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MG Midget and Sprite Technical - clutch master pushrod woes

Just spent a not very enjoyable weekend swapping my clutch master and came across an interesting issue.
You may remember earlier in the year I had to swap out the original master as the cylinder bore was scored so it was weeping fluid into the footwell. I went with one of the generic plastic bodied units on ebay but no matter how much fluid I pushed through it I just couldn't get a decent clutch so I put a new seal kit in the old cylinder and put up with fluid leaks till this weekend.
Not too long ago I managed to get a NOS Lockheed tin can to replace the old one and finally got around to fitting it. However, again with around 1/2 a litre of fluid being pushed through the system (with an eezibleed) I still couldn't get the clutch to throw out fully. This was around 5pm last night and I'd had enough.
This morning I thought I'd compare the length of the master pushrods and found that both the NOS one that was now in the car and the one on the plastic master were at least 5mm shorter than the original.
I can understand there being some difference with the plastic one but neither of the tin cans have any circles stamped on them so I'd have expected them to be identical.
Swapped pushrods on the 2 metal cans, rebled the system and now have a nice clutch.
Just thought it was worth bringing this up in case anyone else is thinking of swapping out master cylinders - check the pushrod length before you spend hours bleeding the bl**dy thing.
Oh - one last thing - I've found it's a whole lot easier bleeding the system with the slave unbolted from the gearbox and hanging vertically with the pushrod right in and tied securely.
graeme jackson

This is an interesting learning process. What is odd though, is that the clutch slave piston only needs to operate through a travel of around 1/2" to 5/8", and yet the slave cylinder is nearer 1 3/4" The reason is that the section of the cylinder that the piston moves along changes as the clutch wears - the travel is the same, but the at rest and fully depressed positions move progressively further back up the cylinder bore. The design is self adjusting. The relevance of this is that, within reason, the actual pushrod length has much the same effect - i.e. altering the section of the cylinder bore that the piston operates along, but has no effect on the actual distance moved. So in theory, the 5mm difference in the pushrod lengths shouldn't actually make any difference.

However, this isn't the end of the story as there are limits. If the clutch driven plate is brand new, and thick, and the clutch release bearing is thinner than standard - maybe a roller bearing is fitted that is slimmer? - then combined with a short pushrod the piston may have been coming up against the end of the slave cylinder bore before it had done its job. The trick to rectify this used to e to slip a 7/16" nut onto the end of the pushrod where it fits into its concave seating in the piston - lengthening it by about 1/4" (=5mm, - coincidence?)
Guy Weller

Guy I know we've discussed slave pushrod lengths ad infinitum over the years on here - but Graeme is talking about the Master pushrod being the wrong length. I don't think anyone else has brought this up before, and if I fitted a new m/cyl I'm pretty sure I wouldn't have thought to measure and compare the pushrod lengths.
David Smith

LOL! Missed that entirely! Thanks David. ;-)
Guy Weller

Pretty sad when 1/4 inch (5mm) is the diffrance between a shop queen and an outlaw screamin machine, thats coolnyou found the issue

Prop and the Blackhole Midget

This thread was discussed on 17/08/2014

MG Midget and Sprite Technical index

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