Inliners International Forums General Discussion Engines 307 Pistons in 250 Engine

Hi all, since I need to replace one piston in my 250 and it seems like I can't get a single piston, only a set, I was thinking of swapping over to 307 pistons to bump up the CR.

Is there anything else I need to do for this to work or can I just swap the pistons? Thanks!

You'll need to make sure the bores have the correct cylinder wall clearance for the pistons you purchase. Forged needs more cylinder wall clearance than cast pistons as well if you choose that type.

MAKE SURE YOU GET FLAT TOPS TOO. MIKE THE CYLINDERS TO CHECK CLEARENCE, YOU MIGHT NEED A OVER BORE. PAT SPECIAL ATTENTION TO FIRST 2 CYLINDERS,THEY WEAR THE MOST

When you say flat top, I assume this is still allowing for the 4 reliefs? I can't seem to find any that have no reliefs.

Mike, all o.e.replacement pistons will have the 4 valve reliefs; the difference is in the design around the circumference of the piston above the top ring. Most of the replacement pistons now have a chamfer around that circumference. The pistons previously available did not have that chamfer (true flattop) and produced slightly higher compression ratio.It would seem that the chamfer would also increase the crevase volume a bit also(which usually results in greater unburned h/c).Can't explain the logic behind the change except that reduced comp. reduces oxides of nitrogen; maybe that was the intended purpose.All I can get in my area is chamfered pistons so thats what I use.........fats

Great! Thanks for that info. I am looking at the fact that if I order Silv-o-lite piston set for the 250, I am looking at 2+ weeks out to get them. If I go with 307 pistons, I can get them shipped today. I really need to get the engine back together so I thought it would be a win-win by getting what I need now and bumping the CR up for a little more power.

I just want to make sure I am not shooting myself in the foot and need a bunch of other things done. My plan is to have my friend press out the pins on the current pistons and swap the rods onto the new ones. Everything mics out okay as this engine only had 5300 miles on it.

You still might need to hone the block to get the right clearance for the new pistons, or worse, they could have too much clearance compared to the old pistons the block was originally bore and honed for. Never assume that you can simply swap them out without having to do this.

I have .040 pure flat top with no valve reliefs instock. Your bore needs to be 3.915-3.916 for these pistons. Check on my site. And mine come with rings. http://www.12bolt.com

Thanks for that info tlowe. I had seen those on your site but they are WAY out of my budget. I am looking to get this back on the road on a reasonable budget.

ebay is a great place for 307 pistons, hardly anybody wants them. last set i got i paid 75.00 delivered for them but you better mike your block before you buy pistons