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MG Midget and Sprite Technical - Rocker face contact with valve stem

Maybe Ive just forgotten how the rockers contact the valve stems on A series engines, but during sorting a fault on my 1275 I noticed that the contact areas of rocker and valve stem were somewhat askew. I would value the opinions of A Series experts out there.
Attached is a photo of the area.

R C Skerritt

I am not a wizard on the MG engines outside of the 1500 so I might be way off base.

On the 1500, the rockers are handed left and right. Any chance you could have the wrong handed rockers on that valve?

Clare
Clare Ravenwood

If I remember correctly, my 1275 has the rockers off set like that. I've never taken my rocker assembly apart but that doesn't mean that Clare is wrong about the handedness. It could be down to "precision" engineering or assembly.
Martin

Bob, that's normal.

The small-bore engines had a circular pad which sat directly over the valve stem. When the 1275 engine was introduced, they fitted larger valves, which meant off-setting them and the rockers no longer sat over the valve stem, so they designed the 1275 rockers with a wider, oval pad.

It does look a bit odd, but that's how it is.
Dave O'Neill 2

I worried about this for weeks, Bob, and schemed systems of shimming and grinding to get them central. Did Clare's thing, too, of swapping them around.

No need, though.
Nick and Cherry Scoop

I would not be happy with the rockers set up like that.

Are the pushrods central in their holes or are they offset the same way as the rocker tips?


Chris at Octarine Services

I'm going to check the "handedness" and centralisation today. Thanks Clare and Chris.
I did find Daves comments about the valve size offset to be logical though, on the 1275. So if they are not handed, I will accept Daves reasoning. my mind is offended by the lack of centralisation on the rocker pads though!
Regards
Bob
R C Skerritt

The offset will translate into additional wear on the rocker bush and shaft as well as wear in the valve guide.

It also loses horsepower albeit only a tiny amount - but all the tiny amounts add up - which is why a fully blueprinted engine is more powerful through minimising internal power losses.
Chris at Octarine Services

"I would not be happy with the rockers set up like that."

Unfortunately, that is how BMC made them.

On my race engines, I would shim the rockers away from the pillars to centralise them. This isn't possible for all of the rockers, as some need to go closer to the pillars. The only way around that would be to remove material from the sides of the offending rockers, but in series whe you have to run standard, unmodified rockers, that isn't allowed.
Dave O'Neill 2

OK.
Pushrods vertical
No offset on rockers
It seems Mr O'Neill has the answer.
Thanks Dave

R C Skerritt

Heres the pushrods.

R C Skerritt

Thinking about it,...if I shimmed the rockers to move them centrally to the valve stems, I would end up with the push rods not vertical!
Good old BMC precision engineering!!! Mind, they did the job.
I'm not going to race this, its a cooker, so I'll leave as is.
Thanks for all the input
Bob
R C Skerritt

Well, the things one can learn on here. I too thought it looked rather odd and I suppose on the 1500 they corrected that by having the rocker designed with a bend to make them handed and sit on the valve directly.

Still, if it works......

Clare
Clare Ravenwood

In the first photo, the left rocker is too close to the support whilst the right rocker is too far from the support. Could swapping them round reduce the mis alignment of the rocker pad with the valve stem?

I dont know if it is likely but could the rockers be bent? I guess this could be checked by using a straight edge from the centre of the adjuster screw through the 'seam' to the centre of the valve stem pad.
D Plumb

I may be (probably am!)completely wrong, but I'm sure I read somewhere that the offset is deliberate.

Offsetting the rocker means that the rocker's contact imparts a slight turning action on the valve thereby evening up the wear and maintaining the seal.

This rotation also prevents the build up of material on the valve with corresponding pits on the seats which can cause the valve to stick.

Edit: After taking another look at the first photo I'm more inclined toward the 'completely wrong!'. :)
Colin Mee

I'm not sure you're wrong Colin. I've watched videos on YouTube recently that showed both the valves and the pushrods rotating. Makes perfect sense to me.
Greybeard

I've read the same idea about the contact point between cam lobes and cam followers. It's off centre and the cam 'wipes' the follower so that it rotates and it avoids it from causing a wear ridge at one point. So maybe the same principle is applied to the valve and rocker.
GuyW

Bob - are some of them central over the valve heads? I recall that some of mine were, which would support the O'Neill theory.
Nick and Cherry Scoop

Has anyone tried using 1500 rockers with the offset on a 1275?
s1

"...but I'm sure I read somewhere that the offset is deliberate. "

Not really deliberate, more a consequence of moving the valve centres, then redesigning the rocker pad to cover the valve stems.


"Has anyone tried using 1500 rockers with the offset on a 1275? "

Never even considered it. The rocker shaft diameter would need to be the same, as would the lengths between centres, not to mention the diameter of the ball on the adjuster and the radius of the rocker pad.

Dave O'Neill 2

Nick
heres another photo showing that some are more central than others
Bob

R C Skerritt

I don't think the offset is a major issue as they were like that on my set and have lasted well but I did reface/harden the pads and rebush the rockers and used a hardened rocker shaft, I think the hardened shaft is important as it can wear more than the bushes. It is noted in the A series bible by Vizard that the standard shafts are quite soft and wear badly, the hardened shafts are available from a number of places. Mine was gas carbo nitrided IIRC so didn't need cleaning as apparently tufftrided ones might to remove any possible salt deposits internally.
David Billington

I wouldn't argue with Vizard, but surely an advantage of a softer rocker shaft is that it wears rather than the rocker bushes, which are more problematic and probably more costly to replace? I have replaced worn shafts rocker shafts very simply and reused the same rockers without problems.

Incidentally, wear on the rocker shafts occurs almost exclusively on the underside and a step can easily be felt with a fingernail if the rocker slackened off and moved to one side. Excessive rocker shaft wear results in too much oil being pumped out onto the cylinder head with resulting blue smoke displays!
GuyW

Well said Guy. My shaft is worn as you describe and it makes it impossible to set clearances satisfactorily.

I noticed however that it is possible to get replacement shaft and rocker sets for the 1500 which feature bushed rockers. 1500 rockers were not bushed originally.

It looks like an improvement to me - has anyone used them?
Greybeard

I don't think the harder rocker shaft will lead to more bush wear just a longer life and a more satisfactory life of the whole assembly, at the extra cost of the hardened shaft.
David Billington

David, I am not an engineer, but usually when two materials abraid against each other, the softer one wears faster. So putting a harder shaft in may not accelerate total wear, but that which does occur will favour the softer material.
GuyW

This thread was discussed between 03/03/2017 and 14/03/2017

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