Clutch Master Cylinder Shim

When I removed the clutch cylinder on my '60 BN7 for a rebuild, I found an aluminum shim behind the lower mounting bolt. This is a commercially made piece that follows the profile of the flange through which the lower bolt passes. It was trapped behind that flange and fire wall. The 1/16"-thick piece is slotted so that it can be slid between the firewall and the cylinder flange without fully removing the bolt. In addition to that slot, the piece has a half circle cut that reaches midway around the circular portion of the cylinder that goes through the fire wall. I know this piece is commercially made, as one side is raised around the outer perimeter, something that would be extremely difficult to do by hand. I have also recently replaced the brake master cylinder, and no such piece was found on that. Naturally, I forgot to install the piece when I replaced the clutch cylinder, and then lost it into never-never land inside the engine bay when I tried to put it back. While looking for it, I found the bolt that I had dropped into that abyss and replaced earlier. Since someone thought it was important to have the shim in there, I made a new one in similitude of the lost part and installed it. Given the generous potential play between the clutch push rod and the clutch pedal, it wouldn't seem a shim this thin would be a necessity. Postscript! Of course the missing original piece belatedly was found and is on my desk as I write this. I looked in vain for an identification of this part in the factory Service Parts List for the BN7/BT7 cars on Larry Varley's comprehensive Healey Web site. Happy to say I seem to have successfully rebuilt the clutch cylinder and bled the clutch slave, meaning that I now have to restore the transmission cover over my Smitty Toyota gear box and the balance of the car's interior. So, has anyone ever seen such a piece? I now have some scan capability, and could likely figure out how to post a picture.

Are you referring to this?:
From the Moss online catalog.

Indeed. Should have checked that resource. The pair of shims aren't shown with the lower MC, which is the type in my car, but that may have been to avoid duplication. I have now successfully replaced the brake MC and rebuilt the clutch MC.

Well, I don't know what they were intended for__did you look for an explanation in the Bentley O/H manual__but it's rare to find them still fitted to the cars, considering the potential number of m/cyl rebuilds in the last 50-60 years.

I do know that they should be fitted in pairs; two (2) shims fitted, to keep the m/cyl mounting flange from being torqued down unevenly.

Not sure if they were ever fitted to the brake m/c too, but I seem to only recall finding them on the clutch side myself.

Speculating, I can only imagine that their purpose was to prevent the stroke from bottoming out the piston inside the cylinder bore.

Randy Forbes said:

...Not sure if they were ever fitted to the brake m/c too, but I seem to only recall finding them on the clutch side myself...

Click to expand...

The Moss catalog drawings show them on both clutch and brake pages.

I have a set installed on the clutch MC. I'm not sure it's because of the shims but the clutch pedal is about 1/2" closer to the floor than the brake pedal. I might remove the shim and see if the pedals are the same height.

Just on the clutch master here as well. Original shims too, none were on the brake master.

I did a review of hundreds of Google images of Girling master cylinders and these shims or packings flat-out do not appear.
This is shaping up to be a major mystery!
Enquiring Minds Want to Know (TM)

On reflection, maybe they're necessary to make the clutch pushrod return far enough to uncover the return port, given the non-adjustable pushrod and pedal setup.

The reason I suspect this is several years ago when I installed my Smitty 5-speed, I had a situation where the clutch MC wouldn't return all the way, trapping the fluid in the pipe and not allowing it to return upstream when my foot was off the clutch. It had the effect of "riding" the clutch all the time, which started to slip in a few months.

I always blamed the little dust seal next to the circlip (discontinued by Girling) - or sloppy manufacturing tolerances - for causing the piston to be juuuust far enough forward for the return port to be blocked.

But can see without the 1/16" shim between the firewall and the MC, that would move the MC backward relative to the piston, having the same effect as moving the piston forward, riding the clutch.

The master cylinder push-rods are non adjustable (unlike many other cars where the clevis is threaded) . The shims allow you to adjust for pedal travel and take up free play. I have seen them on both clutch and brake M/C.

steveg said:

On reflection, maybe they're necessary to make the clutch pushrod return far enough to uncover the return port, given the non-adjustable pushrod and pedal setup.

The reason I suspect this is several years ago when I installed my Smitty 5-speed, I had a situation where the clutch MC wouldn't return all the way, trapping the fluid in the pipe and not allowing it to return upstream when my foot was off the clutch. It had the effect of "riding" the clutch all the time, which started to slip in a few months.

I always blamed the little dust seal next to the circlip (discontinued by Girling) - or sloppy manufacturing tolerances - for causing the piston to be juuuust far enough forward for the return port to be blocked.

But can see without the 1/16" shim between the firewall and the MC, that would move the MC backward relative to the piston, having the same effect as moving the piston forward, riding the clutch.

Click to expand...

steveg said:

Are you referring to this?:
View attachment 37385 From the Moss online catalog.

Click to expand...

Old post.
The shims in the diagram (top MC) are for a BN4 to C.E. 48862
OP has a 1960 BN7. Doesn't that car use the bottom MC? No shims shown.

I have a BJ8. My Throw Out Bearing is in constant contact with the release plate. The new TOB had 7mm of graphite. In 500 miles it wore down to 5mm. I'm not ridding the clutch. It's always in contact. Could this problem be related to what you had?

"On reflection, maybe they're necessary to make the clutch pushrod return far enough to uncover the return port, given the non-adjustable pushrod and pedal setup."

"The reason I suspect this is several years ago when I installed my Smitty 5-speed, I had a situation where the clutch MC wouldn't return all the way, trapping the fluid in the pipe and not allowing it to return upstream when my foot was off the clutch. It had the effect of "riding" the clutch all the time, which started to slip in a few months."

My clutch fork isn't binding and moves freely. The clutch slave cylinder push rod is the correct length and moves freely. When installing the slave, I push the rod in to attached to the fork. The pin then returns to about middle. With the clutch fork gaiter removed, I can move the fork by hand to see the TOB move away from the release plate. Let go of the fork, it returns and makes solid contact with the release plate. I'll let the car run for one minute, not using the clutch pedal, transmission is in neutral, I'll see wear on the TOB when I remove it. Photo attached.

What is causing the constant pressure between the TOB and release plate when the clutch pedal in not being used.

Bump. Anyone?

The Austin-Healey Experience forum has a current thread titled, "What's going on with my BN7 clutch." Similar questions and post #18 has a YouTube video from the MG guru that you may find interesting - same system the Healeys have.