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Rocker Pedestal Shims

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Mickey Richaud

Mickey Richaud

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In the process of rebuilding the MGB, I noticed the shims in the Moss catalog under the rocker pedestals. They weren't on the motor that I'm redoing (after a rebuild by the DPO), and I'm just wondering how important they are. I ordered one for each pedestal, as the catalog calls for, and they are VERY thin. Can anyone enlighten me about them?

Thanks,
Mickey
 
Hello Mickey,
they are supposed to correct rocker geometry when the cylinder head has been skimmed. However, my view is that is
that supposition is incorrect. By skimming the head the rocker shaft will be closer to the cam, therefore the only alteration necessary is to adjust the rocker screws. The geometrical relationship between the rocker and the valve tip is unaltered and adding a shim below the pedestal will uset that geometry. If the head has been skimmed so much that there is no adjustment left then either shorter push rods should be fitted or longer valves plus shims under the pedestal. The ideal geometry gives the rocker horizontal with 50% lift. This gives maximum valve lift for any given cam. Any deviation either way gives reduced full lift.

Alec /ubbthreads/images/graemlins/thirsty.gif
 
Thanks, Alec - I figured if anyone would have the answer, you would. Your input pretty well matched what I was thinking.

Mickey
 
Hmmm..

I don't know if I quite agree with that...although the set secrews allow for adjustment, you could discover, that you bottom out the set screw and still do not have the desired configuration...

because in essence, your pushrods may be too long right now.. I.e. taking. ..050 off the head, makes the rod oversized by the same length...threfore collapsing the valve a further .1 down..

On my race engines, I have used a combination of fly-cutting the block, nd special length push rods...

On my MGA, although there is I believe .09 clearance between the valve and cylinder block, I shaved .05, and promptly started breaking rocker arms from slugging the valve onto the top of the block, again, a fly-cut of the block proved to be the solution.

When I do use the pedestal shims, is when I shave the pedestals for even height.... and they can be made out of simple sheet aluminum, to your own degree of thickness... just trace the pedestal, and trim.. but do make sure you leave the oil galley hole for the rocker shaft open...
 
Hello Bob,
yes, if there is no adjustment left then further measures are necessary. However my basic point still stands, shims do upset the original rocker\valve geometry and should be avoided if possible.

Alec /ubbthreads/images/graemlins/thirsty.gif
 
Shim shiminy shim shim sheroo
If I were a shim I'd shim one too

Sorry - couldn't resist.

The way I remember the rocker pedestal shim thing was that the shims purpose was to put a slight up pressure on the rocker arm shaft and thereby stabilize it and prevent it from chattering in the bushes.
As thin as those shims are, I can't see them affecting rocker arm geometry. What's the tolerance on the bushes? Is it less than the shims?
 
Hello Gt's,
they were sold (or included) for performance cylinder heads, which as a matter of course were skimmed and the quoted purpose was to "restore the geometry caused by skimming the head". As I said before, the valve tip\rocker shaft height is unaltered so these shims adversely affect the valve lift.
How can they reduce shaft chatter?, being accurately flat they do not add any load to the pedestal.
Back to geometry, remember the rocker describes an arc on the valve tip, so valve lift is not linear relative to cam lift, being at maximum when the rocker is around the horizontal.
I don't believe that it is a significant loss of power fitting them, but why add a negative? Mickey asked what they were for and that is my opinion; anyone with a different explanation or evident gain in using them?

Alec /ubbthreads/images/graemlins/thirsty.gif
 
OK - disregarding all the "shim shim shiree" and the "shimmy shimmy coco bop" I have a further question on this:

Did any of the 1800 engines come from the factory with these shims?
 
Mickey
Everyone that I have seen (4) has had the shims under the middle rocker posts and nowhere else.
Bently gives the rocker arm bore as: 0.7485 to 0.7495"
and the bush inside diameter as: 0.6255 to 0.626"
and the rocker shaft diameter as: 0.624 to 0.625"
That leaves anywhere from 0.0005 to 0.002 room in there.
I don't know the shim size and don't have one handy to measure.
 
[ QUOTE ]
That leaves anywhere from 0.0005 to 0.002 room in there.
I don't know the shim size and don't have one handy to measure.

[/ QUOTE ]

Interesting - I don't have one with me here, but as I recall, they seem a little thicker than that. Again, though, I'm not convinced of their necessity, and would still like to know if they were supplied originally from the factory.

The four that you've seen - were the heads cut down?

Mickey
 
3/4 were stock unmolested blocks. The fourth was molested, defiled, abused, drawn and quartered, hacked and all but rendered asunder.
 
My completely unmolested '67 had one under each of the two center pedestals. They were paper thin, literally. Of the half dozen or so engines I have torn down (not of my origin) about half had shims and half didn't. Of course I don't know their history.
Shims will alter the geometry and may have been installed by the factory due to machining irregularities (cigarette papers used to shim the mill machines? No doubt they had variations).

Mike!
 
Sorry to barge in on the discussion, guys, but of course Triumphs are pretty similar mechanically.
I think 2 different types of shims are being discussed here.
The original question refers to shims placed under the pedestals supporting the rocker shaft. It's my view that ideally you want to have the pushrod more or less at right angles to the end of the rocker shaftwhen the valve is closed; as the pushrod rises to open the valve the angle becomes smaller, the "cup and ball" design allowing for this.
If the head is skimmed the angle will change, becoming smaller, perhaps causing excessive wear by pushing on the side of the cup, or producing a non-linear motion of the rocker arm or something.
The solution which was suggested, using shorter pushrods, seems preferable as a mechanical solution, the shims a "cheap fix", though you could get shims in the exact thickness required, pushrods usually 50 thou or 100 thou shorter than stock.
The second type of shim referred to, I think, was to act as a bushing between the rocker shaft and the pedestals. I think a clearance of a few thou would be required for oil film, so would be surprised if such a shim would be used here, perhaps a bearing of some sort.
Simon TR4a (but noses in to other peoples conversations!)Also an MG fan!
 
Thanks to all who contributed. Even with the lack of consensus about what to do, I decided to put the shims in. As I continued to search through all the parts that came with the car, I found two very rusty but otherwise identical shims. So it appears that the engine had them installed. I figure that even though they are very thin, they must have served some purpose, and can't really hurt if they're there. Guess we'll see when I finally get to start it up!

Mickey
 
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