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Rebushing HS2 Carb throttle shafts

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TexasSprite

TexasSprite

Jedi Hopeful
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Has anyone put bushings into SU HS2 Carb bodies? I have a perfectly functional set on my Bugeye, but have a box of spares that need some TLC. This appears to be an interesting machining challenge. For tooling, I have a accurate South Bend Lathe that I thought to use (with a jig) to bore and ream the holes for the bushings. As as an amateur machinist, I always appreciate any technical info or how-tos.

One of the spare bodies already had the job done with Teflon bushings, so is that the material of choice? On the other hand, the carb kit I got from Burlen Fuel Systems (https://www.sucarb.co.uk/) came with two 5/16" OD X .250 ID metal bushings that are supposedly teflon coated. In addition, The carbs on my car were rebushed with brass or bronze (can't really tell which). I guess there are a lot of materials that could work.

I thought I had an easy way to do this when I read an article on rebushing SU carbs using a 3/8" reamer to cut the seats for 3/8" OD X 5/16" ID oilite bronze sleeve bearings. This was for a larger carb with a 5/16" throttle shaft. The shaft of the reamer used in this technique is 5/16", so it rides in the hole on one side of the carb while cutting the seat in the other. Note that the reamer was pulled into the hole with the cutters at the tail end of the reamer blades engaging first. The bearing is pressed in and used as a guide for the reamer shaft while it cuts the other side of the carb body. Very clever and low tech. Too bad the HS2 uses a smaller throttle shaft (.249) There are 5/16" OD by 1/4" oilite bronze sleeve bearings available, but the 5/16th reamer I have has a shaft diameter of .277, which will not fit. So, it's back to the lathe and jig setup.

Given that the idea is to prevent air leakage, what kind of clearance am I looking for between the shaft and the carb body? Is .001 OK or does it need to be .0005?

Thanks!
 
If the shaft of your reamer is .277 then to use it as a pilot you would be using the wrong end of the reamer as the cutting end and it wouldn't work at all.
You need to grind the leading end of the reamer down to .249 dia x 1.5" long or so and resharpen the cutting edges (moderately difficult without the proper tooling).
If you have a milling machine (or milling attachment for your lathe) and a good jig setup then you may be able to do it successfully with a common .250 reamer.
I would shoot for .002 to .003 clearance for the shafts to prevent binding.
BillM
 
I don't know that re-sharpening the reamer would be necessary. If it is a multi-flute straight reamer the back/trailing edge of the flutes will still have the right angle for cutting. However, you may wish to grind a slight chamfer onto the trailing edge of the flutes if they are "square" on their back ends.

Most of the reamers in my collection have centers on their ends. If your 1/4" ream has centers, mount it between centers on the lathe (with a face plate and dog) and turn the shaft down to roughly 1/4" so that it will pass through the carb body with minimal clearance. Then pull it through as you have done before. It should work very well.

The last carb I bushed was a 175CD Stromberg. In that case I reamed the body on a mill by placing a guide rod in the mill's spindle and passing it through the carb body. I was then able to position and clamp the carb in the mill's vice. The guide rod was then removed from the spindle and replaced with the ream to machine the carb body. In that case I used standard oilite bushings (sintered bronze) as I did not know there were other options. The carb is still in service on our GT6 after 15 years. However, the car hasn't racked up many miles since then so there may be better bushing material choices.
 
I've been using Nylatron for bushings in HS2's for some time now. They handle the heat very well, much easier to machine (hold a tight tolerance) and give a low friction, long wearing surface that the brass shaft will never have an issue with.
My method is a bit different only because I have a milling machine as well as an indexing head to work with.
Once the carb body has been centered and indicated to a common centerline, next is to counter bore into the body leaving roughly 1/16” of the original bore at the inside edge of the throttle chamber.
I use 11/32” instead of 5/16” as the outside diameter and essentially counter bore the bodies from each of their respective sides. Why? It gives me a shoulder to press to and I don’t have to fiddle with making the bushing with a matching radius of the throttle chamber. Now the bushings can be made at the lathe, with both the OD and ID concentric to one another. Most of the time the ID of the bushing is reamed to 3/16” in the lathe. The only reason this is done so that upon removing the carb body from the fixture in the indexing head to press the bushings into the carb body.
It can be bolted back to the fixture and lightly push a 3/16” length of rod into and thru both sides of the carb body for indicating the rod true to the carb body again (a double check as well). At this point a final reaming to size is done once the standard shaft has been checked for it’s specific size. I do try to hold a 0.001 / 0.0015 clearance over the shaft size. Yes lots more work but the right equipment and fixturing; it's still very quick work. When someone’s uncomfortable with Nylatron, 6061 alumn bushings are made and inserted.

Dug
 
Havent had any SU's that were bad enough to repair yet. But on some others I did drop a counterbore just big enough for an oring with washer. A spring can be used to keep the washer pushed against body or even a flanged sleeve depending on how much space is available. This works great on a lot of different applications. Gets rid of the airleaks on all but the worst ones.

Its more difficult but I have also done this. Step the inside of throttle bore and press in flanged bushing. Then machine to match radius of bore. This fixes the area where throttle plate wears the body around shaft.

Another option I have put to use is a two piece bushing. Leave a gap between the pieces for an oring to fit. Shaft clearances are not much of a factor this way. Without air being sucked in a little grease will stay a long time on the outer bushing.

A really cool way is partially threading the bores. Then machine a bushing from hex. Thread the OD. Then split the bushing most the way back to the hex
shoulder. As you tighten down the bushing it will adjust the clearance on shaft. Probably need loctite on thread to keep from backing out.

There are also some oil impregnated bushings available that are very thin wall. Actually not much more than the size of worn shaft bore in body.

I recently acquired a complete set of Sunnen hone mandrels for valve guides. I need to double check but they may be just long enough to do an align hone on the bushings for that absolutely perfect fit.

My big Healey is a low milage car, and I havent gotten anywhere near that far on the Bugeye yet. But nice to go over the possibilities so when the time comes, I will have most bugs already worked out.
 
HS2sare bit different aniaml than then their HS series cousins, HS4/6/8 in that hey do have a bronze/brass bushing cast into the carb body, they simply ride on the aluminum of the carb body, and what this does noramlly is cause shaft wear but not shaft bore wear in the carb body, so what you generally see is worn shaft, but good shaft bores, almost all of the time, a new standard shaft will put thing back to whear they need to be, you can test this by, when removing the throttle shaft fell the fit on the shaft to the bores on a section of the shaft that is not worn, and you will most likely find you just need new shaft, and not a bushing the bodies.
 
Haps experience has been the same as mine and I'm sure he's had more of it. Very little wear in the carb bodys on HS2's just on the shafts.
KA.
 
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