Why would it have cracked in the first place if not a stress point... Remove the bolts, see if the leak is less.

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'84 XT600 is now bored to 2nd oversize and new OEM pistons and rings installed. No more smoking.

Good question why it has happend, never heard of it before and havent had that experience with older xt with twice mileage. Just tried to understand what you mean with word stress point, i quess you mean that its alot of movement on the monting, I recon if there is so much weight on that monting they would have done it with more strength. I will call it just my luck and seal up and ride for as long as it last, if it cracks more i can try TIG.

Unless other things are flexing, like the frame, there shouldn't be any hard stresses, but I'm sure these frames flex especially under a load. Tig welding is ideal and it will most certainly warp some, but how much ??? If it is bolted down to a heat sink when it is welded it would minimize warpage , but it really should get remachined after . I'd try some of alustick or JB and maybe leave that bolt a bit loose, I don't know just how much it is critical to have it.Maybe look at making a bracket to mount else ware so that one can be let off. Otherwise your looking at a big fix. This way you "may" get by without , worse case is it doesn't work and you have to weld it.

EDIT : I went out to see how thick that cover is around that mount and have the one off my 3TB sitting there. Well looky here,


Cracked just like yours all the way around to the inside by the sparkplug. I got 2 covers off the older '80's motors and noticed the newer cover is thinner on all the areas for bracing and strength. The dip for bolt/socket clearance right below the hole is deeper and runs all the way out where the older ones just have a shallow droop just below the hole itself . I think we just uncovered a design flaw. I guess I'll be machining this one if I use it on a motor otherwise it's getting gutted of rockers and tossed.

3TB
'85ish

Then there IS stress there and a weaker designed mount shows this up..... How much plainer can it be. Yes, it will be a problem fixing it. Like Jim said, if you can bolt it down tight before welding, it might not warp, preheating the part evenly should help prevent it if it cools down slowly all over....

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'84 XT600 is now bored to 2nd oversize and new OEM pistons and rings installed. No more smoking.

No kidding there is stresses , that's why I said if the frame flexes, which they do, but are these mounts truely needed or are they trying to fight movement that doesn't need to be stopped? Sometimes more mounts fight each other and cause more problems than they solve. maybe the newer frames are thinner and move more also and those aluminum covers are the weakest point , add in hot/cold cycles with vibration. Rubber mounted would probably stop the cracking because solid mount do cause cracking ,just because there can be no movement, maybe that's whats happening.
Two of my bikes didn't have those top mounts hooked up, didn't look like they ever where or were taken off early in life with no apparent ill effects. When looking at the frame geometry it is trianglated without the top mount, they may be causing a pinch point .

We've welded enough aluminum to know there will be warping, even bolted down, the weld shrinks that area and will pull up no matter what. Letting it cool bolted down minimizes things but won't stop it. Stainless is 10 times worse.

Thanks for all the info, this is not anything i have experience with. I was thinking of getting a thin rubber spacer beetween engine and the frame monting to unload some of the vibration. If you put like these on both side of all the screws. I dont know why they didnt make it more flexible from factory. Actually the bolt that hold engine should have been thinner with rubber bushing around, the side that cracks has only support backwards and thats a big mistake from the engineer if you ask me. I can buy brand new cylinderhead + cover and after a short time of riding i will experience a new crack at the same spot after spending 800dollars + all worktime.



Iam not shure its possible to get a good weld inside the cover,is it possible to see cracks inside your cover JJrider?


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Are you 100% sure those are cracks and not casting marks??

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Yamaha XT600E '01, Honda XR400 '03, Yamaha XT550 '83

Mine looked the same before i grinded, it was never in my mind this was crack and possible for oil to come up. After grinding more oil comes up, so what you think is just marks today is maybe a leak tomorrow.

IMHO it's not the vibration making the crack....

not hard to imagine that if that top bolt is over tightened, the two soft alluminum ribs will compress a little bit an cause a bending stress on the ear (that does indeed break). first gen gsxrs had same problem in the their bottom rear engine mount.... dont ask me how i know...

now then, when building my XTZ supermono i had to design a new top motor mount and indeed, passing a through bolt didnt make any engineering sense to me.

i therefore ground a bit TWO separate M10 allen bolts to fit from the INSIDE so no compression stress is put on those ribs / ears. might be wise for others to do in the future to avoid such problems.

now that i see this, I will do the same in my XT supermoto engine!

another solution could be to fit a very tight fitting steel sleeve between the ears to take the compression load.

yep, even perfectionist japanese engineers can get it wrong sometimes,

in your specific case i'd surely go for DEEP welding, also from the inside. if you weld only outside the crack will propagate again.

Well i dont really know what causes this, but its good suggestion to insert a steel sleeve inside. I know i never been overtighten this bolt, if dont remember wrong its 65nm on those bolts and thats pretty damn hard..I think its a more complex problem with movements and heat, bad support on that ear+++. If this thread can lead to others not having this problem its fine, i will first talk to someone that do welding all day long before i decide anything. If its a gamble of the head cover beeing damage i will seal up the oil leak and reduce stress to the ear and ride for as long as it last and buy new. I recon the price for welding is so high that i prefeer buying brand new! Honestly, the last thing i wanna do is going back to the seal problem again with cover not align to head...

I can just add that my cover never had been taken off before, it was the original seal on. I had the bike for 6years before the crack came and i never tighten the bolt before, everything seem normal at release.

G600 , it is most positively a crack and not casting lines, in person you can see it better.

Now that it has been mentioned I remember seeing steel sleeves in my 660 motors mounts.

if you decide to just reseal, would actually do it from the inside.

rough up well and clean with solvents all the area to be sealed and use some high quality Araldite, brushed over in a not too thick layer. big blobs will break off...

that said, i've welded heavily long cyl heads in in-line 4 motors i tune for racing and never had warping problems. see very little risk of bad distortion with the valve cover if you weld.

Ok thanks for info, for now i just use some sealent on top, made a sleeve and see if it stop leaking. Hoping it last this summer and get it welded when its cold and rain again...

I may play with mine to see if it stays, worse case is I throw it away or remachine it anyways. The worst thing is the bores for the rockers, they have tight tolerances and need to be inline, one is right below the bad spot and any movement will make them not fit. It would be more work than it's worth to have to bore out any of those to put a bushing in and then re-line bore it.

I can see the crack on the inside, it goes right betweem the 2 bosses for the rocker shaft, a small TIG torch will fit but there won't be any room for the side to side sweep, MIG would work but not as nice, also welding inside would cause the cover to cup inward, how much again ,can't say, but any little bit will be a problem . If I do mine I'll just drill a small hole at each end of the crack and weld the outside.

I have the same issue on my 1990 but the crack is worse. I got the bike at 24,000 miles and it must have been an issue already because it was caked with congealed oil already and when I re-installed the engine it got worse. I put some oem grey gasket maker on there for now and left the mount loose. I haven't noticed any bad side effects as of yet. Why they would thin the mount out and still have 46ft/lbs on such a fragile part makes no sense...

It kills me to have such a ghetto fix especially since the rest of the bike is totally freshened up with OEM parts and well sorted but I am not taking it apart again unless I have to.

I have about 42,000 miles on it now so those of you with e-start bikes with less miles just throw some rubber donuts on there to maybe keep if from cracking in the future.