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MG MGA - Valve clearance adjustment

A bit of an eye-opener..
The spec for the valve clearance on my engine is .014 hot (It was rebuilt by Lawrie Alexander and the .014 is his spec for that engine).
I've always felt it'd be less hassle to set the clearances while the engine's cold at whatever gap it would be after the engine had cooled down after it had been set at .014 hot.
Anyway, to prove a point to myself, I set the clearances while hot and then re-checked the clearances again when the engine was cold.
What I found was that, after cooling down, an .014 feeler would no longer slide in, but an .013 feeler slid in perfectly. I checked another, and another, with the same result.
And I always thought it was the other way around; ie: set the clearance while cold at .001 above what it was supposed to be when hot since the metal expands and would close the gap as the engine heated up.
The original shop manual seems to infer this; in Section A.14, 2nd paragraph, it states "An additional .001 in. must be allowed when the engine is cold."
I don't even know if I'm looking for an answer here; more like throwing it out there for what it's worth & fish for any comments, insights..
Rick deOlazarra

Yeah
The only thing I can add is that there is usually a bit of difference between up to temp on the gauge versus taking it for a blast and getting the valves and all nice and hot -- thats when clearances tend to open up a tiddle, maybee you didn't have it hot enough

willy
William Revit

" after cooling down, an .014 feeler would no longer slide in, but an .013 feeler slid in perfectly."

and "And I always thought it was the other way around; ie: set the clearance while cold at .001 above what it was supposed to be when hot since the metal expands and would close the gap as the engine heated up.
The original shop manual seems to infer this; in Section A.14, 2nd paragraph, it states "An additional .001 in. must be allowed when the engine is cold."

So if you followed the factory manual and set it 0.001 higher when cold (i.e. 0.015" not 0.014"), then it would be 0.014 hot no? same...same....
Chuck Schaefer

No, it would be 0.016" hot.
Dave O'Neill 2

Thanks for the comments, insights..
While hot, I set the clearances at .014.
The next day, engine cold, it reads .013.
When I start it up, cold, and run it until
it's hot, the clearances become .014 once again.
I know it goes against how I thought it should work,
but the evidence is undeniable.
I let it run long enough to insure that the engine
was indeed quite hot when I set the clearances to .014 & dead cold when I checked again to find the .013 figure.
So, in the absence of anything contrary, I'll be setting my clearances while the engine's cold at .001 under from now on, but given the strangeness of it all, I'll be double-checking the hot reading just to make sure until I'm completely convinced.
I'd be curious to see what results any others out there have.

Rick deOlazarra

Interesting one, personally I just adjust the valves as recommended. They sound quiet enough to me so that's that.
Thinking about it though, when you adjust them cold everything is at the same temperature. When 'hot' everything is a bit different. The hot head will be something over the water temperature and where the valve stems are in the guides the temperature will be about the same, so no real difference there. But where the exposed section of valve stem is getting the full blast of the exhaust gases they must be longer due to expansion in that area - reducing the gap. This would make your findings seem illogical.
But then we have the cold air/fuel mixture passing the inlet valves, surely they cannot have expanded the same as the exhaust valves.
But as I understand it, all your valves change similarly.
So that theory goes out the window...
What maybe happening in our B-series engines is a bit different as the rocker shaft pillars are made of aluminium alloy (correct spelling over here). So when hot the rocker shaft will become a lot higher as the thermal expansion of these alloys is several times that of the steel valve stems. That will give you a bigger gap when hot! How's that for a theory.
Pete
PeteT

Don't forget expansion of the tappets and pushrods!
Andrew Dear

Last time I looked my rocker pedestals were steel
dominic clancy

Dominic-

Original MGA pedestals were aluminum... The iron MGB pedestals fit and are an improvement. Barney has it covered here:

http://www.mgaguru.com/mgtech/engine/ch104.htm
D Rawlins

Dave said, "No, it would be 0.016" hot."

I stand corrected. Thanks for straightening me out. That is what I get for not thinking it though thoroughly before typing......
Chuck Schaefer

Like Pete I was also interested in Rick’s finding and and have given it some thought. Basically Rick is saying that the difference, cold to hot, between the valve clearances was positive (by one thou) in the old days, when our manual was written, but negative (by one thou) for his recently refurbished engine.

As the valve clearance is always measured with the engine stopped and probably at least five minutes later, by the time you have removed the rocker cover and found your spanners, etc., then a hot measurement is when all the engine is at a uniform temperature as it is the cold measurement. It is not denied that when the engine is running the valves, especially the exhaust valves, will be considerably hotter than the surrounding metal parts but we can forget about this valve temperature difference for these measurements.

If the valves themselves are made from a material of different expansion coefficient to the surrounding cast iron or steel components then the hot valve clearance will be different to the cold clearance. If the valve material coefficient is higher then the hot valve clearance will be less than cold and if it is lower then it will be more.

The fact that today (or for Rick’s engine) the valve clearance difference cold to hot is negative could mean that today’s valves (or perhaps just Rick’s!) are made from a material having a lower expansion coefficient than the surrounding metal than they were in the old days.

Looking on the net at some valve manufacturer’s websites it appears that a common present day valve steel is EN59 alloy which, when compared to the specification of an earlier valve steel EN52, is described as a low expansion coefficient steel, presumably lower than that of EN52. Unfortunately I wasn’t able to download any tables of these expansion coefficients but the fact that it was specifically described as low expansion must be significant.

Therefore Rick’s finding may just be a result of present day valve manufacturers using a different lower expansion steel to what was once used. I believe these modern valve steels are chosen for their hot gas corrosion resistance and not particularly for their expansion coefficient. I’m assuming that Rick’s engine builder used new, present day valves and did not keep the originals or use NOS.

I did a calculation of what difference in valve material expansion coefficient would be required to change a valve clearance difference hot to cold by 2 thou and for a 70 degC temperature difference and a 4.5 inch valve length it would need to be around 6 x 10 >-6. So if, say, the cast iron coefficient is around 12 x 10>-6 then the valve steel coefficient would have to be 6 x 10>-6.

Anybody know what it is for EN59 or present day valve steels?.........................................Mike

m.j. moore

Mike

Most thought provoking. Also, many of us have now inserted hardened valve seats. Would these have any bearing on matters?

Steve
Steve Gyles

Nice thinking Mike,
I think I've found why you are having problems finding the coefficient of expansion for En59 - I found and copied the following:-

EN59 (No longer available)
This material had a lower thermal expansion rate and good heat conductivity properties which gives much better resistance to high temperature scaling. This steel is resistant to Chlorine compounds but not Sulphur. It is suitable for use where resistance to high octane rated fuels and oils containing tetra-ethyl-lead.

I guess this wasn't any good with unleaded fuel then... I guess any new valves won't be En59!
Over to you.
Pete
PeteT

Pete, Yes I saw that comment about EN59 being 'no longer available' but it was from a UK valve manufacturer. It seems that is still readily available from Indian and Chinese valve manufacturers. I imagine that if Moss is the main MGA valve supplier then they will be made overseas due to price.

Steve, I can't see valve seats having much influence on the valve clearance mainly because they are so short (1/8" ??)...........................Mike
m.j. moore

Mike

Yes they are small but as we are are talking about just the odd thou I thought they may have some influence compared to the standard head material. But there again my knowledge about coefficients of expansion left me 50 years ago in grammar school.

Steve
Steve Gyles

Steve, I don't know much about valve seats but assuming they are made from Stellite No.2 (Co/Cr/W)for which I have information they would have no effect whatsoever on the valve clearance difference hot/cold because its exp. coeff. is identical to that for typical cast iron/mild steels. Perhaps this is a requirement for seat material otherwise they may work loose in the head but as I'm on very weak ground I'll leave it at that! ............................Mike
m.j. moore

This thread was discussed between 30/11/2014 and 04/12/2014

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