tighter or looser?

[Seeker]

There are some topics that I have stored in my brain that I carry along with me confident in the veracity of my thoughts. Then I verbalise them to someone and my firmly entrenched foundations are shaken.

Valve clearances - do they tighten or loosen with mileage?

I always thought they tightened as the valve seat wears. My MT07 friend contradicted this and claimed the dealer said they would get noisy if they needed reshimming.

Googling it gives mostly (but not completely) that they tighten.

[Sir Trev]

I'm with you on tightening. The Mundaneo I had with a LPG conversion did not have the lubrication unit fitted and all my clearances tightened due to valve recession. The garage lightly ground something or other, so they did not have to take the head off, and it ran WAY better afterwards.

[glang]

I think the confusion is because both can happen in different cases. If theres valve recession then they will get tighter but if not and you do a lot of high revving miles the shim can get a wear spot in it where it sits on the valve stem plus the cam/bucket themselves wear all of which increase the gap...

[Craig380]

Originally Posted by glang

I think the confusion is because both can happen in different cases. If theres valve recession then they will get tighter but if not and you do a lot of high revving miles the shim can get a wear spot in it where it sits on the valve stem plus the cam/bucket themselves wear all of which increase the gap...

Exactly this. Regular high rpm usage will accelerate valve seat recession (thus clearances closing up) simply because the valves are slamming more frequently into the valve seats per hour of riding.

However, if the cam lobe's opening angle is aggressive, or the contact area between cam and bucket / follower is relatively small, you get cam / follower wear which opens up the clearances. This was a problem with Honda's early VF750 with the 'chocolate cams' fiasco, the cam opening angles were very steep and the friction forces this placed on the cam lobes & followers broke through the oil film on the mating surfaces, causing very rapid wear and in some cases, written-off engines.