Head skim

[80Sprint]

Sorry to repeat a main forum topic but as i had no response i thought someone may only read this section and has more knowledge on head mods.

My Sprint head has been skimmed before. The standard cam journal to main face was 120.5mm from the factory but mine is 120.0, so about 20 thou less.

At what point do you need an extra thick head gasket ? I am running a reprofiled cam and 45's.
It had a thick one before but i am not sure it needed it. I have a standard one and hoped to use that.

[trackerjack]

I would just slap it on, the arguement that it changes the cam timing is used, though this is true a blind man would like to see it!

[gmsclassics]

I certainly wouldn't be concerned about minute changes to cam timing unless you are running a 220+bhp engine.

However the risk of a valve clipping the piston top or rather the valve cut out on the piston is real. As there are so many variables involved in this, especially cam lift, best way is to place a thin layer of putty or plastecene (or howerer you spell it) on top of the piston, over the valve cutouts. Smear a thin layer of oil over the valves to avoid sticking to the putty, bolt the head up carefully to the correct torque with a standard gasket (one you had on before will do) and slowly turn the engine over by hand two full revolutions without the plugs in. Take it part and measure the thickness of the putty after it has been squashed. That will tell you the exact clearance. I was told never to go below 5 thou to allow for heat expansion and movement, but interested in what others think.

Before you start this though check all of the valve clearances are correct and / or adjust them.

The longer term solution is next time you have the engine apart and pistons out, or are fitting new pistons, is to have the valve cutouts made a little deeper. It doesn't cost much to have this done and then it solves this problem forever, also allowing higher compression ratios (but also higher octane). I did this last time had to rebuild the engine and have been able to dispense with the double thickness gasket, back to a normal gasket which of course lasts longer and requires less frequent re-torquing of the head.

Geoff

[jeroensprint]

I certainly wouldn't be concerned about minute changes to cam timing unless you are running a 220+bhp engine.

However the risk of a valve clipping the piston top or rather the valve cut out on the piston is real. As there are so many variables involved in this, especially cam lift, best way is to place a thin layer of putty or plastecene (or howerer you spell it) on top of the piston, over the valve cutouts. Smear a thin layer of oil over the valves to avoid sticking to the putty, bolt the head up carefully to the correct torque with a standard gasket (one you had on before will do) and slowly turn the engine over by hand two full revolutions without the plugs in. Take it part and measure the thickness of the putty after it has been squashed. That will tell you the exact clearance. I was told never to go below 5 thou to allow for heat expansion and movement, but interested in what others think.

Geoff

Hello, electronic solder tin does also the job. Put a few lines on the pistons and here you go.

Jeroen

[trackerjack]

In the pastic mould tool business (me) we use plasticene with selotape round it and stick it to a surface and squish away!

[80Sprint]

Sounds sensible advice guys and happy to give it a go. No original gasket to lay my hands on so i will have to use the new one. I assume that using this along with bolts that will be stressed by tightening them will not be an issue for re-use.

Could you also measure the piston at full lift down to the valve cut out with a straight edge and also measure the valve base height protruding from the head. ? Take one measurement from the other and hope it's more than 5 thou ....

[gmsclassics]

I think you might find that the cumulative errors in each measurement, however small, might make this of questionable value.

It is also worth remembering that as you skim the head surface, because of the angled studs that are being torqued down first, the head is being pulled further and further across the block surface, rather than simply straight downwards onto the piston. Best I think to use the trial and error method as above.

After having being supplied from 'a reputable source' replacement studs with nuts that weren't able to withstand 55ft lb torque without ripping the threads out, I have now moved to cap screws (best) or just ordinary bolts to keep the head on (8.8 not 5 grade). Stretching does not appear to be an issue. I lubricate them with copper grease and they come in and out easily.

Geoff

[80Sprint]

Geoff

Where did you get hold of 8.8 grade caps or bolts ? If they don't stretch to your knowledge could they then be re-used as well making them more cost effective ?

[xvivalve]

I have ARP studs supplied by Ken Clark; recommended