stainless steel valves in cast iron guide

I'm not sure if this is the best thread to ask but my problem is actually on a 1930s Moto Guzzi engine with exposed valves. I had new guides and valves professionally fitted in a cast iron cylinder head. Expert advice is that a stainless valve will 'pick up' in an iron guide so they fitted phosphor bronze guides. The exhaust valve stuck after 2 miles. The guide was reamed out to give more slack. this time it lasted 20 miles then stuck. Originally this engine had iron guides as they are self lubricating which the bronze ones are not. Any advice please? Thanks.

Interesting ! Bronze guides are norm in better engines. Is the cause lack of lubrication. Noel.

What clearance did they ream the guides to, between the valve stem and guide? A lot of those old engines ran massively larger clearances than what modern automotive machine shops are used to providing. Same with piston to bore clearances, which have caused problems for several restorers I know over the years.

Those old engines with exposed valve gear and no positive lubrication could run .003" to .005" (0.075mm to 0.125mm) exhaust valve guide to stem clearance, which would be considered "worn out" in a modern engine.

The stainless valves I have used in bike engines all had hard-chromed stems, presumably to prevent "picking up". And usually not phosphor bronze for valve guides as it is said the phosphor can tend to leach out under extreme heat in an air cooled engine. Aluminium bronze such as Ampco 45 is more commonly used. But that factor is unlikely to be causing you problems at such short mileage. Probably has more influence over length of service life. So I would look first at what clearance you have.You already found that more clearance lasted longer before seizing so I would look at trying a bit more.

Edited By Hopper on 16/02/2023 21:57:48

Edited By Hopper on 16/02/2023 21:59:09

I run my BSA Sloper's with exposed valve gear and 0.004"/0.005" stem to guide clearance, guides are cast iron made by me.

I once had a BSA M20 sidevalve that needed 0.010" clearance on the exhaust valve to stop sticking.

Agree with Hopper and Chris above, old engines had much larger clearances than modern valves/ guides, I took a 1964 Triumph Bonneville head to a so called expert on classic bike cylinder heads who insisted that bronze inserts in the existing guides and machining the seats on his massively expensive Serdi machine was the only way to go. After 20 miles the an exhaust valve siezed in the guide and pulled it out of the head. Damaging the bore for the guide on the way out.. Turned out he used the same clearances as for a modern engine as an "improvement", Made a new set of guides ( one with an oversize O/D) with much more clearance and it ran fine with no smoke or siezures, this was with Nitrided valves rather than stainless, but I imagine the principle is the same. Looking at some forums found that others had had the same experience with the expert , now retired I believe.

An exhaust valve will need a lrager clearance than an inlet, since it is exposed to exhaust gases , will therefore be hotter and expand more. hence the sticking.

Increasing the stem / giude cleance should msolve the problem, but at the expense of leakage until the vale warms up. Maybe an inverted bucket over the ouiter (cooler ) end of the valve giude?

Howard

I have owned Opel cars and they had rubber cups that fitted the valve stem and covered the valve guide. They stopped oil leaking into the guide, very effective. Also supplied in top overhaul kits.

I would suggest ream a further Thou out of the guide.   It does seem an odd combination of materiels though.

Edited By Clive Hartland on 17/02/2023 14:55:13

I confess that I have no idea how valve springs on the outside of the engine survive with all the dust and rain that can get thrown at them.

Might there be a small weep hole now blocked by the new guide? Or are you supposed to oil up everything steam engine style before you start it.

Years ago I used to run the spares organisation in a vintage car club. We found a batch of stainless steel valves for one side valve model. They were run in cast iron guides and there were no reports of problems. I wouldn't like to use bronze guides though with the poor lubrication.

Russell

On my Slopers I use spray grease before every ride. The modern bike (1932) has a crankcase pressure fed oil mist feed to the inlet valve only, the older bike does not have the feed. My theory as to why the inlet valve is to maybe counter fuel wash ?

Thank you so much for all the replies, some good information there which is much appreciated. The Company concerned is South Cerney Engineering near `Swindon. They have an excellent reputation, working on the highest quality and rare vehicles so I took them at their word. I'm sure the chap doing the job said he'd opened the guide up to give .003". I've since found a Moto Guzzi chart from the period which recommends nearly .008" at max tolerance! So that's the first thing. Is there a way to tell if a valve stem is chrome plated? or is it obvious?

Incidentally, the inlet guide has an oil drilling for valve lubricant but the exhaust has to just get on with it.

Thanks again for the knowledgeable and informative replies.

replace the valve guides with the correct iron guide ream them to the original manufactures clearance. end of problem

Can I suggest that the use of stainless steel may be contributing to the problem? It is known (from real life experience, mainly) that unlubricated stainless can gall - pick-up into a mess that sticks the two surfaces together, rather than (as you hoped) produce a nice sliding surface. So, any idea about a return to original should include valves made of the original stuff.

Cheers, Tim

My Royal Enfield twins had bronze valve guides, they must not have any sort of seal fitted like most cast iron ones do.

Bronze and stainless steel are not the best mixtures because of the chrome in the stainless.

Edited By old mart on 17/02/2023 21:08:03

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.

Clearance of .003" on an unlubricated valve stem could quite possibly be too tight, I reckon. My 1977 Harley with cast iron heads has a factory spec of .0035 to .0045" valve stem clearance, new from the factory. Then they allow another 2 thou of wear beyond that before replacement is called for. And that is in a much more modern engine with higher revs, enclosed rocker gear and pressure fed oil to the rocker spindles and arms which then sprays out over the guides and valve stems.

So I reckon you could try something like .005" clearance and see how it goes. Maybe even a thou more. Just make sure they give the valve stem a darn good polish up after it has nipped up so there is no roughness left to start galling again.

I don't know if the chrome is really visible on stainless Harley valve stems, I never really looked at them that hard. What brand of valve did you use? I think most of the high performance aftermarket valves like Manley come with chrome stems if they are stainless. But they are made for high rpm high power use with heavy valve springs etc so your lovely olde-worlde single banger will probably not need it.

It is a common problem with modern machine shops working on true vintage machinery. They think that because they have done work on Honda 4s and Kawa 900s from the 1970s, and maybe a few old Norton Commandos or BSA twins and the like, that they know all about "vintage" bikes. But those old engines from the pre-War era ran everything loose and are a whole different world again. They were still operating very much in the steam age to a certain extent . They did not have the machining or the metallurgy let alone the lubrication, to run modern tight clearances.

Edited By Hopper on 18/02/2023 04:17:31

Edited By Hopper on 18/02/2023 04:29:00

Hopper, What surprises me is that this company work mainly on "old" stuff back to the dawn of motoring so I'm disappointed to say the least, that they didn't take the attitude of "looser the better." The valve has a little pad on the end to take hanmmering from the rocker. Each time it sticks open, that pad is lost somewhere on the road. They have to come from Italy at euro5.73 but plus euro22 for post. Its like rubbing salt in the wound🥴

At the price (not the postage!!) and at the news of the stellite pad coming off the end of the stem, even under severe hammering, I would have to wonder about the quality of the valve. Good quality Harley valves run about $50 each and Norton valves can be as much as almost $200 each for the super magic ones. But I suppose you don't get much choice with ancient Guzzi parts. I have seen a few hung up valves on Harleys and Brit bikes but have never seen a pad come off the tip of the stem. But not sure they even have such a thing. I don't suppose you have an alternative supplier ?

Yes surprising your machine shop would make it so tight on such an old machine. If they opened it up to 3 thou after the first seizure it must have been 2 or 2 somehting to start with. Did you find what the original Guzzi clearance specs were?

Edited By Hopper on 18/02/2023 07:30:34

Ah OK that makes more sense.

Can the supplier of the valve give you any info on what clearance to run?