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MG MGA - Twenty one

What does the number 21 signify, on the head of the exhaust valves - B head?
Also, it seems that some B heads had oil deflectors inside the valve springs. Mine does not. Is it important?
Art Pearse

Oil deflector was deleted after (e)18G4384. Without the deflector it uses a slightly different spring cup (cap) with larger pilot diameter for the inner spring.
Barney Gaylord

Does that mean the oil deflectors were not / are not necessary to stop oil loss via the valve stems?
Another observation - there is a flat washer at the bottom of the springs, about 1/16 thick which sits in the spring pocket in the head. What is it's purpose?
Art Pearse

Hi Art,

The use of oil deflectors is different for different engine designs. The best way to approach the problem is to do what the engine designer intended. This does require you to also have the valve stem/valve guide clearance to the design specification.
Valve springs have hardened caps at the top and hardened washers at the bottom. This is to prevent wear which would be caused by spring movement at both ends. A coil spring is only a compact design of a torsion bar, the coils twist along their length and would cause wear at the spring end mating surfaces.
Also, the thickness of the washer at the bottom affects the spring tension.


Mick
M F Anderson

Mick, looking at the Moss catalogue, it shows the bottom spring cup as one piece; mine appears to be in 2 pieces, the washer and the small spring cup. I was wondering why. Good point about the hardness.
Art
Art Pearse

The washer was probably used to set spring height
gary starr

Hi Art,

You say that your cylinder head is a MGB type. I have attached the diagram from the MGB Workshop Manual.
It shows that there should be a cap at the top and a cup at the bottom of the spring. Both should have a ridge to keep the spring located and straight.
The washer appears to have been added by a previous owner, as Gary says to set the spring height, to increase spring tension.


Mick


M F Anderson

More to this mystery.
Here is a picture of the base washer. It is a manufactured piece, with radial grooves on the underside (shown in pic) On top is stamped THIS SIDE UP and VSI 4-200.
The small spring collar that sits on it is a thin stamping, not a machined part.
Spring lengths are inner 1.934 and outer 1.900 averages. The inner ones appear to be .034 short of standard (1-31/32), but the outers are 0.147 short (vs 2-3/64) Assuming MGA and MGB spring lengths are the same. This is strange b/c you would thing the springs would age at the same rate.
I'm going to contact the vendor of the head for the engine No it came from to try and get to the bottom. I hate mysteries! Again, any input welcome. Head casting is 12H906, which is MGB, plus 18 cast in.
Art


Art Pearse

That looks very much like the spacers that automotive machine shops use to correct valve spring height/tension after grinding the valves and seats. When the valve and seats are ground the valve fits deeper into the head which puts the valve stem higher on the top side of the head. Then spacers like the one you show are placed under the springs to give the correct spring tension.
Ed Bell

Could be Ed, but this washer is no thicker than the outer part of the standard collar. I could see it if this were inaddition to the standard collar. Also the valve seats are not sunken too badly.
Art Pearse

I just managed to get them all out, the washers. All the same type / number. They are no harder than the CI head. I'm going to replace with the proper collars and I think new springs
Art Pearse

I found them on the web! Woodward Valve Spring Insets (VSI)
http://www.cylinderheadsupply.com/valve-spring-inserts---vsi-type--a-.html
Looks like it has been to a head shop at one time, who turfed out the original collars and substituted these shims plus different collars for the inner springs. This makes no sense to me as the spring height is not altered if that was the intention. Is it really necessary to compensate for the miniscule amount of change after lapping a valve? I doubt it.
Art Pearse

Hi Art,

Talk about selling "snake oil". In my 50 years of working on car engines, many in competition, I have never seen any such rubbish before.
They say the serrations are to prevent heat transfer. Which way? Heat from the head to the spring or from the spring to the head? Neither makes sense. Nearly all the valve heat is dissapated through the guide and the valve seat, and very little would reach the head through the spring, and if it did more heat transfer would be best, not less. The head is not hot enough to affect good valve springs, even in the head area immediately adjacent to the exhaust valve. The mga twin cam does not have them, nor does any full blown racing engine that I have seen.
As you day, they fit the washers and then a thinner base cup washer to finish with the same spring height.
The correct place for them is in the trash can.

See image.

Mick


M F Anderson

This thread was discussed between 24/02/2009 and 27/02/2009

MG MGA index

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