04 YZ450f timing and valve clearance questions


I have been trying to diagnose rough start, idle and restart issues for some time now. I bought the bike a month ago, am unsure of the previous owners hours, and have put 1hr on it in the times I've gone around the neighborhood.

I just took a look at my valve clearances and found this:

Missing camshaft cap bolt (exhaust)

ImageUploadedByThumper Talk1462656748.579891.jpg

ImageUploadedByThumper Talk1462656778.000963.jpg

1 exhaust valve pretty tight

Valve clearances:

ImageUploadedByThumper Talk1462656720.846059.jpg

My timing seems to be off, if I'm not missing something. ImageUploadedByThumper Talk1462656830.954196.jpg

I was going to remove the caps, reset the timing and remedy the tight clearance, but wanted to make sure I am being thorough enough now that I saw the missing bolt...possible damage? Is it common for just one exhaust valve to be so tight?


Looks like the timing is off, it seriously needs that 4th cap bolt, one valve out of spec shim it if it's within 1 shim size you should be good, if not its probably time to start changing valves and cutting the seats in the head.

If the engine was at TDC when that picture was taken, both cams are correctly oriented to each other, but they are both one tooth advanced. 

If there is an equal jump (both a tooth off) wouldn't that indicate a bad chain? Or less likely, a bad tensioner?

Edited by NorthernMNSurf

More likely bad assembly, but the chain bears inspection.  The tensioner itself simply needs to operate smoothly throughout its range of motion, not snagging or sticking in any small way, and impossible to push back against without rotating the drive screw manually.


As to the chain, ANY even slightly tight links are intolerable, and it should not bend sideways more than about an inch or so.  At only $25, if it doesn't look like new, replace it.

Thanks gray, having seen the PO, I should've assumed bad assembly....

I am camping in the olympic peninsula for a week; will be back to MN to inspect everything and follow up soon

Would it be beneficial to replace all camshaft cap bolts at once in this scenario (I ordered enough) or would it most likely not matter?

Edited by NorthernMNSurf

It shouldn't matter, but you have them, so....

Makes sense. I didn't think to check for a snapped off cam cap bolt, but sure enough...

So my only option seems to be tapping it out while it's still on the bike or removing the top end to put it on a bench. If I can leave the top end in, I was thinking of putting masking tape over the entire gasket mating surface so no shavings fall in anywhere, but does anyone know if the threaded cavity goes all the way through? (which would have shavings falling through)

Any ideas or experience with this would be greatly appreciated!

Edited by NorthernMNSurf

Also, anyone know if lucasoil assembly lube (says it has moly, zinc etc..) would pass for the molybdenum disulfide grease that the manual calls for?

Nvm about the moly, I found a Yamalube tube online. Any guidance or experience on the broken bolt would be much appreciated!

Edited by NorthernMNSurf

Snapped camshaft cap bolt successfully extracted. I decided to check the cylinder rings and surfaces and found an absent dowel pin underneath (gasket surface that mates with crankcase)

A few questions I had:

Should the wrist pin be very hard to get out? (New one + circlips ordered)

Could something like that missing dowel pin potentially cause damage/warping? (Unsure about hours on this rebuild)

Edited by NorthernMNSurf

What you often see is that there will be a little ledge develop at the snap ring groove.  The pin will shuffle between them fairly freely, but needs help to get past them.  If you need to resort to a hammer and punch of any kind, have a helper push the piston toward you as you do that.

I did have to use a (plastic) dead blow hammer and a plastic-handled sq2 driver to get the wrist pin out (before knowing that) but I was very gentle and really only needed to skip the pin in 1/2" increments a few times...

Should I be ok replacing just the wrist pin and circlips (after checking everything with a micrometer)?

Would I most likely be covered If I honed the cylinder with a spring ball hone and replaced the piston rings w oem (+the wrist pin and circlips)

+ new gaskets of course

Parts come in a day or two, still would like to know if the above changes will be enough or if I should start looking into a new piston and jug? Any help would be swell!

I might be talking to myself here, but in the interest of contributing to TT I will continue!

Bought a digital caliper, measured the old wrist pin, piston and sent the cylinder to be honed/checked at a local shop...

The shop told me the cylinder repair was beyond their purview (lack of muriaric acid to get the alum. off the walls??) and needed to be sent out for $300.

Found a cylinder works top end kit w with cylinder, vertex piston, wrist pin, circlips and gaskets for $270 on rocky mountain atvmc. (I think that was pretty cheap!)

Cosmetic gaskets any good? (I have oem ones in hand)

My main question is how do I check the connecting rod for clearance? So far, I can't find anything in my manual about tolerances or specs on the top of the rod itself. At the shop, (taken w a grain of salt)they said they wouldn't even measure the wrist pin/piston because visually, the pin had symmetrical wear lines (rings) on the non-contact surfaces of the piston, and therefore the connecting rod will, with %100 certainty, be damaged beyond repair.

My general feeling from the service Dept of this shop is if you're not already paying them big $$, then they will try to steer you in that direction, so of course I am skeptical. Again, any help would be much appreciated!

Edited by NorthernMNSurf

You're talking about small end clearance on the pin?  First, just replace the pin.  Then look at the rod.  If the interior of the wrist pin bore is scored, or if the original cross-hatch finishing is polished off, it may be unusable.  The new pin should be a smooth slip fit with no detectable clearance. 

ImageUploadedByThumper Talk1465570253.487647.jpg

ImageUploadedByThumper Talk1465570274.901318.jpg

ImageUploadedByThumper Talk1465570289.238453.jpg

You're talking about small end clearance on the pin? First, just replace the pin. Then look at the rod. If the interior of the wrist pin bore is scored, or if the original cross-hatch finishing is polished off, it may be unusable. The new pin should be a smooth slip fit with no detectable clearance.

Yes, I am talking about the small end of the connecting rod, and more specifically the viability of reusing the connecting rod. In the above pics, is that the type of cross-hatching I should look for? I do know what scoring looks like, but it's hard to tell if it's actually pitting or excessive wear...

I have a new pin that I inserted into the small end of the rod lightly, without the piston. There was no up and down clearance but it was ever so slightly able to "wobble" (like a see-saw motion)

Besides looking for scoring and the original cross hatching, how would one know if their connecting rod is usable or not?

Grey, in your opinion, does a (standard bore) cylinder works cylinder kit meet oem specs in terms of durability, service intervals etc? Cosmetic gaskets any good?

Edited by NorthernMNSurf

That rod is gone.  It has been run too long with too much clearance.  The pattern you see in the wrist pin bore is due to the pin "rattling" in the bore and beating metal particles into the surface.  Replace or rebuild the crank. 

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now