stainless steel valves in cast iron guide

[Dave HalfordParticipant @davehalford22513]

Posted by c wastell on 17/02/2023 21:09:20:

I'll see if some extra clearance solves it first. If it doesn't then yes, I'll have to shell out for a one off valve. Thing is, other people are buying these valves presumably and I've never heard of anyone having problems with them.

Thing is how many 'other people' actually ride the bikes to find out the shiney new valve is only that and not much of a valve at all. You may be the only one in a sea of trailer queens.

[SillyOldDufferModerator @sillyoldduffer]

As has been pointed out, the original engine ran the valves in iron self-lubricating guides. I doubt these can simply be replaced with phosphor bronze on this type of open valve engine, because – as far as I know – PB needs some source of lubrication – oil or grease.

Maybe the repairer got it half-right. Believing a modern stainless valve is incompatible with cast-iron, which could well be true, they've fitted bronze guides, but not provided a way of lubricating them. Or possibly the type of Phosphor Bronze used to make the guide is a problem. The PB alloy used for bearings is softer than most Phosphor Bronzes, many of which are very hard. Does the bike run on leaded petrol? If not, unleaded will make the problem worse because it dissolves lubricants aggressively. Old engines don't have the technology to cope with unleaded fuel effectively!

Increasing clearance will reduce metal to metal contact, but the purpose of a guide is to guide, not to be a wobbly dust-cover with a big hole in it! I think if enough Bronze is left to function as a guide, then the valve stem will hit it, and either wear rapidly or seize due to lack of lubrication. Increased clearance might be "good enough" if the bike only does occasional low mileage at moderate RPM; don't expect to tour Europe on it though!

There's a historic echo in the question in that aero-engine developers during the 1930s really struggled with valves disintegrating, burning out, and the stems seizing or only lasting an hour or two. The issue was that the simple technologies that produced acceptable performance from a Moto Guzzi failed miserably in a high-performance aero engine. Engineers had to develop new technology, and modern engines are full of it. Not all developments since 1935 are straightforwardly back compatible with older kit, and its easy to lose sight of what's needed to keep the original working well.

Just how far technology has come is illustrated by the Ford Model T engine. In it's day, super reliable, truly a work of genius! But to modern eyes a 2.9 litre engine that only produces 20HP is a grossly inefficient gas guzzler, the performance is terrible, and the exhaust unacceptably filthy.

Dave

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