Tight valve clearance on rebuild

**qwertyur** · 01-22-22

Hey y'all,

I'm putting my motor back together and I've reached the stage where I am checking inlet and exhaust clearance. After measuring each one I've noticed the clearances are low and some are below spec. Is this a normal thing when rebuilding the top end? Do the clearances expand once the motor has worn in again?

My tolerances are as follows
BMW Specs: Inlet 0.05...0.12mm/Exhaust 0.22...0.29mm
My Specs: R-inlet 0.04mm/L-inlet 0.08mm & R-exh 0.20mm/L-exh 0.15mm

It appears I could get away with the current clearance for the intake side but I'm not so sure about the hot side. If anybody has guidance on whether I should reshim both intake and exhaust or just one please let me know!

Thanks squad

*Edit: Let it be known that I am reusing the shims that came out of this motor prior to the rebuild.

**Tims1572** · 01-23-22

Are you sure you didn't get the semi sphere shims for the Intakes mixed up? Swap them and see what you end up with.
Were the cams followers removed during service?
They should have a witness mark from the side that rides against the head and the opposite side that was against the retainer C-clips.

I would definitely open up the valve clearances on the Exhaust side.

Fire the engine and within the hour, while the engine is still a bit warm recheck all of them.

Just for some additional information.

The Husky's 449/511 TE and TXC models have valve clearance specs very close to our G450s and the cams are very similar.
However, the Husky TC 449 models which run hotter cams uses valve clearance specs way outside the G450 specs.

My spare big bore engine started out life as a TC 449 5 spd. and it had the G450X wider ratio 5 spd. transmission swapped into the cases during rebuild.
It got full head porting work done to the TC head and it still has the hotter TC 449 motocross cams with a new 101mm cylinder and ProX piston.
When it got reassembled the intake valves were both a little bit tighter than when I took it apart and everything was clearly marked and separated when it was taken apart.

It hasn't been off the engine stand to be plugged in and run yet as the original G450X engine in the bike now probably only has less than 15-20 total hours on its rebuild and fresh top end.

I'm in no big hurry but I still need to get a few thinner intake valve semi-spheres to loosen the intakes up on the spare big bore engine just a little before I ever run it.

The only difference in their valvetrain is that the 449/511 Husky models use a small lash cap on the tops of the valve stems to decrease wear on the Titanium valve stems.

The TC valve clearances are below:

Intake: .20mm - .25mm
Exhaust: .28mm - .35mm.

**qwertyur** · 01-23-22

Hey Tim,

I thought that it was odd that the clearances were low and I originally thought the intake shims were switched, so I had already switched them and remeasured them. However, the tolerances were still the same on all 4 valves.

Cam followers were never removed from the head and I only touched the valve assembly, stem seals, and cams.

There is nothing to worry about in terms of damage if I were to run the motor for a short amount of time before rechecking the clearances like you suggested, correct?

What is the best way to measure the shim size? I have a digital caliper. Is a micrometer the ideal tool?

**Tims1572** · 01-23-22

A micrometer is best, but a digital caliper will work fine too.
The Intake shims themselves not being where the feeler gauge is used to check the clearance, calculating the difference required to adjust to the actual shims doesn't have a formula that I know of.
However, the distance measured with a feeler gauge between the cam and the top of the follower is likely a lesser amount than what you need to increase the clearance by on the semisphere shims or as the CD-Rom refers to them as oil sinks themselves. Since the shims are further out on the ends of the followers compared to the wiping position of the cam against the followers. I don't know if there is any hard and fast rule to actually doing this other than check the clearance, remove the shims, measure them and change them by an amount just over the previous measured clearance needed. You may find out it takes a couple tries at doing this to get them right. Look to see what sizes are available and select a few sizes thinner than what you now have.
I can't remember what model big BMW road bike uses these same semi-sphere shims but they shouldn't be any problem to get, just ridiculously expensive.