What does machining the head do?

If the head was machined to increase compression and now requires the use of an additional washer on the spark plug to keep it from hitting the piston, is this a normal procedure and what actually would be the result? Also, what would be necessary to do when it is time to do a top end? thanks.

2 sep areas of head machine-ing.

* lowering the squishband area for :

increasing comp # , and overall combustion effic.

*dome shape and dome cc volume value vs the desired comp # for your aplication.

IE : sometimes you lower the squish so much the comp value is much higher than desired , so the bowl /dome material is machined to achive the desired comp #. And yes sometimes a sparkplug washer is req , not so much as for it not to hit the piston ,, but just so it does not stick down into the combustion chamber impeading flame and high speed gasses path.

this is my understanding , and hope i explained it ok.

more comments welcome , i'm just a race dad.

OK- I sorta get it. So will I need to do anything special when it is time for a new top end or do i just buy the kit and put it in remembering to reinstall the washer on the spark plug? I assume the mod. adds gains somewhere in the powerband, yes?

Yeah , the same top end procedure + parts and all , and yes if it has the washer by all means install it.


now requires the use of an additional washer on the spark plug to keep it from hitting the piston,

That is not normal (for a MX bike anyway)

The head mods would probably not effect anythign with the top end job. I cannot imagine anyone doing anythign that would change the basic piston kit. I would probably measure the squishband just to be 100% sure. It only takes a few minutes. http://www.rb-designs.com/

Without knowing what the person working the head over did, it woudl be tough to guess at where they were trying to improve power...low, mid, or top. My best guess would be material removed from the top of the head would result in larger volume. This would favor top end.

Additional thought on the whats needed for a top end question,,.

I would think it would be important to use the same thickness clnyd base gasket , and perhaps the same type piston.

That way whatever has been done to the top end / engine ,, eveything same same except fresh.

Here is a quote from the guy who sold me the bike and is the reason for the question I asked.

"I will show you on Sunday but I want to make sure that I do not forget to tell you that Pro Circuit machined the head to increase the compression. Therefore there is a gold spacer that you have to run on the spark plug. If you ever change the plug you will need to move this spacer to the new plug or you will damage your piston. Just did not want to forget to tell you that."

I also understand that PC did a port/polish job on it and suspension work. The whole bike is basically a pro circuit race bike. He told me the PC Works pipe was different from what was available to the public and since I just bought and installed a new PC Works pipe, it was definetely different looking (not just the coloring). So i guess when i do the top end I will report back to you guys so you can help me identify what piston is in there. The top end was done last at 51hrs and the bike now has 75 hrs...how much longer can i go?

. The top end was done last at 51hrs and the bike now has 75 hrs...how much longer can i go?

35 to 40hr

I would think that this is also done for warped heads, if it doesn't sit on straight they take of a bit and give you a performance increase as well in the low end. I think it's a bad idea personally, you'd probably have to run 97 octane (AKI) to prevent detonation.

I don't know much about this area, so someone please correct me if I'm wrong.

35 to 40hr

No way jose!

Any race built bike should NEVER be run that long!!!

IT'S A RACE BIKE...rebuild it OFTEN.

I highly suggest you at a min inspect every ten hours.

rjb . 35-40 hrs on that sounds about right. judging from the sig picture. lol

I'm sorry RJ, did you say something...I was too busy studying your avatar:worthy:!

Ok- so as the cold weather sets in i will go ahead and do a top end for peace of mind even though she is running just as strong as when i got her. What do i need to look for to identify what piston is in there so i can get the same as a replacement? My buddy thinks i should just replace the rings every 10-15hrs until i notice some drop off in the power, but I think I'd rather be safe than sorry.

On a side note, if anyone followed my other thread about my bent/cracked pipe, i replaced the PC Works pipe with a new PC workks pipe and my throttle response cleaned up a bit. It also has a much smoother power delivery. I like it even though it doesn't feel as fast as it did with the old pipe. Not sure whether to attribute this to the fact that it was dinged up and had a crack or whether it was a one-off PC race pipe...dunno

"The road less traveled, and that has made all the difference.."

mr robert frost.

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