Heads And Valves

Round Three – Head Re-building and Valve Clearance Setting.

With the heads all stripped and cleaned the next job is to rebuild them. Valves will need to be lapped in to ensure an accurate seating is achieved. Lapping in valves is a bit of a chore in some respects, but an absolute essential if you are to get the full performance out of your engine. When Bill started to check the valve clearances he gave me some great news. If you remember I mentioned in the first phase of this project that the compression on No 4 cylinder was well down. The reason was that there was no clearance on the exhaust valve, therefore it was never shutting properly, hence the lack of compression.   PHEW !!!!!

Some of the essential tools you’ll need for the rebuild. Feeler gauges, shims of different thicknesses, micrometer and out of shot spring compressor.

Head Rebuild valve setting 001

Bill doing his ‘favourite’ job of lapping in the valves.

Head Rebuild valve setting 005

Lapping tool in situ. With lapping paste on the valve and seat you now employ the boy-scout fire-starting reciprocating stick rotating technique which grinds away small amount of the seat and valve against each other ending up with a perfectly mating valve and seat.   It would be much easier to find a boy scout and pay him to do the job in the first place!

Head Rebuild valve setting 007

Everything is now ready to go back into the head now.

Head Rebuild valve setting 010

The bottom couple of coils of the valve springs are coiled slightly tighter than the remainder, so be careful to put them back the right way up.

Head Rebuild valve setting 012

The valve clearances can be a bit of a game. Each clearance needs to be measured with the cam in place. If the clearance is wrong the way you correct it is to ‘calculate’ the difference between what it is and what it should be and then measure the shim currently in place. (The shims are located on top of the spring assembly under the cam follower/bucket). By ‘adding’ the difference to the dimension of the current shim, you will now have the thickness of the required shim to give you the right clearance. Clear? Not a chance! Here’s an example which hopefully makes it a bit clearer. Current valve clearance = 25 thou. Required clearance = 18 thou.  Difference = +7 thou. Current shim thickness = 48 thou. New shim required = 48 + 7 thou = 55 thou. Clear?

Head Rebuild valve setting 017

While Bill was busy working on the valve clearances I spent some time cleaning up and painting the various bits that we’d taken off.

Ancilliaries Clean and Paint 003

Inlet manifold looking good.

Ancilliaries Clean and Paint 004

I put a block of wood under the front cover to support it whilst I belted out the crankshaft seal. When I say ‘belted’ I of course meant to gently persuade the seal to come out.

Ancilliaries Clean and Paint 005

Seal removed

Ancilliaries Clean and Paint 006



PLEASE NOTE – these are NOT instructions on how to undertake this job just some pics and words of our experiences, always consult professionals when undertaking any repairs or restorations. (please refer to the disclaimer on our about page)

3 thoughts on “Heads And Valves

  1. Restoring a 1976 stag valve spring tension is 30lb tested spare springs also 30 lb seams too soft thought they should be about 70lb. Could you advise, finding it hard to get enough specs

      • Hi David, I have had this response from Bill, as will all advice from owners its his own opinion.
        “I don’t have any data for valve spring pressures, they are not published in any of the Stag workshop manuals, but I think 70 lbs seems much too high!

        When checking spring tensions, you need to know the fitted length of the spring when assembled in the cylinder head, then using a special spring pressure gauge you compress the spring to the normal fitted length and observe the pressure showing on the gauge.

        But I usually take a new spring and compare the length with the ones removed from the head, if they all look the same length they should be ok.”

        I hope you also had some useful advice from the forum too

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