How much wear is too much on a wr426 crank timing gear?

I recently replaced my timing chain and a month later I have mashed the valves.

Prior to timing chain replacement I was getting aluminum bits in the oil. I duly noted on replacing the timing chain wear in the aluminium of the timing case in the head so this put me at ease thinking a nice new chain should fix this problem.

Anyway I was spinning on the spot on slippery ground attempting to climb a hill at about 2 -3000 rpm. I released the throttle quickly and my engine made a chainy type clunk and it it was all over... I'd snapped off an exhaust valve bent an inlet valve cracked a guide... and smashed a cam bucket and a few put a small dings in the piston.

I did check the tensioner was working correctly when I replaced the TC and it was released an.d the timing was correct. Id ridden 2-300 miles after changing the timing chain.

I dide not after replacing the chain I was getting a bit of timing chain slap when I backed off...

So my intended solution was to install a manual chain adjuster.

I didnt get round to it though... :(

Should this fix the problem or is there more to it?

I have duly noted the crank shaft timing chain sprocket isnt as good as it could be...

Any assistance much appreciated...

Cheers from Australia






Edited by rileyp

Pics added

Judging by the photo it still looks fine, I've seen a lot worse on running bikes.

What is the chain tensioner like now.strange how you replaced chain with new one ,then could hear it rattle when you back off.see if it works correctly now.put it under pressure on your bench and see if stays out,or collapses.did you see chain nice and tight when you set cam timing.did you measure old chain to new chain to see it was infact worn.

The old chain had same number of links . It binds up in hand and is visibly damaged (swaged looking)  on its inside faces. I didnt measure it. On releasing the tensioner the chain was tight with very little give between the cams (maybe 1 or 2 mm if pushing very hard under thumb.)


I also duly noted when attempting to turn the engine over via the timing chain by hand from where the cams normally sit upon reaching a compression stroke and it getting very hard to turn the chain skipped...

Whats the point of the pressed metal guide on the bottom of the slipper if it does not sit close enough to the chain to prevent a skip?

I'm thinking I need to make something out of 3/6 or 1/4 thick flat bar  that I can bolt in there that sits close enough to prevent a skip.

Edited by rileyp

The pressed metal guide is intended to prevent the chain from "drooping" a link under the crank gear while it's free of the cams.  the tensioner is supposed to prevent skipping.  You cannot rotate the engine in either direction beyond the point where a cam lobe contacts the lifter unless the tensioner is in place and totally functional.  I suggest you replace yours.


Also, since you mentioned having aluminum ground out of the chain well, be sure all 3 of your chain guides are in place and in good condition.

I have replaced the 2 long guides that sit in the timing case and tensioner and the one on the rocker is barely marked.

Thanks everyone for your help! I really appreciate it. The tensioner old and new seem Identical in their operation.Press on bench won't moved back in. The two old guides in the timing case were a bit worn-chewed looking on edges on the non tensioner one.

It it feasible the new chain is stretching as it beds in and the auto tensioner isn't responding fast enough. I have counted turns of the screwdriver on the tensioner from wound fully out to fulling in at around 4.5 if I recall correctly. When I release from the fully out clicked position with the new chain on it it moves in about 2.5 turns again iirc. It was a few weeks ago now.

Edited by rileyp

The tensioner is a perfect example of effectiveness through simplicity.  It works on the simple principle that you can't push a worm gear backward, any more than you can pull a screw straight out of a threaded hole.  The plunger has a spiral cut groove in side it, like threads that are square instead of V shaped.  The drive screw fits into this, and is driven by the tension of the spring.  Basically, ANY slack that appears takes pressure off the plunger and allows the spring to advance it.  The chain cannot push the plunger back.  The trouble starts when the chain gets tight at the pins and starts kinking, as this "fools" the tensioner by not allowing the slack to develop on the back side as it should.  Otherwise, they are almost too simple to fail.

I am very impressed with design of the auto tensioner and have pulled it apart (my old one)and rebuilt to get a better understanding of how it works . With my old chain it never gave any trouble apart from a rattle every now and then and a bit of aluminium swarf from the timing case when it hit in the oil filter. Now I have replaced the chain it has skipped 3 or 4 times. The first skip is normally a warning where compression is lost. If 2 skips occurs it's crunchy valves and a mashed piston. This has occurred and I simply can't afford for it to happen again. I get timing correct at 13 pins between marks and crank at tdc and release tensioner and timing chain goes nice and tight. it starts first kick with greT compression and all seems perfect then ride for a while and I get the dreaded rattle which is the precursor to a skip.

I'm going to go with a manual tensioner and if I ever hear the slightest rattle pull over and nip it up a bit.

It's a brand new genuine yamaha chain. Do they stretch a bit as the bed in?

It's the only thing I can think of that can be giving me enough slack for a skip.

Edited by rileyp

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