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MG MGB Technical - clunk when shifting?
|There seems to be a load clunk noise every time I shift gear and gear down.Is this a problem with the rear pion gears,rearwheel bearing or drive shaft? I cant isolate the noise.|
|If you have wire wheels this could be the clunk as worn hub splines make this sort of noise. Also check the "u" joints for play.|
|Chasing noises on a 'B can drive you crazy. Wouldn't think pinion or bearing are the source, but I have little experience here. I would expect them to either whine or roar - constantly. Drive shaft - possibly. Check the universals. Worn motor or transmission mounts are another possibility. I once chased a clunking (at shift) for nearly a month. Eventually discovered that my soft spongy mounts allowed enough engine-transmission twist that the head pipe was bumping up against the left frame rail. Although generally experienced on bumps, a loose inner fender mounting on the rear shock can generate a clunking sound.|
A little more details will help. What year and style car Roadster or GT? Wire or steel wheels.
Steve has listed the common components that cause "Clunk" but as he pointed out, going through each one can be a pain. Start with easiest to fix and check. U joints would be my first place to start. Then depending if you have wire wheels or steel, I would check these. (wire will give a clunk if not tight enough or if splines are worn.)
Then I would look at mounts. While harder to change, a lot less expensive than getting into the differential.
Next would be the Differential. There are brass thrust washers that wear inside the differential. Last for me would be the transmission.
As also pointed out, this could be one part hitting another such as exhaust pipe. Has anything been changed recently? Are the exhaust pipe brackets in place? I had the rubber on one pipe bracket come off and it was not noticable until I actually pulled down on the pipe.
|Getting under the car, with a good light, can help you determine the state of the various rubber mounts. They should be firm. If they feel spongy, they need to be replaced. With the currently available mounts, I find them going spongy in only a few years. The motor mounts do not seem to hold up particularly well and I have seen them delaminate after two years of service.|
Bruce mentions the copper thrust washers. There are also a set of fibre washers involved and both can wear/self destruct. When I replaced the copper washers the fibre washers were still in place and seemed in relatively good condition. Others have found them destroyed. My copper washers were worn to .022" on the sections making contact. Thickness of the new washers was .034". This amount of wear allowed a significant thunk in the rear end, mainly found when first starting off or when backing up after moving forwards. More significant wear might allow the sounds to be felt/heard when shifting.
The point about U joints is a valid one and they should be examined while under the car. Try to move them in relation to the yokes at the front and rear of the prop shaft (drive shaft). If there is any movement, they need to be replaced. Also check for movement where the front yoke slides over the prop shaft. This is a splined section which moves inwards and outwards as the body of the car goes over bumps. It has been known to wear to the point it can cause clunking.
One thing which can cause significant problems is if the yokes are out of alignment with each other. I have only found this on one car, but it caused significant problems--badly worn U joints and wear on the sliding section of the prop shaft. Tom Sotomayer told me he has seen it on a number of cars and has written a tech article on it which is availabe under the MG/articles section of my website, www.custompistols.com/ should you care to read it.
This thread was discussed on 10/12/2006
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