Cylinder head gasket

[RogerRust]

I'm not happy with the suggestion that 40 thou is the correct amount to pocket the valves. I have measured the amount the valves move up and down with the valve clearance set at 12 thou.
It varies a bit valve to valve but averaged out at 7.82mm with standard rockers.

I also measured the thickness of the compressed head gasket by bolting down the head without a gasket then setting the valve clearance to 0 then head off put the gasket in and tighten it down and checked the gaps again. Then divide the gap by the rocker ratio 1.25 to give me the thickness of the gasket = 1.7 mm

The measurement of my cylinder head when I first had it gave 6mm from the valve face to the face of the head.

So 6mm plus the thickness of the gasket 1.7 = 7.7 my valves would have hit the block by 0.12mm = 5 thou in old measurements

40 thou pockets would only give a clearance of 35 thou
As the valve clearance closes up when the engine heats up it would be even less. so my 50 thou pockets are safer.

I strongly suggest you make careful measurements and avoid assuming that 40 thou is the correct figure.

I suspect that my head has been skimmed which may explain the difference between my figures and bmcecosse's, but be careful.

[bmcecosse]

But Roger - 20 thou clearance is enough! And there are those on here who say there is NO need to sink the valves into the head. I'm very confident my 40 thou 'sinking' gives me plenty of clearance - but hey - what's another 10 thou!!
Ray - interested to hear you say that the Austin head seats were only 'hardened' - because this is exactly my view too - yet many others insist that proper hard seats were fitted. I have yet to find any! My view is that this mod can be done very inexpensively - and it works. I used a head that cost me £10 - the valves were already slightly recessed - so nothing to be lost by sinking them a bit more.

[rayofleamington]

And there are those on here who say there is NO need to sink the valves into the head.

Really?? Never seen that myself. Most people say each head should be measured as they often differ.

[bmcecosse]

The only difference I have found is in the thickness of the valve head. And of course - due to wear and tear the exhaust valves do tend to recess slightly with age. But any I have measured up have shown an approx 20 thou interference - hence 40 thou sinking to give 20 thou clearance.

[minor_hickup]

I think our trav may have a dodgy Head gasket now!. Loads of rocker mayonaise. I cleaned it out a few weeks ago when setting valve clearences and now its full again. The cars not overheating, but it seems the coolant has gone down a bit (me being paranoid?)

the car has the old swan neck tappet chest and rocker cover to air cleaner breather, could it just be the design and short runs?

[bmcecosse]

Aye - short runs and low engine temp are not good. Fit a n 88 degree thermostat, and it's best to have some positive crankcase ventilation ie rocker cover breather connected to the inlet manifold (usually via the carb - but can be direct through a 2mm restrictor hole). Change the oil - clean the rocker cover - and take it out for a good run!

[minor_hickup]

Done and done, will take it for a good run at the weekend and see if anything accumalates.

[bmcecosse]

Good luck! But take a spare gasket and a few tools along - just in case.

[Kevin]

This is an old issue, years ago when the 2 main oils were Duckhams 20w50 and Castrol GTX during the colder months the mayonaise was always far worse when using Duckhams than Castrol if most milege was short runs, and it always seemd worse on BMC cars than Fords so I can only assume that some modern oils may have the same effect, as a matter of interest what oil are you using.

[minor_hickup]

Silkolene 20w50, I will try and sort out some positive crankcase ventilation and see how it goes.