1962 shortstroke 350 head spigot, gasket and bolt torque

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1962 shortstroke 350 head spigot, gasket and bolt torque

Postby Peter Wright » Thu Dec 07, 2017 10:25 am

I am working on a friend's 1962 short stroke model 16 Sceptre. After a bout of blown head gaskets it was fitted with a copper head gasket but it continued to blow. I have had the head off and determined that it wasn't flat. After some file work and testing with blue on a flat surface it is now pretty good and I consider an original style composition gasket and a bit of Loctite 510 should do the job.

Being a suspicious type I wondered whether the head might have been settling on the cylinder spigot rather than compressing the gasket adequately. I lightly bolted the head down and measured the gap between head and top of cylinder where the gasket goes using feeler gauges.

At its widest the gap is 36.7 thou. and the gasket itself measures as 34-37 thou. Clearly the head will bottom on the spigot without compressing the gasket and this is confirmed in the second photo where the gasket is shown slid into the gap up to the spigot.

Clearly the spigot recess in the head needs to be deepened. Does anyone know what the clearance here should be when fully assembled and tightened? I was thinking in the vicinity of 10 thou. (i.e. increase the depth by about 12 thou.). How much would the composition gasket be expected to squash when the head is tightened down? Obviously the compression of the earlier type copper gaskets would be negligible.

I am mystified by this because I don't think the small mount of metal removed to true the head could have created this deficit. To the right hand side of the cylinder the measured gap is 27.7 thou. which should work with the composition gasket. Prior to this exercise a copper gasket had been made and fitted and this was thick enough to ensure the head didn't bottom and the leakage was undoubtedly due to the uneven face.

And finally, what is the correct torque for the head sleeve bolts. The workshop manual (for 1957-64 models) says 40 foot-pounds for the scrambler and short stroke (page 78) and 35 for other models. This seems high and has caused crushing of the alloy under the sleeve nut heads. Or does the high figure assume some loss from settling of the composition gasket?

I hope someone can advise.

Regards,
Peter
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Re: 1962 shortstroke 350 head spigot, gasket and bolt torque

Postby pkr87 » Fri Dec 08, 2017 7:37 am

Many years ago I had a similar problem with my16M and I did not then have the facilities to machine the head or barrel, so I spent some time with grinding paste to make sure that the head would seal against the barrel and fitted the head without a gasket. It still works so I leave well alone. The head bolts I tightened with a ring spanner about 'that much'.

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Re: 1962 shortstroke 350 head spigot, gasket and bolt torque

Postby Peter Wright » Fri Dec 08, 2017 10:58 am

I did give that notion a bit of thought but eventually decided that the Factory must know best, although I have to seriously wonder about this at times. Not so sure grinding in an alloy head would be the best idea but my vintage Ariel is done that way. And I always tightened it "that much" with a ring as well.

This particular head seems to be made of a particularly soft alloy and the pedestals under the head bolts had become quite dished along with the washers. On this engine, as well as the competition models the sleeve nuts to the through bolts mean there is less bearing area under the head than with a bolt so this issue becomes worse. When I took the head off I discovered the left rear was down to about 22 ft-pds. but this can't have affected it as it was actually blowing on the other side, between the pushrods. You will see on the photos that the pedestals have been milled down to tidy them up and extra thick washers made.

My next job is to find the composition head gasket that was on it previously and measure the thickness, to try and get an idea of how much these will settle in use so I can work out how much needs to be taken out of the spigot recess. I think the gap wants to be as small as possible.

Cheers,
Peter

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Re: 1962 shortstroke 350 head spigot, gasket and bolt torque

Postby pkr87 » Sun Dec 10, 2017 10:34 am

I started by making sure that both the head and barrel had flat surfaces with straight edges and checking the spigot depth a Vernier gauge. It did not take a lot of work or grinding paste to make sure of a decent seal between the two by checking with Engineer's Blue. My engine is an all alloy trials unit with the sleeve nuts.

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Re: 1962 shortstroke 350 head spigot, gasket and bolt torque

Postby GOLDSTAR » Mon Dec 11, 2017 8:24 am

should there be gasket? I ask as the short stoke alloy comp engines were just 'lapped' in Kind regards

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Re: 1962 shortstroke 350 head spigot, gasket and bolt torque

Postby pkr87 » Mon Dec 11, 2017 10:28 am

Goldstar, you might be right there. Mine is an earlier long stroke alloy engine which I originally put together from a box of bits in 1965. I did not have access to any spares supplier so made it up as I went along, using some bits from an ex army Matchless engine. After years of abuse, including some grass track racing the gudgeon pin escaped and tore a hole in the cylinder. The engine lay under my bench for 20 years till I found a genius who had a special sleeve made for it when I was able to rebuild the engine again, so I made it up as it had been before without a head gasket.Perhaps I got it right in the first place.Paul

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Re: 1962 shortstroke 350 head spigot, gasket and bolt torque

Postby Reynard24 » Mon Dec 11, 2017 10:45 am

The roadster short-stroke 350 (74mm and 72mm bore) engines do have a gasket - Steve Surbey supplies a copper gasket and the club supply a composite one both are listed as part no. 028100.
I've just measured a used spare copper gasket and it is 40 thou but there should also be two "O" rings (022518) fitted into the pushrod tunnel apertures on the gasket.
I torqued my 1963 350 short stroke sleeve nuts to 40ft/lbs and yes the alloy pillars do compress, it also happens on the long-stroke models.

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