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MG MGA - Bronze Mangeneze Valve Guide

OK, I'm just about to drop off the valves and the valve guides with the machinists.

I read that they require additional clearance to .003".

I want to double check than number and ask a dumb question.

Does that mean that the diamter should be .003 larger (i.e .0015" on each side of the valve

or does in mean the radius should be .003 larger.

Thanks....measure twice cut once.
T McCarthy

The "clearance" is the difference between the valve stem diameter, and the valve guide diameter. The original spec for clearance (on inlet valves) is .00155 to .00255 inches, and if you want .003 additional clearance, the machine shop should be aiming for .00455 to .00555. exhaust valve guides, with .003 additional clearance over original spec, would be .005 to .006 inches.
P. Tilbury

What is the reason for the extra clearance, bronze vs iron valve guides ? Is it higher coefficient of expansion ? If so, .003 more sounds quite a lot.
A Pearse

The clearance is increased to .003" not by .003
They do nip up when hot more so than iron guides.


The total clearance I mean is .003"

There is a "to" buried in there.
T McCarthy

Unless I'm mistaken (never happened before..:)).003 clearance means the diameter of the guide should be .003 greater than the diameter of the stem. For example, if the stem diameter is .250 the guide diameter should be .253.
Interesting though, I have bronze guides and I don't remember haveing them clearanced greater than spec. But it was a looooong time ago in a place far, far away....

G T Foster

When I responded with dimensions, I was concerned about the "additional" clearance aspect. (Just shows how the English language can be misunderstod) DENIS4 has it right. But going from .002 to .003 clearance(original specs for exhaust valves) to .003 nominal clearance may not be enough to avoid sticking valves.
Our local shop does not recommend bronze guides in an MG head, but that is a whole other topic!
P. Tilbury

I just talked with my machinist about this yesterday. The trick is to have a machinist who is experienced with MG heads, then he'll know exactly how to machine the bronze valve guides for your engine. If your guy hasn't done this before, I might not go with the bronze guides.

I know someone who had guides from a Chevy motor put in their head, they were another alloy, not bronze and not iron. Can't remember now what they were at the moment.

What exactly is the bottom line of cast iron vs. bronze guides anyway? What are the merits of one over the other?
Mark J Michalak

Bronze guides are required for unleaded fuel.

M F Anderson

This thread was discussed between 20/08/2008 and 21/08/2008

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