Straightening a McGee throttle shaft

[Harv]

The photo below shows one of the three throttle shafts for the McGee red motor set. The parts marked as “bearing” normally live inside the throttle body casting. The right-hand side of the shaft pokes out the casting, and normally mounts a bronze throttle stop. At some stage, the throttle body has had a big hit, and bent the shaft (casting is thankfully OK). In theory, I can still use the bent shaft, though the throttle stop would be wonky.

Question is, how do I straighten the shaft without misalligning the “bearing” bits? I suspect if I clamp it into a vice and whale on it with a bfh that it will end in tears.

A job for my fumble-fingers, or find a steel magician who can?

301EC516-8AF5-4EE4-929A-C6F81808C907.jpeg (60.97 KiB) Viewed 785 times

Cheers,
Harv

[EK283]

Harv,

If you have access to a lathe you can clamp the bearing surface in the chuck rotate it and hammer it until its straight.

You could also heat it up at the same time to avoid large hammer blows.

Greg

[ardiesse]

Harv,

(Greg beat me to the punch).

The shaft is only bent after the third bearing surface, so you're in luck there. If you can drill a hole of exactly the diameter of the throttle shaft axially into some round bar, and if you can get access to a lathe, then I'd suggest chucking the shaft into a collet of the right diameter. Then slide the pre-drilled round bar onto the bent end of the shaft. With the lathe in neutral, you'll be able to turn the shaft, and know exactly which direction the shaft's bent in. Lean on the round bar as appropriate, and slide it bit by bit onto the shaft, straightening as you go. You'll be able to get pretty close.

The classic approach, I believe, uses a dial gauge, a copper hammer, and the "knack". The copper hammer is used instead of the pre-drilled round bar, and the results are monitored with the dial gauge.

Failing everything, it wouldn't be too hard to reverse-engineer the shaft and get a new one made.

I got some experience of the whole shaft-straightening game repairing a coaxial sliding load (used for calibration in microwave measurements, fragile and about $5000+ to buy new.)

Now I remember. I used the tool rest with an aluminium bar in it to push against the bent bit. With the dial gauge I could correct for spring-back when straightening.

Rob

[Harv]

Thanks gents.

Cheers,
Harv

[rosco]

Tricky issue - no matter how good the result, there will be marks.... miniscule, but marks.
Sadly, if these are in the area of sealing surfaces - there may be issues.

I was going to suggest heat and the use of a fine tolerance steel tube to bring the shaft back into alignment... haven't done it, so probably best if I keep my clever thoughts to myself. I would hate to have the shaft thermally bind to the sleeve during the cooling process..

I can't understand how that shaft could have been damaged.... was it a result of engine removal/replacement with the carby still attached?

Keen to learn how you got out of this one...

frats,
Rosco

[Errol62]

I would look for some thick walled tube to slide over and try and hold the bearing alignment. Maybe peen the out side at the bearing positions if necessary to take out any slack or take up the slack in a vice. Possibly another bit of pipe on the end as a lever or drill out some rod as Rob suggests.

You could heat the end and let it cool slowly to anneal it. It’s already been bent cold once and might fail on the compression side. Then re-temper after straightening.

Worth a try. Preventing the bearing surfaces from getting out of alignment is the challenge. I suppose they could be linshed if necessary to re-true them.


FB ute fixer upper, EK van on rotisserie

[Harv]

Not sure how it got bent, as it was posted to me that way (disassembled, so not Aussie Post’s fault). The whole set has had a hard life... methinks it is likely to have been in roundy-round service. All the throttle body and lots of other bits were painted with gloss black enamel... over the top of dirt. Someone has cleaned up most of the throttle body, but it will need blasting to get rid of the last of it. The drive pulley has a huge chunk out of it (looks like the ally was hit with an angle grinder)... then was painted gloss black too. Maybe it spent some time as a “display engine” . It’s a red motor unit, but someone has tried to redrill it and use on a blue/black motor.

Lots of love needed to make this one sing again.

Cheers,
Harv

[rosco]

It's in good hands with you, Harv - you'll gently nurture and guide this shaft back into service.... and non but us forumites will be any the wiser.

Yes, there are some ham-fisted monkeys in our ilk.... failure to appreciate or understand componentry being one factor, attempting service/adjustment/removal etc when fogged by a Friday afternoon's social 1/2 slab being probably more the culprit..

With the amount of black paint you indicate - I'm surprised your items didn't also come with feathers buried in the "tar"...

I hate seeing parts which should be bare metal - painted.. and black is one of my favorite hate colours.. it conceals just so much detail that I would go so far as to walk from an item if up for sale that attracted my interest... black is a very heavy basic colour (if we don't debate both black and white as being such)... it does not have a colour of its own - most are either a brown or a blue... but or eyes see the pigment as black - it reflects very little hue even under the strongest of light sources.. I was led to believe it actually "absorbs" light.. like a black hole.... and people use it to hide issues... hence the love of some people to fall in love with the rat-rod livery ..... panel work on these looks pretty darned good - until it gets wet enough to reflect.....

I'm keen to learn what action you take with the shaft, Harv... I do like the idea of spinning it in a trued up lathe - but I balk at any attempt to true it up with cutting tools... those bearings are of a pivotal (pun intended) criticality..

My thoughts now turn towards replicating a replacement... and tempering the result.... you'd be guaranteed of a true shaft with as close a tolerance as your machine is capable of.... whilst you're at it, don't stop at one... never know, you may need a replacement down the track...
Making two always results in a "good" one - and a "not so" good one.... law of practice...

My thoughts,

frats,
Rosco

[Harv]

I got it straight. At least mostly.

I took an LPG torch and heated up the bend to a dull red, then let it air cool. The shaft was indeed tempered, as you could see the colours run up the shaft about 10mm as I heated it.

I took a piece of 3/4" mild steel rod and cut it to length slightly shorter than my drill bit. The shaft was a perfect imperial size, but too close... a test hole in a short piece of steel showed the shaft would go in to the imperial hole, but would struggle to come out. I was wary of the need to belt the hell out of the shaft to get it out of the imperial hole, so chose a metric bit. It was a good firm fit, but not tight like the imperial hole.

I marked the centre of the rod, counterpunched it, then set it up in the drill press. It quickly became evident that my drill press was way out of alignment. It is a good Hare and Forbes unit, one of the earlier ones with steel casings. I replaced the bearings years ago, but only have a cheap (sloppy) vice on it. I played around with the adjustment a bit, but the end results show that it is still pretty misaligned. One day I will learn how to line up a drill press properly. It was a deep hole for steel (full depth of the bit), so lots of cutting fluid and in-and-out to clear the flutes.

I took my wonky drilled steel rod and put it into a vice. I put the shaft into it up to the bend. This was deep enough to cover the end and centre "bearings". It would have been nice to have a hole deep enough to have both ends and the centre, but I do not have a drill bit long enough. I then rotated the shaft back and forth and eyeballed where the bend was coming from. Got it rotated to the worst part of the bend, then gave it a love-tap with a copper hammer. The shaft would then move in to the rod by a mm or so. Rotate, tap, push in. Repeat. The end result is OK, but nowhere near perfect. Given it is just floating in the air with a thottle stop on it, I am happy.

I still have to re-harden the shaft, but that was when my LPG burner crapped out. It's a good Primus unit that runs off a standard BBQ bottle, and I've had it for nearly 30 years. Pulled it to bits, but can't see why the check valve is jamming shut. Fearful of replacing it at Bunnings with COVID going on (it would be an expensive $1300 burner, and not exactly essential). For now, I've screwed the injector all back together to let me check the rest of the assembly, and I'll re-harden the shaft later. Picture below of the reassembled unit, alongside it's grey motor brother. The red arrow shows where the un-bent end of the shaft sticks out.

Bent shaft.jpg (465.15 KiB) Viewed 679 times

Cheers,
Harv