after rebuild exhaust valves touching piston!!!

Ok bit of a problem here. this started a couple of posts below when asking advice on valve replacement.

I replace the intakes on my 05 exc 400. used OEM valves and had them cut in. did not do anything to the exhaust except remove them to clean. After putting it all back together the exhausts are touching the piston - I could hear it and on tearing into it again tonight you can see very subtle contact, no damage - really just the carbon layers got polished off but still hitting.

I timed it this way:

-insert TDC bolt on front of case

-install cam lobes down

-have the two dots level with top of head.

*I also installed a new plug

I do not have a leak down tester but the compression with a normal gauge was 90.

When I took the head off I checked the valves for seal by filling the head with acetone and checking to see if anything leaked through valves - nothing. Good seal.

I reused the head gasket and the base gasket.

Previous to rebuild the bike ran rough and would generally start and run for a few seconds then die and never start again. This is what it did after the rebuild too , except with the clacking noise that was caused by the contact with the piston.

Here are some things that maybe could cause it:

-base and head gasket old and too thin and now too thin causing contact?

-maybe F-ed up and timed it one tooth off??? can't see how but maybe

-cam chain decided to get real long suddenly and put timing out?

Other than that I can't figure out what has changed.

Help!

You may have missed the tdc slot on the crank.

Soda straw in the spark plug hole and/or flashlight down the stop bolt hole.

Realiy bad rod or totally bad mains.

Prox piston?

Is the adc stop at the top of the cam gear with the lobes down?

Doc bob

after doing some reading I am also considering the chance of the crank bearing being out. After a few tough enduros I noticed a growing vibration in the pegs and tank. I assumed I was making it up. Can I check the health of my main bearings without cracking the cases?? just reach in and wiggle it? I have never actually had a bike with crank bearing go.

You did a rebuild, did you replace the piston? I have seen this happen when someone installed the piston backwards.

-maybe F-ed up and timed it one tooth off??? can't see how but maybe

That's my bet. There's a tight side and a slack side to the chain, you could easily be a tooth off if you get this wrong.

That's my bet. There's a tight side and a slack side to the chain, you could easily be a tooth off if you get this wrong.

I really like this idea.

I never touched the piston - it had zero visible wear and I have been told these RFS are usually good for two sets of valves for every piston - so far that seems about right.

We never really looked at the timing dots after we put the link on. if we set the cam in properly, laid the chain on, then took of the slack with the tensioner it could have easily rotated it clockwise one tooth.

I have not checked the length of the chain and I was stupid enough to remove the cam without first checking to see how far the tensioner was extended - meaning how much stretch there is in my cam chain.

Anyone have the acceptable specs on cam chain lengths??

I would still really like to investigate the bottom end bearing considering I have the top off for the second time this week.

That being said, is one of the reasons you always clay your engine after a new rebuild. Things can change, either the piston dome could be off or like others have said, a tooth off on your cam timing.

Either way, it's always a good idea to put it all together and crank it over by hand to make sure nothing is hitting before you go and just start her up.

Good catch!

That being said, is one of the reasons you always clay your engine after a new rebuild. Things can change, either the piston dome could be off or like others have said, a tooth off on your cam timing.

Either way, it's always a good idea to put it all together and crank it over by hand to make sure nothing is hitting before you go and just start her up.

Good catch!

its touching so lightly I could not feel it when I did turn it over by hand! I saved an XR80 from certain death after the pro rebuilt it with incorrect timing - he never even turned it over after putting it back together, gave it a gentle turn and thud, good thing I never tried to kick it!

I picked up a new link and will put it back together and watch the cam closer as to what it is doing when I re-install the chain.

anyone know the specs on a tired cam chain?? will my manual tell what is too long.

problem solved. When we did the timing the first time we had the piston at visual TDC and inserted the TDC locking bolt, but after some TT research that TDC bolt can be tricky, it is VERY sensitive to being in the right spot and it can easily drop into a slot only a tooth away - that's what I did. so this time I checked and double checked. I got out a flashlight and looked down into the cases and saw the bolt go into the TDC slot. Then set the chain on the cam and checked and rechecked before i swedged the new link that even with the chain tightened it was still in the right spot.

put her all back together and she fired up on the first rotation and purred just nice. wow what a weight off my shoulders.

That is awesome. I am glad you didnt bend the valves. Happy Riding

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