Home re-bushing for SU throttle shafts
Some while ago I copied this from a web site - but cannot remember
where.
Apologies to the Authors, if they feel agrieved by my circulating it
here.
The original has some colour photos to illustrate the technique.
Anyone interested, I can send off-list as an attachment and include
the photos . Havn't tried it yet myself but it sounds good to me!
Guy
(Copied text)
Just a Simple Wrench
by Ann & Jake Snyder
Doing It Yourself
It seems like decades ago, and it was, when a peaceful weekend
afternoon meant tuning the twin SU carburetters. It was wonderful how
each instruction in the shop manual could be followed perfectly, and
the engine would idle perfectly, and then we would blip the throttle,
and we could start all over again. This was a perfect game, because it
was entertaining and cost only a little for a whole afternoon's
gasoline. And it was never over, because each time the throttle was
pressed, the car needed to have the carburetters' mixture adjusted and
be synchronized again. And we understood that many others enjoyed the
same entertainment, and all was as it should be. Rebuilding the
carburetters in those days meant changing the valve in the float bowl
(there were no Grose jets in the beginning) and replacing the various
gaskets that came in the "rebuild kit". These kits were carefully
assembled by the supplier to avoid any component like throttle shafts,
main jets, or needles that might actually correct any real problems
and end the peaceful afternoon entertainment.
Then came a revelation, when we took the University Motors Ltd. course
on tuneups. This was really a course on rebuilding carburetters and
distributors, and there was no screw or spring or gasket that was left
attached in the rebuilding process. And the most important concept was
that throttle shafts were not only replaced, but the worn carburetter
bodies were made like-new again by reaming them and fitting bronze
bushes. There are many warnings in the books about the impossibility
of repairing the worn-out bodies that have egg-shaped throttle shaft
holes in them from tens of thousands of miles of use. "Just give it
up", the books say, "and shell out a few hundred for new
carburetters". The reason the re-bushing is supposed to be impossible
is that the bushes must be in perfect alignment, which means that
holes in the carburetter body must be align-reamed and then
align-reamed again on the inner diameter to make certain that the
holes for the throttle shafts line up perfectly. University Motors
Ltd. dared to accomplish the process with custom-made reamers operated
on an expensive metal lathe that actually did ream both sides of the
carburetter body simultaneously. Needless to say, custom-making a
reamer for one or two sets of carburetters would not make any
financial sense at all.
We used the set of carburetters we had taken for the tuneup course at
UML, and, not unexpectedly, one the other cars started providing the
same entertainment that had so engrossed us years before. Knowing what
the problem was, however, took all the fun out of setting and
resetting carburetters that had air leaking past the throttle shafts,
almost never ending up with the same leak after using the throttle.
Thus we were faced with sending the old carburetters (or at least the
bodies) out for rebushing, buying new carburetters for hundreds of
dollars, or having a custom reamer made and buying a metal lathe for
thousands of dollars. While idly glancing through a machine shop
catalog, just to see whether there was any way that an expensive metal
lathe might have some other use, like mixing paint or bread dough, we
came across a listing of reamers. One was particularly interesting
because it had a smooth 5/16 inch drive end, reamed a 3/8 inch hole,
and cost five dollars. This was definitely in the right price range,
and actually works with just a few modifications that take no more
than an hour.
The trick is to use the reamer in a backward sense, with the 5/16 end
acting as the pilot, and the reamer being driven from the front end
ordinarily used to enter the work. Naturally the reamer must be turned
counter-clockwise to present the cutting edges of the flutes. The
modifications are to grind cutting edges on the trailing end of the
flutes, and to attach a nut on the front end with set screws so the
reamer can be turned with a wrench or socket.
What you need to do this:
One reamer, 3/8 inch (comes with 5/16 shaft) (Enco number 331-1124,
Enco Manufacturing Company, 1-800-873-3626); one 3/8 inch nut; two
8-32 set set screws 3/16 long. Four bronze bushings, 3/8 outside
diameter, 5/16 inside diameter and 1/2 inch long (HIF carburettters)
or 3/4 inch long (HS carburetters). The shorter bushings will work in
HS carburetters, but longer is better because there is more surface to
absorb wear and control air leaks. These cost anywhere from
twenty-five cents apiece to three dollars apiece depending where you
buy them and whether they come pre-lubricated, The most economical and
fastest place to get them is from a company that specializes in
distributing bearings.
A five-dollar reamer and a 20 cent nut are the main parts needed.
Two 8-32 set screws will hold the nut in place.
How you do it:
Step one. Bore out the 3/8 nut with a 3/8 drill so it will pass over
the reamer. Bore and tap two holes on two flats of the nut for the set
screws. Fasten the nut on the front of the flutes of the reamer and
tighten the set screws. Grind them off if they protrude. Grind cutting
edges on the trailing end of the flutes: This will become the new
cutting end.
The assembled reamer, showing one of the set screws. We have also made
some reamers by welding, but the high temperature is probably not a
good idea for a precision tool. Note that cutting edges have been
ground at the original trailing end. We used a die grinder on this
reamer, but we have also had perfectly acceptable results using a hard
white stone.
Step two. Disassemble one of the carburetters-do one at a time so
there is a reference when something does not make sense. Clean the
carburetter body with solvent and a pick. Holding the carburetter body
securely in a cushioned vise, and, turning the reamer "backwards",
pass the shaft through the body into the hole on the other side. Ream
the near side. Check fit a bushing, then remove it, smear with
thread-locker and refit. Then ream the remaining throttle rod hole and
fit a bush in the same manner.
The carburetter body is held in the vise with brass strips cushioning
it. The reamer is best driven by hand, withdrawing frequently to
remove the swarf with a gentle air blast. We used WD-40 for
lubrication.
The reamer is held by the old bushings for the first pass, and by the
old and one replacement bushing for the second pass. This process has
always worked for us and for several other MG owners for whom we
explained the procedure.
Step three. Rebuild the rest of the carburetter as detailed in Bentley
or Haines. Use a new throttle shaft, main jet, needle, Grose jet,
gaskets and seals. Lube the new bushings so they do not wear out right
away. Rebush and rebuild the other carburetter.
Step four. Replace the carburetters, set the mixture and air flow as
given in the books, and take the rest of the peaceful afternoon off.
The procedure really is simple. We have used it for several years, and
others have produced acceptable results as well. The key for us was
taking the course at UML, which gave us the confidence to attempt the
rebushing job with our home-made tool. The advantage of this procedure
is that it can be done quickly, standard-size throttle shafts (that
is, inexpensive) can be used, and really bad wear in the carburetter
body can be corrected without using an oversized shaft and oversized
reamer (both expensive). And you did it yourself!
----- Original Message -----
From "Gerard" <pixelsmith at gerardsgarage.com>
To: <spridgets-mod@autox.team.net>; <spridgets@autox.team.net>;
<midgetsprite@yahoogroups.com>; <bugeye@yahoogroups.com>
Sent: 31 May 2002 04:48
Subject: [midgetsprite] SU loose throttle shafts
> Anybody devised any good home remedies for loose throttle shafts on
SU's? I
> have a few I'd like to see more snug.
>
> Also, I have a set that are very clean, but the aluminum is dark
from
> cleaning is TSP. Any suggestions for something to dip them in to
brighten
> the aluminum? I'd like to avoid the really toxic carburettor cleaner
dip.
>
> Gerard
>
>
>
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