stainless steel valves in cast iron guide

Communicating with the Italian suppliers is a process best described as slightly wearing. They are friendly and helpful but oh, so Italian; I might get an email out of the blue in 10 days time, with the answer.

Ah yes. Tutto bene!

I get involved mostly with JAP V twins on Morgan Three Wheelers and one or two other odd balls, and more recently JAP 500 singles. Safe valve guide clearances for these engines are considered to be 2 thou on the inlet and 4 thou on the exhaust. Some practitioners opt for 2 thou all round, and some get away with it.

Valve guide material was Meehanite, although Colsibro is currently offered as an alternative. Valve guide lubrication is a bit hit and miss - there are drillings through the head into the valve guide and small pipes feed oil from a collector on the back face of the rocker box into these. Lining up the hole in the head with the hole in the guide is important. The rocker box may or may not be positively fed with oil. The worst case is no positive feed (like my engine), oil “mist” is thought to find its way up the pushrod tubes from the timing chest which communicates with the crank case, into the rocker box, some then finds its way to the valve guides. I add a bit of oil to the the exhaust valve guide each morning before setting off but the rocker box is always wet inside.

The little cap on top of the valve stem we know as lash caps. These are currently being machined from EN24T rather than EN36 hardened.

Measuring valve guide bores reliably can be tricky enough, but parallelism is critical. One engine that we dealt with recently that nipped up when working hard was found to have plenty of valve guide clearance at the ends but the middle of the valve guide was tight - the bore was barrel shaped. I think also that adjusting valve guide bores can be a game for any number of players....reamers like to have a bit of work to do and taking an extra thou out with a used reamer, especially in the bronze like materials, may not work. Adjustable reamers don’t always behave either.

Ian

Thats very interesting, thanks. I didn't know I could make them myself, I have a little bit of EN24T in stock. I don't mind buying them but I do object to the Euro22 postage plus the delay at customs.

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.

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