×

Sale ends todayGet 30% off any course (excluding packages)

Ends in --- --- ---

# Cam timing and TDC after Head and block milling

### Tech Articles

Discussion and questions related to the course How to Degree a Cam

Hi,

Engine is a SR20DET (Timing chain driven), head and block were milled about 0,3mm in total.

How does the milling job affect cam timing (retard or advance) ?

About TDC point, in my case the deck clearence is about 0 or slighly positive after milling the block, does this affect the real TDC ?

It does retard the timing, but by a tiny amount - IF I have my sums right (if someone would be as kind as to double check?), then...

1mm of head and/or block skimming, with a 100mm effective diameter, will mean the camshaft sprocket will be 1.14 degrees retarded and, as cam' timing is in crankshaft degrees, that will be ~ 0.57 degrees retarded. This will be approximately in proportion - half the skim and half the angle, half the sprocket diameter and double the angle.

No, TDC is down to crankshaft position - well, crank and/or gudgeon off set alters it a miniscule amount, not something to really worry about unless you getting into some VERY advanced building. You would be well advised to check piston to head clearances, though.

Hi Gord,

I dont understand the calculations :) is there any calculator for this ?

Sorry about the delay, it's very basic trigonometry - this is a quick calculator for it - https://www.omnicalculator.com/math/arc-length. Use your camshaft sprocket effective* diameter or radius, use double the amount removed for the chord (because were're bisecting the chord and angle) and then half the resultant arc for the precise camshaft sprocket angle change (because we need half the arc), then double that for actual timing change at the crankshaft as it needs to turn twice as fast as the camshaft.

For example, for the calculator, using a effective* sprocket diameter of 100mm, 0.3mm removed (which we double to 0.6mm for the full chord), we get 0.687553 degrees which has to be halved to 0.3438 degrees for the actual camshaft angle (half the arc) and that's doubled in turn for crankshaft degrees (which the actual timing is done in), for 0.68755 degrees of timing at the crankshaft.

*Effective is going to be through the middle of the chain pins for a chain drive.

We usually reply within 12hrs (often sooner)

#### Need Help?

Need help choosing a course?

Experiencing website difficulties?