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MG MGB Technical - Valve rocker refacing

I am considering grinding the valve end of a set of rockers to remove wear and optimise the profile. Can anyone tell me is face that impacts on the valve is case hardened or not?
Clearly grinding through the case hardening would be a pointless exercise.
David Witham

Hi David

Shouldn't be a problem refacing. Careful not to alter the profile, if you carry out the op on an offhand grinder (bench grinder) do not finish with a flourish on the part nearest the tip....it will lose valve lift for sure! To optimise the profile to get the correct lift for standard race classes you need to flatten the backpart of the pad. Be prepared to scrap a lot of arms getting it right. Standard lift is nearer 340-348 thou rather than the quoted 364.

Peter
P Burgess

Agree with Peter. If it is so worn that you go through the hardening, it is junk anyway, but the hardening is quite deep, not "case hardening" which is shallow. I've done quite a few very badly worn ones and not gone through the hardening while maintaining the profile. I was timid at first and did it with a stone, but that takes a very long time indeed! Just don't grind aggressively and get them hot, because that will soften or cause cracking.

FRM
FR Millmore

Thanks both of you.

I have at least 2 spare sets. Doing this was brought to mind when checking the clearances. I realised the tightest point was always nearest to the shaft. Perhaps I will try puting a set of 4 shims under the pedistalls to correct for the skim first.
David Witham

I don't think that you need to install shims. Bear in mind that you are only grinding as far as the current low point and your current rocker height is OK for that so nothing is going to change.
Iain MacKintosh

I would use an oil stone, better finish and easier to control. Also look at the wear and you will get an idea of how squarely it was bearing on the valve, and so compensate. Setting the rocker, shaft and posts on a surface plate with "blue" will get you even more accurate.
Allan Reeling

Allan, I am not sure I follow you about the surface plate given that you put a shim under 2 posts to induce a distortion to bind the shaft in the posts anyway.
David Witham

Another though I had, being an amateur wood turner. I grind my tools with a 'wet' grinder. The wheel turns slow in a bath of water. No heat build up, and it is very controllable.My tools are High speed steel, which does not like being heated up as is the risk with dry grinders. It is a slow process if the tool profile is wrong or being corrected. But it strikes me that this might be the ideal way to grind the rocker pads? Mike
J.M. Doust

Another though I had, being an amateur wood turner. I grind my tools with a 'wet' grinder. The wheel turns slow in a bath of water. No heat build up, and it is very controllable.My tools are High speed steel, which does not like being heated up as is the risk with dry grinders. It is a slow process if the tool profile is wrong or being corrected. But it strikes me that this might be the ideal way to grind the rocker pads? One could even make a jig to hold the rocker firm and in just the right angle, Hmm, I think I'll try making one! Mike
J.M. Doust

Mike
"One could even make a jig to hold the rocker firm and in just the right angle" True, but you must remember that there is no fixed angle. The contact face must remain tangential to the valve stem at all positions of the rocker travel. This results in a curved rocker pad. To do this, the rocker must be mounted on a pivot that slides perpendicular to the stone face. thus mimicking the valve stem motion. Such a device was a standard attachment on valve refacing machines back when people actually repaired stuff.

FRM
FR Millmore

FRM You are absolutely right, all the more reason to get my head around this problem. You know that someone said all Dentists have OSD ( obsessive compulsion disorder ) Yep, that's me. I give myself a project and cannot stop until I have a sort of solution. What's the worst that can happen? I cannot grind into another pushrod hole! Mike
J.M. Doust

Mike -
Evidently you also have Obvious Spelling Disorder, since ( obsessive compulsion disorder ) would be OCD to most of us! Sorry, couldn't resist. You grind people's teeth but go through a port wall? OMG.

Anyway, as you are afflicted with this or these, consider all this, noting Peter's above comments re pad contour etc. As I stated,the pad face must always be tangential to the stem, but it should also always make contact at the stem centerline. You can fudge that a bit, but too much causes evil things. The pad face is then a cam, and the shape of this cam, together with the regular cam and the rocker geometry, will determine the lift rates and amounts. So, if you are obsessed, figure out how to optimise it will you? No doubt a true math nerd would figure out the equations for marrying all this and sell it as an app for some damned electronic device that I don't have and couldn't operate.

This has been for many years a thought exercise as I drive down the road, (used to drive a truck) but I've never tried to really figure it out in metal or on paper.

FRM
FR Millmore

I was at the dentist yesterday afternoon. As I was in the chair she was telling me about her trip on the London to Brighton run and that her husband had recently restored a Triumph Stag. Anyway I could not help thinking that a dentists tools must be ideal for head work.

FRM, do we want the contact point to be on the stem centre line? To minimise friction and wear that is correct. However to maximise lift I think you want to go nearer to the edge away from the rocker shaft without actually riding on the edge.
David Witham

I used to go to a dentist that was an MG owner, too.

We always talked about the car but what amazed me was that the dentist actually understood my mumbled words caused by the dentist tools in my mouth!
Willem vd Veer

David, as I understand it, you are grinding away the metal that does NOT contact the stem , because that is the area that is NOT worn. So, as long as you grind down to the area which IS doing the work and you don't grind that area, the exact profile surely does not matter. Of course by this reasoning, why are you doing this at all?
Art Pearse

Dave W -
Right but, side loading the stem results in high friction and wear of guides, all worse with high lift and heavy springs. The losses, immediate and coming, soon exceed the minor gains of higher lift. Part of the theoretical optimization would be to balance the off center loads, so you might have the pad contacting near the shaft at valve closed, but progressing across the stem as it opens further. The only time things are "right" is when the axis of the shaft center through the pad is at right angles to the stem, since that is the only time the force is in line to the guide. It might be beneficial to have this happen nearer full lift, since the spring load is greatest there. Or something!

Art -
Sorta, but the contour is what determines exactly where - or which bit of metal - makes the contact. In general and barring lubrication failure, the most severe wear is at the valve closed point, since the clearance makes it act as a hammer/anvil where the rocker and valve beat each other to death. Engines that had no oil changes show a scored region extending from the usual valve open wear point, corresponding to the "wipe" of the rocker across the stem.

FRM
FR Millmore

This thread was discussed between 04/05/2011 and 13/05/2011

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