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MG MGA - Non-metallic carb bushing problem

Guys,
A friend who just got his A back on the road finds the throttle hangs up when it gets hot. We found the carb throttle shafts binding. Pulled the carbs apart and they appear to have some type of non-metallic bushing. It's very dark (black, dark grey) in color. Cleaned the shafts, burnished the bushing and movement was fine. As a test, heatrd the carb body to about 200F in an oven and it began binding again. I suspect these non-metallic bushings. Carbs were done about 20 years ago and this is the first the car has been used. Any ideas what these bushings might be? I have Delrin bushings in my carbs and they work great.
Cheers.
Gerry
G T Foster

Gerry-
Dark grey would likely be moly filled nylon, black could be a number of different polymers. I would expect moly filled nylon to be OK, if it is fitted correctly to start, but could be wrong. One issue is that they can be fit very tight, since friction is so low, but the heat could tighten them up. The only way to fix it would be to ream them out a few tenths, but it is extraordinarily difficult to take a very light cut on that material. Best bet is to replace with Garlok DU bushes from a bearing supply place, plain bronze as second choice.

These plastics can behave very strangely. My big truck had nylon (not MOS2 filled, but had grease fittngs) brake cam bushes, and when the brakes got hot the bushes shrank and seized on the camshafts, which left them loose in the axle flanges. So, the flanges wore out and they are welded to the axles! And it left the cams loose which caused brake chatter and rapid wear. I replaced them constantly, but first time down a mountain they did it again. Earlier versions of the axle used bronze bushes reamed in place. I made steel bushes and brazed up the camshaft bearing surfaces, end of problem.
On another occasion, I made a dovetail slide out of UHMW PE, about 3"x5", nicely fit to about .002" clearance. Next morning it was seized, so I gave it another .005. That worked until the next morning, when it was seized again. So it went every day for a week, by which time it had about .040 clearance and stayed free. Turns out that UHMW PE shrinks a lot with temperature drop AND absorbs moisture from the air! The shop would get cooler at night near ground level where the part was, about 40F, but our movement stirred up the air and warmed it to 65F or so, plus I would take it off to machine, which got it warm. Then serious winter came so the near floor temp was colder, and if we left the thing stopped with the slide at its low point, it would seize again, but if it was at the top of travel it wouldn't, so I gave it another .010, which fixed it.
And,my daughter built a science experiment, which involved measuring the coefficient of expansion of different materials. All went as expected until we measured a random piece of what I think was acetal (Delrin). Stuff turns out to undergo a phase change around 100F, which gives crazy expansion figures that are not the same on cooldown. That is, there is a variable and very consequential hysteresis that changes with rate of heating and cooling.

FRM
FR Millmore

There is no such thing as a plastic part in a precision application. Sintered bronze (Oilite) bearings are the cat's meow for throttle shafts.
Barney Gaylord

Sorry to bomb your list, being mainly a TR guy, but the topic caught my attention. I have re-bushed many SU carbs with Delrin with only 1 problem. The problem occured when the car is brought up to normal temp., shut off and re-started a few minutes later. The throttle shafts would bind in bushings, causing the throttle to stick. It seems that heat soaks the carb on shutdown and the expansion of the Delrin causes the binding. The problem only occured if the bushing was a tight fit in the carb body. If I relaxed the fit of the bushing and lightly staked the carb body, the binding problem diappeared.
Berry
BTP Price

This thread was discussed on 26/04/2011

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