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MG MGA - Valve guide clearances

The WS manual gives clearances for inlet and exhaust guides as .0015 to .0025, and .002 to .003.
On the other hand it gives guide ID as .34425 to .34475. Since the stem is .3420, this implies a clearance of .0022 to .0027 for both.
I am installing the manganese bronze guides. Suppliers of these say you can reduce clearances compared to steel because of the improved lubricity. On the other hand, I have read somewhere in these threads that you should increase the clearance with the bronze guides.
So what is the best? I was going to ask for a .344 ream after installing.
Art Pearse

Bronze has higher thermal expansion rate than iron, and the guide is held in the fixed bore of the iron head. When it all heats up the guide will expand in wall thickness to make the bore smaller. With standard bore a bronze guide may constrict enough to make a valve seize. This is most likely to happen with hottest running conditions, full throttle at high rev's (maximum power output). To avoid seized valves the bronze guides should have a bit more clearance, like 0.0010 to 0.0015 larger than standard spec's for iron guides.

The bronze material is self-lubricating in contact with a polished steel valve stem, so it needs no oil and will exhibit very little wear with time (you can use good valve seals). At 100,000 miles the bore of a bronze guide will be smaller than a worn iron guide, and most likely good for much more service after the iron guide would be totally shot.

I have a cylinder head with hard seats and bronze guides that had accrued 100,000 miles. I touched up the exhaust seats only to narrow the lands slightly, installed new valves (without changing the guides) and put it back in service on the next rebuild block.
Barney Gaylord

I can attest to what Barney says. The first time I used bronze guides, I did not give sufficient clearance for the valve stem. Every time I would rev the engine under load (70 plus mph and full throttle) the engine would "go dead". The valves were hanging up! The solution came with slightly more clearance and the problem never repeated itself. Go with Barney's recommendation.
James Johanski

This thread was discussed on 16/04/2009

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This thread is from the archive. The Live MG MGA BBS is active now.