This applies to the MGF only.
Having recently had the rear damper off the car i thought I would investigate why the bottom bush so often quickly deteriorates.
First, I removed the inner metal sleeve and measured the bottom fitting on the damper. You can see it is 20mm across.
You can see that the bush within protrudes about 1-1.5mm each side. I also measured a new bush and that is 22/23mm across.
The inner sleeve however is 26mm long
With the nut/ bolt/ washers and sleeve I was able to 'mock-up' the actual setup on the car.
I think you can see that the bush can move around when the sleeve is clamped between the two washers.
So even if you tighten it up completely, the bush is not compressed and actually has a couple of mm to move about on the sleeve. Initially I think a new bush will feel reasonably secure, but after a short time the bottom fixing of the damper will be able to move around on the inner sleeve. Result - quickly knackered bush.
Those of us who are F owners will know that this is often an MOT fail. Usually the rears are worse but the same problem applies to the fronts.
I have just had to replace the front bottom bushes after hearing a knocking noise on bumps. Investigation showed the bush had deteriorated enough to allow the damper fitting to hit the inner sleeve. Which is one of the reasons I decided to investigate WHY?
It seems strange to me that the inner sleeve should be 6mm longer than the bottom damper fitting.
To me the logical solution is to reduce the length of the sleeve.
So, using a grinder, I reduced it to approx. 22mm. I was careful to clean off any burred metal - we are trying to prevent damage to the bush after all!
I replaced the bush and inserted the shortened sleeve, Then I refitted the damper to the car. The bush is now slightly compressed between the two washers when nut is tightened up. The bottom of the damper is now much more secure.
I admit that until some time has passed I won't know if this solves the bush problem or whether it actually causes other problems, but to make a proper comparison I have modified one front and one rear damper bush sleeve.
Just a quick update.
This modification is showing good results, so far. We've done several thousand miles and there is no play in the bottom bushes and the rubbers look as good as new!
I have also modified the other two bottom bushes now.
edit:- I can also confirm that although Rimmer Bros, B&G etc only list a replacement bush for the rear damper, the same bush also fits the front dampers. MGR o.e. part number NAM9017.