help with wossner hi comp

4 replies to this topic
  • ehoward381

Posted September 04, 2013 - 01:14 PM


hi im just rebuilding my yz400 1998 and fitting a wossner hicomp piston and have run in to troubble

to start with whan i fitted it and bolted it up and turnd over engine with out cams in it hit the head so i removed that problom with the die grinder checked it for clearance it was ok

then when i put the cams in the valves hit on both sides in and out and they are we land truly touching its not just a bit

i thought the piston would just bolt in and go now ive fitted it i carnt send it back ither


so how much can i mill of the piston to gain valve clearance iv tryd wosner but no responce


i think the head has never been skimed so carnt quite understand whats going on


would be greatfull of any help at all on this one manny thnks edwin

  • f150jokerstyle

Posted September 06, 2013 - 04:05 PM


I'm sure you have checked this but you did not put it on 180 degrees off did you? Other than that I think they gave you the wrong piston. I would not go milling down a piston dome without knowing the limits. If you go to far and run it and there is a problem you are most likely looking at more than just another new piston. Also you should throw an old park plug in there for grins and see how bad that might be.


Good luck, sounds like you are going to need it.

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  • jakeh_16

Posted January 27, 2014 - 03:11 PM


sounds like piston is backward or complete wrong piston.


do not mill or grind the piston, you will reduce wall thickness and significantly reduce its strength.


if the head has been ground you will have a higher compression ratio, if valve seats have just been cut, or dished valves installed u will get a bigger combustion chamber (lower comp)


ALWAYS measure piston dish volume, combustion chamber volume, head gasket thickness and deck clearance and calculate a compression ratio.


if you lack the tools or knowledge to do this you shouldn't be engine building, pay a professional.

  • grayracer513

Posted January 27, 2014 - 04:44 PM


Any well made piston should already be cut to clear the valves at beyond the stock lift and duration.  Sounds more like a simple case of cam timing being off.  If you are using the marks on the flywheel as a guide, you may want to verify that the flywheel hasn't sheared it's key, or that the key was not dislodged during flywheel installation.  Do this by checking for TDC with a probe down the spark plug hole, comparing real TDC to the marks on the 'wheel.

  • ehoward381

Posted March 18, 2014 - 02:27 PM


Actually I found that the head had been skimmed and probably the barrel as well and the bridge of the piston is a tight fit and also after market head gasket all had caused this used a spare head I had and all was good

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