Hi,
I have a 1999 Honda 270 big bore , stock stroke motor. It had a stock head on it when I bought it, But I switched over to the ESR cool head for big bore Honda motors. I went to check the squish and without the soldier in on the piston it wouldn't turn over. It felt like the piston was touching the head. I took the head off and found that the piston is coming up .023 " past the top of the cylinder. How do I fix this so that I can get around a .040 squish ???
Thanks,
George

Are you sure the dome in the cool head is for a big bore cylinder ? Pop the dome out of the head, and check the numbers on it. I believe Hyper has a list of dome numbers that will show you whether it is stock bore/ or big bore, and what rpm range the dome is best for. I have had problems in the past with Wiseco pistons when it comes to big bore engines. Either no squish clearance, or way too much. If the dome is correct, you'll have to find a thicker gasket.

Hi,
Here is the dome numbers. 72FT68566A and 72FT68555A That came with the kit.
The Hyper book calls for this to be the big bore stock stroke domes. The top of the piston has thease numbers on it :

A37A5

2.00 mm

7238p8

Is this the correct piston ???

Also, how thick a plate do you think that I would need ???

Thanks,
George

www.papermanracing@yahoo.com

Hi,
I have a 1999 Honda 270 big bore , stock stroke motor. It had a stock head on it when I bought it, But I switched over to the ESR cool head for big bore Honda motors. I went to check the squish and without the soldier in on the piston it wouldn't turn over. It felt like the piston was touching the head. I took the head off and found that the piston is coming up .023 " past the top of the cylinder. How do I fix this so that I can get around a .040 squish ???
Thanks,
George

are you sure that its a stock stroke if it is check to see if your piston fits inside of the ridge on the dome if it does than you have to use more base gaskets sounds like something is wrong i have same dome and only need a 1/7 base gaskets the number for the piston you gave does not tell me anything i have a 68.43 piston and the number is 3428a its a 2mm overbore if you can get a hold of wes hastings he could figeur it out good luck

It would be best to get the engine to a engine builder or someone who knows what they are looking at. It could have been worked on by a few different people before you got the engine. this could mean that no head would work but a custom cut one. The port timings should be checked. The squish and the cc's of the combustion chamber should also be checked. When set-up correctly the engine will perform much better than if you just get the squish correct. I have found that all of these things need to be set up to compliment each other. Squish clearance, Squish width, Squish angle, CC combustion chamber, Port shape and timing, Piston type-dome or flat top, pipe and header length, Ignition timing curve. I've seen an engine with two of these off a bit when corrected picked up 8-12hp! But most of all a good usable power band.

Hi,
Piston fits inside of the dome ridges. Also, I measured the stroke the best I can with a digital micrometer and with the amount the piston rises above the cylinder, it is at 72mm.The part number on the box that the new piston the guy put in this is Wiseco # 677M06850 If that helps.
Thanks everyone for helping me.
George

Hi,
Piston fits inside of the dome ridges. Also, I measured the stroke the best I can with a digital micrometer and with the amount the piston rises above the cylinder, it is at 72mm.The part number on the box that the new piston the guy put in this is Wiseco # 677M06850 If that helps.
Thanks everyone for helping me.
George

I think your whole problem is the piston, you should have Wiseco 702M06850... And you shouldn't need any spacer plates w/ a stock stroke motor. I don't have any stock stroke engines to check the measurements for you but the spacer plates are made for when you increase the stroke...

That part # that you gave for the piston you have is for a Yamaha engine, and I don't know how you ended up w/ that when you have a Honda Engine. It probably wasn't a stock head you took off, it probably was some "afro" engineered head. Last winter I was talking w/ a guy that was using Yamaha pistons on Hondas b/c he could save a few dollars....
IE
* Part 677M06850 fits the following machines:
* 1992 Yamaha WR250Z
* 1993 Yamaha WR250Z
* 1994 Yamaha WR250Z
* 1995 Yamaha WR250Z
* 1996 Yamaha WR250Z
* 1997 Yamaha WR250Z
* 1992 Yamaha YZ250
* 1993 Yamaha YZ250
* 1994 Yamaha YZ250
* 1995 Yamaha YZ250
* 1996 Yamaha YZ250
* 1997 Yamaha YZ250
* 1998 Yamaha YZ250

I am looking for some old used 68.5 Honda pistons to try if anybody has any cheap that they don't need anymore. I want to figure out this problem before I go and buy a new piston set up. The one in the motor right now is new.
thanks,
george

Also, C and G , the head he had in it was a stock head that had been extensivly machined to use on the big bore cylinder. I stuck it back on to see if I could get a squish and it didn't even squish a .060 pc. of soldier.
george

Thats pretty nice, you'll probbaly have a lot more power when you get it right! My two primary motors are both at .038... 68 bore x 74 stroke (DBY), ESR head w/ ...55A dome

Throw a post on the classifieds to find a used piston...

actually c&g just running a 68.5 piston with a stock stroke is like 265-266 CC

actually c&g just running a 68.5 piston with a stock stroke is like 265-266 CC

Sorry bout that, was looking at my sheet wrong.... I edited the post....

:)Hi,
Everyone, thanks for all the help. I went down to Hyper today to pick up some items and I waited and talked to Mike Dicey about my big bore problem.
He said the piston isn't my problem. He thinks that the previous owner had the cylinder machined down the amount that the piston comes out of the top of the cylinder. He gave me 3 possible options to fix it.
1. put in about 5 stock base gaskets. ( this was not one he would do for fear of messing up the port timing )
2. use a stroker dome for a 68 mm piston
3. have my big bore domes machined to allow for the proper squish.
I am going with option 3 as he can do it while I wait for $ 15.00 a dome.
Thanks for all the help.
George

paperman77, if your looking for a cheap piston just seen one on ebay listed @ 39.95

IE
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=290239864258&ssPageName=ADME:B:SS:MOTORS:1123

Already bidding on it,
thanks
george:):D:):D