Valve lapping

[Albert Donaldson]

Good morning,

After porting K-series head and cutting new valve seats I am struggling to get a reasonable seal between valve/seat. I have 2 valves out of the 16 that will hold a column of WD40, the rest leak off at varying rates, I can see a sliver of light between valve and seat. I am wondering how to correct.

I am thinking that maybe the seats are too wide and considering increasing the inside diameter and then relapping, or perhaps I just need to lap some more.

Advice appreciated.

Albert.

[Roger King]

I'm afraid that the seats are badly cut. Your only options are to have them re-done (far and away the preferred choice) or to lose a significant percentage of your life grinding them in (not a joke - modern seat materials are very very hard).

In my experience, most machine shops are not capable of cutting seats accurately. Specialist (and expensive) equipment is needed for this job. I used to use two different machine shops who I knew I could trust and this meant that I simply had to have a quick lap with fine paste on each valve to check. Typically, a 16V head could be lapped in less than an hour.

You should end up with a light grey seat of even thickness all the way round the insert. The final check is to use a very thin film of engineers blue on the valve itself; this should transfer evenly to the seat in the head when you insert the valve and gently rotate it a little.

Production engines generally are not lapped at all these days; the equipment is accurate enough to guarantee that it is not needed.