Replacing Shims

Frank 55

New Member
Hi my name is Frank and I just have a question.
I am currently replacing the shims in my 1hdt motor.
My question is with the motor at TDC I find that the ex shim in
cylinder 5 going by the manual should have pressure on it
but it has clearance. Is the shim just over worn or is there a
problem with the valve lifter.If the shim is just over worn how do i
calculate the clearance into the new shim thickness.


Active Member
There are 2 TDCs on a 4 stroke motor because the cams spin at half crank speed and you probably have it 180 (cam) degrees out.

You'll need to make sure that #1 is at TDC on the compression stroke which you will be able to tell by either a mark on the cam gear will match up to one on the head, or by checking that no valves on #1 are being opened, its lobes should all be pointing upwards. At the same time #6 lobes will be pointing down, exhaust valves will have just closed and intakes are just about to open. Now you have true TDC.

As for calculating shim size, below is a link to a spreadsheet I made years ago that does all the math for you. You'll have to make sure the min and max clearance are correct for your engine as it wasn't originally for a 1hd. Once you tell it the current shim size and current clearance it will give you a range of what shims will work on that valve.


Frank 55

New Member
Hi Andrew thanks for the reply.
Cam lobes on 1 are pointing up,cam lobes on 6 are pointing down
and mark on the cam gear and head are lined up. The ex valve on cylinder 5 has clearance when the manual says it
shouldn't. Is the shim just overworn.


Well-Known Member
Its rare that shims wear, the wear is most likely to be firstly valve head and/ or seat. This will cause the clearance gap to close up. Next is the rocker arm, bucket or lobe on the cam. Sometimes the the case hardening will wear thru on the camlobe, or the wear surfaces of the rocker,or the flat surface of the bucket where the rocker rubs. This kind of wear will cause the gaps to become bigger. You should be able to see camlobe wear without disassembly, otherwise, unfortunately it will require removal of the rocker gear.
But first make sure the engine is in the correct position. To check the gap for #5 cylinder, it must be on TDC. That means the lobes for that cylinder must be 180 degrees from the rocker arms ( rockers contact surface is touching the lobe on the opposite side to the point of the lobe)
#5 will be on TDC when #2 valves are 'rocking', that is ex is just closing and inlet is just starting to open.
It is possible that the last valve adjustment wasnt done properly, and was left with a bigger gap than specs, but a gap is better than being too tight, which will burn valves after awhile