skirt a bit but I don't see that as a problem. I've done similar things in two stroke engines and it worked. Since four stroke pistons run in solid liners without ports I don't really see a problem running a piston a few mm one way or the other so long as it doesn't bottom out.

I'm sure it will work. The thing is, I can't find any information anywhere about anybody doing this to one of these Honda clone engines. Surely SOMEBODY besides me has wanted to stuff bigger pistons into a Benly?

stevo as b4 · World Chat Champion Joined: 17 Jul 2003 Karma :

A shorter piston might be a lighter piston and have less friction too. We need engine designers to come and explain all the possible factors, considerations and variables like skirt loading, Rod angles thrust surface areas etc.

The reality is that I don't think like you said it's going to make much difference in practice, and your not looking to build a big bore engine to last 100'000miles etc anyway.

All your example pistons look of a similar design and I think your biggest concerns will be other factors like crown height, compression ratio's and making sure valve pockets in crowns are the right diameter and depth and in the correct position for clearances.

With taking a 250cc engine to 300cc I'd be early on looking at stuff like how to upgrade the cooling system and things like clutches and gearing alterations, as these are the first likely real world issues that could need addressing.