Problem lapping head to barrel on a '54 VH500

[Roger Gwynn]

Julian is correct, the head joint is sealed on the top of the spigot, there should be a small gap between the head and cylinder. As a general rule the change from a head with a stub for the exhaust pipe (over) to the push in type took place at the same time as the reduction is size of the ex valve but I have seen at least 2 heads with a mix of features.

I still do not understand using metric measurements on parts made to imperial dimensions. If one is not used to using imperial dimensions it is fairly easy to pick up and I would have thought part of the interest in owning old stuff. I grew up with imperial but have 'learnt' metric and am reasonably happy using it but measureing things Ariel using metric does not make sense. It is a well worn maxim that before asking the question you should know the answer, eg measure a worn shaft in metric say it measures 22.15mm, or in imperial it is .872". What size was the unworn component, using metric, not a clue, but imperial it is obviously .003" under 7/8.

OK dismounting from hobby horse!

[dennis.t]

Now I am confused. So the combustion chamber sealing is via the spigot seal only?
I read threads and technical advice on this forum that indicate grinding a mating surface on both the spigot and the barrel to head surface until a matte surface is evident on all of the head surface i.e. no gap between the head and the barrel. Is there a definitive reference in a workshop manual, for example.
I am worried that torquing down the head with a 0.7mm gap may distort the alloy head.
Dennis

[dennis.t]

Roger,
I take your point about mixing units of measurement i.e millimetres and inches but my vernier caliper and feeler gauges are metric. Also, not wishing to give offence but original imperial measurements that include fractions of an inch and decimal inches are also mixing units
I will endeavour to quote imperial measurements in future even if they are converted from metric using the internet.
Dennis

[dennis.t]

Just read Charles' reply and Julian's so the gap between head and barrel may be correct.
Many thanks, Dennis

[Simon.Gardiner]

'Sealing in the spigot' is the method used with these particular joints that don't have gaskets.
This is usually achieved by 'double grinding' the joint with coarse paste on the outside and fine paste in the spigot - keep the coarse paste renewed, let the fine become really fine (I've been known to finish with metal polish on the spigot).
You end up with a slightly greater gap at the outer surfaces than on the spigot, when tightened down the difference is enough to seal is on the spigot while still allowing the outer surfaces to pull up to each other and reducing the chances of the head distorting.
See for example 'Tuning for Speed' (Phil Irving') p110

There will be references to this 'double grinding' scattered about this forum. I've not seen anyone recommend leaving a substantial gap between the surfaces outside the spigot before (which is why I say the usual method is this 'double grinding') but that's not to say it's wrong - if the head is reasonably robust it should be fine.

(I wouldn't worry about mixing metric and imperial as long as you're not dealing with Roger - I think it's all been a bit of a grey area since cylinder-bores and capacities started using mm and cm as far as I'm concerned )

SG

[dennis.t]

Simon,
Many thanks for your reply. I will go with your method.
Roger is entitled to his opinion. If you think engineering is bad at mixing units try the building industry. I once bought some lead sheet and was told it was a yard wide and sold by the metre!
Dennis

[will_curry]

The cylinder head nuts don't need to be that tight - they'll tighten up due to differential
expansion as the engine warms up anyway.

The tricky bit fitting an alloy head comes with the pushrod tube seals. These are invariably
too big and hold the head off the spigot and make a nonsense of what I've just said about
tightening cylinder head nuts. There has been a mention in this thread about making
alloy spacers and using viton 'O' rings in place of conventional seals. This is well worth
doing as it greatly simplifies subsequent assembly.

[dennis.t]

Will,
Thanks for that, I will investigate.
Dennis

[nevhunter]

I think a SMALL gap would be advised on the Alloy barrell with the sleeve pressed in. That's pretty normal with other makes. The "double grinding is the go with CAST IRON . You lap both then wipe and oil the inner face and grind a bit more on the outer. That way you get extra pressure where you need it and good thermal contact on the outer. The alloy heads seem to have problems with the seals rubbing on the alloy push rods and I suggest a thin steel sleeve to prevent that. It can have a step on one of the outer diameters to move freely in the other. IF the seals are too bulky they have nowhere to go. Maybe a suitable VITON "O" ring is better. Nev

[Julian Murphy]

The fact that the seals rub on the pushrod was the reason that I made up the alloy sleeves and used O rings.

I strugged getting the head off and discovered a polished section of the pushrod.

There has to be a gap between the head face and the barrel face or else it will not seal down on the spigot.

You are not using a gasket to seal onto.