Author Topic: Valve guides  (Read 1213 times)

beezermacc

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Valve guides
« on: 24.10. 2015 13:35 »
I've just been organising some parts in the stores and come across an issue which I'm sure somebody amongst us can resolve. I dare say this has been covered before so maybe just point me in the right direction.

According to the parts books The A7's and A10's from 1954 onwards used cast iron valve guides in everything except the early A7SS and A10RR (up to A7SS 4024  CA10R 5602 which equates to autumn '57 ish). However, nearly all the alloy heads whether A7SS or A10RR / SR in my hoard have phosphor bronze guides. It is likely that over the years the guides have been changed by previous owners. I would like to know what the advantages or disadvantages are of using cast iron or phosphor bronze in an alloy head and what the technical experts would recommend. Why did BSA start to use phosphor bronze and then change back to cast iron.? Which is the best material anyway? Thanks.

Offline terryg

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Re: Valve guides
« Reply #1 on: 24.10. 2015 14:24 »
That's a big lion's mouth you've opened Andrew but I'll stick my head in it.

Relative coefficients of thermal expansion...
Aluminium, Duralumin 22-23
Cast iron, steel 10-12
Bronze, phosphor bronze 16-18

So one reason is the better match over a temperature range for the bronzes.

Edit - another reason quoted is that iron guides can crack, losing pieces that may find their way into the engine. Bronzes tend not to suffer in the same way, being less brittle.

On the other hand carbon containing steel is rated as ' self-lubricating', which bronzes are not.

Why did BSA switch back?
Possibly cost or possibly a copper shortage , which would also have raised bronze prices.

There may be other answers to your question!  Perhaps from a tribology expert.

This could be a long thread.
Terry
'57 'SR', '59 SR, '63 RGS

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Re: Valve guides
« Reply #2 on: 24.10. 2015 14:52 »
I am no metalurgist, and I do not know how relevant this is, but I was told that cast iron valve guides have better lubricating properties. Ally heads do have a reputution for valve guides becoming loose in the heads. Perhaps fitting bronze guides was an attempt to cure this tendency? My head was re-built profesionally by the Cylinder Head Shop, and I was told that when the head was warmed up in preperation for removing the guides that one of the exhaust valve guides literally fell out

Offline terryg

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Re: Valve guides
« Reply #3 on: 24.10. 2015 15:22 »
Sorry A101960 - you posted while I was editing the bit about self-lubrication and beat me to it.
Terry
'57 'SR', '59 SR, '63 RGS

beezermacc

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Re: Valve guides
« Reply #4 on: 24.10. 2015 16:17 »
Just speaking from experience I had a vague understanding that the normal procedure is to fit bronze guides in alloy heads and cast iron guides in cast iron heads, purely for the thermal expansion reasons. When I've worked on iron heads the guides are often not very tight yet bronze guides usually put up a bit of a fight. It wouldn't surprise me if BSA were being guided by price. Anyway, more ideas please.......

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Re: Valve guides
« Reply #5 on: 24.10. 2015 18:47 »
Yup, I  thought 'bronze for alloy'was the usual rule of thumb, but 40 years of continuous running with AMC twins alongside whatever else I've had has taught me otherwise. Iron guides in alloy heads throughout the production, very long life, low rates of wear, tight stem to guide clearance -  that mix of bits can work perfectly OK.
Maybe due to good design and materials, good machining standards and good geometry, dunno - but those engines rev, to 7000rpm mostly. Of course, some might say 'huh, they need to, if you want to get anywhere'  . . . but that would be a bit harsh, as they like revs and they do get there.  But, to paraphrase, AMC is another place, and they do things differently there.

Bill

Offline Lightningpower

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Re: Valve guides
« Reply #6 on: 18.11. 2015 02:27 »
Not to hijack your thread, but to continue the discussion on phosphor bronze vs steel/iron. What about phosphor-bronze valve seats in an alloy head? I have one from a BSA single, but I don't get the logic behind it.
1953 Star Twin - Racing
1956 Road Rocket
1958 Super Rocket
1959 Spitfire Scrambler
1967 Hornet

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Re: Valve guides
« Reply #7 on: 18.11. 2015 18:03 »
Hi,
I don t think the valve seat would be phosphor bronze??
more likely some type of aluminium bronze`("wrought not cast" according to Phil Irving)
This is suitable for exhaust seats in alloy heads (Tuning for speed)

Theres bronze and theres "bronze" for valve guides,
Of course theres always a snag or compromise to be made ???
The better the materials the more difficult they are to size properly after fitting and so tend not to be availble to the home engine builder  *sad2* as some require diamond hones to finish the bores

Colisbro guides seem to be in fashion nowadays, I have had  them fitted to several BSA and Tri***h alloy heads and no issues
http://www.columbiametals.com/products/trademark-alloys/colsibro-trademark-alloy
If you want to read up the blurb on the material
It says on the web page suitable for valve seats but I have not come across this before?????

Regards
John



1961 Super Rocket
1963 RGS (ongoing)

Offline jjbsa

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Re: Valve guides
« Reply #8 on: 27.11. 2015 22:49 »
Hi, just spotted this.   The cast iron used for valve guides is not your general engineering cast iron.  It contains about 2% Nickel as well as some Chromium and Molybdenum and is much more hard wearing, though still brittle.  Bronzes such as Colsibro conduct heat a lot better than iron and are very hard wearing.  Modern practice is to run smaller guide to stem clearances than of old because this reduces oil pull-down past inlet valves and it keeps exhaust valves a lot cooler too.  For this reason, the valves with plasma coated stems available today for BSAs are a little larger in stem diameter than the BSA originals, to offer these modern advantages.  Regarding fitting, in my experience, an interference fit of about 2 thou for alloy heads and 1 thou for cast iron, is good for Colsibro guides.  Straying outside this range can cause bad problems. The tricky bit is getting an accurate measurement of the hole for the valve guide, this is important.  A set of old fashioned hole measuring wedges is what I've come to favour as these offer a very good feel and give a sort of minimum diameter reading too.