Valve stem seals on CS175 motor - details - MVAgusta.net
 
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post #1 of 4 (permalink) Old 02-17-2018, 03:31 PM Thread Starter
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Valve stem seals on CS175 motor - details

Ivan (Hulagun) was interested in how I'm going to fit valve stem seals to my CS57 engine, so I'll explain.
I started by measuring the area above the valve guide and it looked like I'd be able to fit a valve stem seal below the hairspring support casting, as long as I could find one of the right size, to fit inside the 16mm diameter - see sketch in attachments.

Fortunately, I found some from a 2 litre Rover engine that looked like they would fit. I just had to trim the height to suit and I did this using a sharp blade mounted in the cross slide of my friend's lathe. The valve stem seal was pushed onto a nylon mandrel and spun at low speed, so I could get a nice square cut. The seal was cut to 1.4/1.5 mm high so it will be squeezed in the 1.36 mm gap to prevent oil getting beneath it and it should hold in place, as there's a 2 mm radial step above it, to the hole through the Hairspring casting.

They haven't been tested yet to be fair, as I decided to fully strip the motor on the recommendation of Ivan who advised this engine is easily wrecked by debris in the oil. And it's just as well I did, as the crankcases had been assembled with too much RTV sealant - see photo.

Several pieces of RTV were breaking away, so thanks to Ivan for his advice. Hopefully my motor will survive when I ride the Motogiro D'Italia in 2 months time!
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post #2 of 4 (permalink) Old 02-17-2018, 10:38 PM
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Looks great! I look forward to hearing how these do! Thanks for really clear explanation and photos.


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post #3 of 4 (permalink) Old 02-18-2018, 09:11 AM
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Nice job. Wondering if the stems and guides are materials that will be happy with no lubrication??? I have seen some engines with valve seals on the intake but none on the exhaust.

In my mind, the intake valve stem is cooled with incoming fuel and intake port has low pressure that would draw oil inward, while exhaust valve stem is super heated by combustion gasses and has a high pressure that would not draw oil down the stem....

My perceptions may be wrong. At any rate, your modification is ingenious. But I wonder about stem to guide seizure possibility, especially on the exhaust valve.

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post #4 of 4 (permalink) Old 02-19-2018, 07:39 AM Thread Starter
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Good point esq'z me. I think the risk of seizure is low as the valves are bedded in, with probably a little more clearance than they should have but not excessive.
However, it's not a risk I want to take so I plan to:
1/ very lightly polish the valve stems (up and down not round and round) to remove any high spots and
2/ put a small groove in the top of the valve guide to let a small amount of oil beneath the seal.

I understand motor manufacturers consider this when they design valve seals. SKF for example, say they have designed the profile of the seal lip to let a little oil through. May be just marketing spiel - who knows?

My motor was sucking a lot of oil down the seals for several reasons - a little too much valve stem clearance, O rings fitted to valve stems acting as a reciprocating pump and very high oil feed to the cam box due to a stretched pressure relief valve spring - see photo below.

Does anybody know the correct spec for oil pressure relief valve spring?
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Last edited by Mondello175; 02-19-2018 at 11:03 AM. Reason: typo
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