Valve shimming

How much will your valve clearance change after hand lapping your valves (approx)?

Clearance should decrease by 2 * cosine of the angle between the valve stem and the contact annulus multiplied by the increase in the width of the annulus caused by lapping. For example if it's a 45 degree cut valve and you lap an increase of 0.02mm the clearance will decrease by 0.02mm*2*0.707 = 0.028mm

Using max minus min recommended width of the seal will give you the max variation.

It's more handy to use change in clearance to tell how much you've lapped than the other way around.

It's easier to just lap and then measure the new clearance.

-Duck

Post edited by: Duck, at: 2005/12/05 05:14

Let'roll, I agree with Duck, regarding "measure after the event"!

The thing to watch if you are doing a valve-grind is that:
You probably have had the the valves machine ground and "tipped" intially. In this case, the "tipping" facing process, where they (supposedly) just take a "lick" off the opposite end of the valve, can cause the biggest alteration of clearance.

In summary the valve face grinding reduces the clearance and the valve "tipping" increases the clearance. Too much grinding at the tip can interfere with the collet/ bucket clearance.

RegardZ

Personal observation... I have found that if you lap properly, you won't change the clearance much at all... maybe one shim size smaller shim needed. That is only .05mm... This is only if you don't have a valve job done... only lap to seal things up. Don't lap too long... most folks tend to feel more is better.