XR500re keeps blowing head gasket

[buttle]

I have a 1984 XR500RE with a radial valve head. About 15 years ago I installed a high compression Wiseco 10.25 :1 piston.

 

Ever since then, I have never been able to run it in without blowing the head gasket.

 

I have rebuilt it several times myself and also had it done by professionals.

 

I cannot find a standard compression piston again... only 10.25 :1. After measuring from the top of the gudgeon pin to the top of the piston, it appears to be the same as the new piston offered by the local dealer.

 

After some Googling I have established that the piston I have currently installed (4330P2 XC ) is actually the correct piston for my bike.

 

WHY does it keep blowing the head gasket?

 

Do I need to tweak the head bolt torque at intervals over the run -in period? I am really getting frustrated over an issue that I do not believe should be an issue.

 

Please will someone help me?

with kindest regards
Peter Butler
Johannesburg

 

 

[pcnsd]

Install a stainless steel o-ring to the top of the cylinder. Any performance motor shop will know how and have the materials.

[SDet]

Or, see if your head is flat. Could have been warped at some point. Any machine shop should be able to check.

[EnglertRacing]

you should be able to run as high as compression as you want

 

are you re torquing after the first few rides?

are you using the proper torque sequences?

have you checked for flatness?

are you using new head gaskets?

[buttle]

you should be able to run as high as compression as you want

 

are you re torquing after the first few rides?

are you using the proper torque sequences?

have you checked for flatness?

are you using new head gaskets?

I was always under the impression that retorque is required AFTER running in. I just never got to finish running in because the head was blowing already. What would you mean by 'first few rides'? How many km... how many hours?

 

yes, using correct torque sequence.

yes, head checked flat.

Yes, new head gasket.

 

I have also been advised to replace the original stretch bolts with standard high-tensile bolts. I have checked for clearance under the bolts & have ascertained that they are not bottoming out

[SDet]

Is it failing at the same spot?

[nsman]

 Check very carefully for detonation picks at the top edge of the cylinder.If there is damage in this area the combustion process will quickly cause  head gasket failure.

[DEATH_INC.]

Do you have pics of the blown gasket and head and cylinder mating surfaces? It may help peeps to see what is going on.

10.25-1 isn't really that high compression ratio.

Have you checked the combustion chamber volume to make sure the head hasn't been machined too much at some point?

Are you using the stock gasket, or an aftermarket one?

[highmarker]

 detonation can fail ahead gasket also,  10.25 may not seem high but if the cam timing is short (early intake valve closing) it can make a lot of pressure.

[grayracer513]

More than combustion pressure? Hmm?

[buttle]

I have a 1984 XR500RE with a radial valve head. About 15 years ago I installed a high compression Wiseco 10.25 :1 piston.

 

Ever since then, I have never been able to run it in without blowing the head gasket.

 

 

OK, I think I had better clarify something here. It is not actually burning or damaging the gasket in any physical way. By 'blowing' I mean it is blowing past the gasket and out the side of the cyl/head junction.

[SDet]

Something is very wrong.

[kx450f63]

I would start with re-machining both mating surfaces.

[jmd426]

I would start with re-machining both mating surfaces.

This is going to be your fix along with new head bolts.

 

I used to do alot of 440 kits on 400ex's and xr400.  The recipe for success was to always resurface both the cylinder witch was being done because of the re sleeve anyway, and also resurface the head. I also upper the torque from if i remember correctly 33ftlb to 40-42ftlbs lube the threads well.  Some people had problems with pulling threads, i never did but keep that in mind.

 

With doing this method i never once had a head gasket failure and many did, about once every few weeks at the local track there would be somebody with this problem. No one was resurfacing both and some even had expensive head studs.

 

 

Best of luck!!

[JoeRC51]

It could also be something as simple as too long of an alignment dowel.

[highmarker]

More than combustion pressure? Hmm?

 

if it's causing detonation

[grayracer513]

Detonation is a combustion pressure, not compression.

[EnglertRacing]

Detonation is a combustion pressure, not compression.

 

yes, i think the point he was trying to make is that with a short enough cam, the relatively low 10.25-1cr could be enough to get into detonation.