ET: Engine (Twin) Leak down test

Bill Cannon

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Bill, it might be the quality of the photograph but I cannot see either the two holes or two cut out to take a tightening tool for the upper part of the guide. If you have never seen the recess for a seal then it is a cut out below the lock ring, perhaps 2 to 3mm deep. All the lock rings I have seen designed to take a seal have the two circular holes for the tightening tool, rather than the milled slots.
Hi Norman,

Don't forget these are TPV heads, think the guides are just shrunk in, there are no locking rings.
Cheers Bill
 

Bill Cannon

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Bill, it may be worth looking down the guide to see if there is an o ring in a groove trying to impersonate a seal. Several repro race motors have them. Roy.
Thanks Roy, I had a quick glance and couldn't see any but I'll try to have a better look with more light.
Cheers Bill
 

Cyborg

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I used the modified SKF seals on the twin, but for some reason.... probably thinking it would save time, I bought the seal setup from Coventry. I assume it is the same as the ones sold by VOC Spares. It has a different retainer (which must be reamed) that is machined out for an O-ring. Also uses a different driver that doesn’t require removing as much meat from the top of the retainer. Next time, I will go back to the SKF seal.

If there is an O-ring down there Bill, you probably will need a light in order to be able to make it out..... but would be surprised if there is one.

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Comet Rider

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Bill C,
The TPV heads do NOT have valve seals as standard.
On my late dad's Comet he machined up 2 seal carriers not dissimilar to Cyborg's and counterbored the top of the guide slightly to take the seal carrier

Best of luck
Neil
 

Cyborg

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When exploring the use of combustion chamber cleaner, we found that you could also get reasonable results by just taking the bike out and beating the ever living daylights out of it. Not all that convenient depending on where you are. Ultimately the cleaner is easier on the pocket book.
 

davidd

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I agree that Terry has not used valve guide seals in most of his heads. I would use cast Iron guides with 0.001" to 0.0015" clearance if you want long wear and good performance. Ni-Resist is what I have used in the past. No seals are necessary.

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You may have to channel the oil away from the guides.

David
 

Bill Cannon

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VOC Member
I agree that Terry has not used valve guide seals in most of his heads. I would use cast Iron guides with 0.001" to 0.0015" clearance if you want long wear and good performance. Ni-Resist is what I have used in the past. No seals are necessary.

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You may have to channel the oil away from the guides.

David
That sounds like a good idea as the top of the guides is flush with the machined casting, so the oil could easily pool around the guide.
Cheers Bill
 

Bill Thomas

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So why can't we use more interference guides when we have damaged thread on our standard heads ?,
And do away with the threads ?.
 
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greg brillus

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Bill you can but the lock ring was a safety measure.......I've seen pre war alloy heads where the guides came loose.........this on a TTR big fin head (I have it here at present and it is a real mess) My guess is the factory played around with the press in only guides pre war and had issues with them, so decided on the post war ones to add the lock rings. The later TP heads do have stem seals, these are the ones with the tiny decompression hole between the valve seats.......they have flush guides as well but have a small rebate cut down from the top and the button sized seal pressed in. It is very marginal and I do wonder how they will last.........This mod could be done on the Heads like Bills one above. If the bike has a side stand only, then it will readily pool oil around the guides and on a hot engine this will run past the stems easily. It would not be too hard to make a simple spot facing tool using an old valve stem and a piece of tool steel to carry out a seal mod.
 

davidd

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Greg,

You are correct. I forgot about the late heads with seals as I have to pull them out for the cam clearance. Bill has the 500cc TPV heads Note that the cylinder machining does not pierce the head stud holes. The heads are the same as the 600cc except for the machining of the recess for the cylinder.

Bill,

You don't need the lockring. You can bore out the threads and make the guide to fit. As long as the interference fit on the whole guide is fine, they work fine. They were like this on the big port head also. The prewar aluminum heads had lots of problems holding their shape.

David
 
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