Nickp
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| posted on 3/3/15 at 06:43 AM |
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Stainless valves in cast iron guides.
Had a disaster this weekend when starting up my freshly rebuilt Twin Cam in my Monte. I'd bought a very nicely ported big valve head c/w new
valves, guides and Alquati cams. I put it all together, over about 1yr, with HC pistons, forged rods etc etc.
I spun it over to get oil pressure up and then it started first time. I ran it at 2krpm initially to run the cams in, as that's how I'd
always been told gives them the best chance. After 10 mins and things seemed to be warming up nicely I gave it a little blip to about 3.5krpm and
immediately got a bad top end rattle so turned it straight off. Totally gutted!! 
When I took the cam covers off no4 exhaust had a huge clearance so I knew straight away it had bent that valve. So I whipped the head off and removed
all the exhaust valves. Each one showed scuffing and signs of seizure on the stems, it just happened that no4 went first. The inlets look perfect. The
valve to guide clearances were spot on, not tight at all.
It appears that the exhaust valves are plain stainless and the guides are cast. I've researched this combo now and it seems stainless must have
nitrided or coated stems to work in cast guides or preferably be used in bronze guides or similar. Bit late to find out know but at least I know the
cause and how to sort it.
Just wondered if anyone has experienced similar? I'm just surprised how quickly this all went wrong
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MikeRJ
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| posted on 3/3/15 at 08:43 AM |
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Yes, my brother had a similar experience with his mini, though since the valves are vertical in the A-Series the piston tends to push them back in
rather than bending anything. The valve stems were very clearly scuffed up where they had picked up on the guides. Bronze guides fixed the problem,
but they don't last anything like as long as cast iron guides IME.
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Paul Turner
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| posted on 3/3/15 at 08:53 AM |
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Back in 1989 a so called expert fitted me new stainless Paul Ivey valves and new cast iron guides in a x-flow head. Guess I was lucky they did not
seize but they only lasted a few thousand miles. Got some bronze guides fitted and they were still in when I sold the head 15 years later. In truth I
did have a new set of valves after about 9 years since the stems had worn quite badly, probably due to the 254 cam and 8500 rpm. In all honesty I
expected the guides to wear out faster than the valves but in my experience the opposite happened.
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Nickp
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| posted on 3/3/15 at 09:48 AM |
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Phew, it's not just me then!! My engine's running very tight v-p clearances (around 2mm) due to the cams and HC pistons so I think
that's why the issue showed up so quickly TBH. I'm not too fussed about mileage as it's hardly my commuter car 
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Angel Acevedo
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| posted on 3/3/15 at 09:29 PM |
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quote:
Originally posted by Paul Turner
Back in 1989 a so called expert fitted me new stainless Paul Ivey valves and new cast iron guides in a x-flow head. Guess I was lucky they did not
seize but they only lasted a few thousand miles. Got some bronze guides fitted and they were still in when I sold the head 15 years later. In truth I
did have a new set of valves after about 9 years since the stems had worn quite badly, probably due to the 254 cam and 8500 rpm. In all honesty I
expected the guides to wear out faster than the valves but in my experience the opposite happened.
I read once that when two dissimilar metals are subject to galling, the softer metal gets harder particles embedded in it and this causes the harder
metal to wear faster.
When I read what I wrote it doesn't makes much sense though.
AA
Beware of what you wish.. for it may come true....
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