Moss Motors, Ltd

Hello All!

I need a little advice from people in the know.  My SU throttle shafts have about 0.015" clearance.  The shafts are worn about 0.010".  The thottle body is worn somewhat less.  A 5/16" drill will go through the hole, but a 21/64" is not real close to going through the hole.  What is the acceptable clearance for the throttle shaft to the throttle body?  Will replacing the shaft with a new one be sufficient to idle properly (should have about 0.005 or 0.006" clearance) or will I have to use the oversize shaft.  OR is it better to put in a shaft with a bushing?  What is the diameter of the oversized shaft that is available from Moss (part number 376-040)

I am certain that the old shaft clearance is too much since I see that the idle stops were completely pulled back so that the thottles were completely closed which indicates to me that the previous owner could not get it to idle down.  (However, I never ran the engine before tearing it down as there were MANY other issues).

 John from kc

John - Remove the throttle disk and slid the shaft out enough to position the unworn portion in the bushings of the body and see how much play there is.  If the play is very slight, then a new, standard shaft is all that is needed.

Thanks for the reply!

As I said, the throttle body has 0.005 to 0.006" wear.  The Shaft is showing around 0.009" wear.  I measured the shaft diameter directly compared to the worn areas.  I measured the total clearance of the shaft in the hole using a dial indicator on the shaft wiggle in vertical and horizontal directions. 

I did use your suggestion and use a dial indicator with the throttle shaft moved to unworn shaft area in the throttle body and verified that the wiggle was .005 to .006".

Thanks,

You could probably live with the  0.005" to 0.006" of play that a new standard shaft would give you.  The oversize shaft is 0.010 over, which would entail line reaming the body bushings to make it fit.

For the present, I decided to build up the throttle shafts with solder.  I used a wood stick to keep the solder under control.  I ended up with the play caused by the out of round worn carburetor body hole wear (about .003 to .004").  This should work for a long time.