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MG Midget and Sprite Technical - King Pin Reamer Sizes

I am sure this has been done to death but I simply cannot find a definitive answer as to what size the king pin bushes are reamed to (or what size are the two diameters on the actual reamer)

I have access to a fully equipped workshop so just need to know the sizes the bushes should be cut to.
mc Wilks

Done to death...?

No not really... give moss a call, they are helpful with tech details... and they no longer sell the reamer... im guessing because it cost around $500 last time they sold it

I want to say its in the bently shop manual also, but dont quote me as fact on that

Now ... wheel bearings thats some killing words...haha

Good luck

Prop
Prop and the Blackhole Midget

Drum or disc stub axles? the sizes are different between the 2. I did the job years ago with standard size fractional inch hand reamers by placing the reamer in the headstock and supporting the other bush with a centre in the tailstock. The bushes came out snug but lasted far longer than the looser ones done later at a local shop with the correct stepped reamer, actually the old bushes still fitted the kingpin better than the newly reamed ones. I didn't see snug as an issue as they don't really rotate to any degree, one could always give the bushes a light home after reaming if concerned.
David Billington

I used the stepped reamer to do my MGB bushes and they felt quite loose.
Dave O'Neill 2

I think the Sprite/midget kingpin reamer diameter sizes are as follows:

Sprite and early Midget (ie front drum brakes):
Kinpin
top journal diameter 5/8" (0.625") (15.85mm)
bottom journal diameter 11/16" (0.6875") (17.46mm)

Later Midget (ie front disc brakes):
Kingpin
top journal diameter 5/8" (0.625") (15.85mm)
bottom journal diameter 25/32" (0.781") (19.83mm)

I hope that helps.
Andy Hock

A bit weird but I measured the old kingpin from one side of my front suspension earlier and the upper journal came in at 0.605" and the lower at 0.738", neither is a standard fractional inch measure or not close enough. I also checked the bushes and they're not much bigger, only around maybe 0.005" bigger. As I had modified the front suspension to improve the lower trunnion I required the OE kingpins with faced sides and that is what these were so presume the journals were of the right size, they fitted the old bushes well. I've checked the Mitutoyo digital caliper used for the measurements against a 0.7500" slip gauge and spot on. I remember, although it was a while ago, that the kingpin journal sizes were a standard fractional inch sizes and the reamers I used were from the college set of standard reamers. I guess we need someone with a new kingpin to measure. I can't remember if the kingpins were supplied to the last company who reamed them last so don't know if they have been adjusted but I find the dimensions I measured odd.

BTW I looked in the archives and found some posts that fit the search criteria I gave but none mentioned the dimensions, Maybe I didn't look hard enough.
David Billington

David, thanks for your findings. I have some new/unused Quinton Hazel kingpins, and a new QH kingpin reamer in my hut. Tomorrow I'll measure them. (Obviously it will be pointless measuring the new bearings that are designed to be pressed into the steering hubs).

The old adage is proved, I guess. Measure 20 'identical' kingpins, and you'll get twenty different sets of readings. And I won't even get onto the topic of 'how much clearance should I allow for grease penetration?'.

In the meantime, I'll see if I can find any original specification dimensions for the kingpins from BL literature.
Andy Hock

Thanks gents, I too could find nothing that gave a definitive size hence the question.

i did however find a posting somewhere in the ether that quoted a size of 15.85mm & 19.83mm (0.624" & 0.7807") which tally's with your measurements Andy, but you can't beat original spec's.

Martyn Wilks

I've just had a close look by eye and at 10x at the kingpin I measured and I can see why the sizes are odd. It looks like the company that did them cleaned up the bushes and ground the kingpin to suit. The 2 journal surfaces are quite obviously ground more roughly than normal and the accurately ground upper section of the pin where the upper knuckle fits is a much finer ground finish. I had heard the company didn't do the finest work and I understood they had the reamer for the job but I guess not and just bodged it.
David Billington

Just been measuring my brand new (several decades old) kingpins and reamer. The kingpins are from repair kit Quinton Hazel QP624TC (kit of king pins etc with no fulcrum pin) and kit QP624TCF (as above but with supplied fulcrum pins!). Oh, it takes me back . . .
Designed for Midgets with front disc brakes.
Reamer:
UPPER cutting flutes diam 15.48mm (narrowest neck); 15.89mm (widest neck).
LOWER cutting flutes diam 19.7mm (narrowest neck);
19.9mm (widest neck). (NB Because the vertical cutting flutes are twisted, it's very difficult to measure a precise diameter).

New kingpin journals:
upper 15.86mm
lower 19.83mm

Compare to my previous posting:
---------------------------------------------
Later Midget (ie front disc brakes):
Kingpin
top journal diameter 5/8" (0.625") (15.85mm)
bottom journal diameter 25/32" (0.781") (19.83mm)
-------------------------------------------------

In summary, I will bet good money on the following being 100% accurate:

====================================================
Sprite and early Midget (ie front drum brakes):
Kinpin
top journal diameter 5/8" (0.625") (15.85mm)
bottom journal diameter 11/16" (0.6875") (17.46mm)

Later Midget (ie front disc brakes):
Kingpin
top journal diameter 5/8" (0.625") (15.85mm)
bottom journal diameter 25/32" (0.781") (19.83mm)
=====================================================

Andy Hock

Andy,

Thanks for the figures now something conclusive in the archive.

If you read my previous post you'll have read that my kingpins are bastards modified in size by the company that did them so disregard the dimensions of mine. As regards your previous post mentioning differing sizes of the kingpin journals from pin to pin I would actually expect very little deviation between pins, likely in the order of 1/10th of a thousandth of an inch. The kingpin have centres at each end and I would have expected them to be precision ground on a cylindrical grinder to fairly tight tolerances, after all the bushes have to be reamed and the kingpin fit with an appropriate clearance consistently, too big a tolerance on either could lead to cases of not fitting at all or a sloppy fit.
David Billington

Hi, David. Thanks for your post. I agree with your comments re. 'expect very little deviation between pins'. But my earlier comment ("20 different measurements for 20 different pins") was alluding to several likely vagaries:
(i) variance in accuracy of measuring devices;
(ii) human error in machining (or lathe/machine set up) of the pins;
(iii) variance between each manufacturer.

I wonder how many UK manufacturers of kingpins there were at the height of A30/Midget production? Where were they made and who made them? Who supplied BL? Who supplied QH?

Indeed, who makes them now? Is China involved?

Andy Hock

As a general thing, bushings of this type should be reamed to .0005" to .0015" over the actual diameter of the shaft to be fitted. They must be reamed after installation as the inner diameter of the bushing will close up after being press fitted into the stub axle or other device. For things like pilot bushings, the average clearance would be about .002" to .003" over the diameter of the input shafts. Dave B. is perfectly correct about the quality of machine work done on such things as king pins and distributor shafts--the ones I have measured have been within .0001" between a number of examples tested. The Brits were capable of some very fine craftsmanship when they thought is was desirable to do so.

Les
Les Bengtson

Even with the factory reamers, you cannot assume the reamer will cut to t he right size. As I get near the finished size, I will test fit the king pin several times. I am looking for a tight fit with no interference. To anyone who posts regularly stateside, I have a factory reamer for both sizes of kingpins and will ream your uprights if you pay postage both ways. Before I get a flurry of posts saying there is no reamer for bugeyes, I do have one. After Christmas would be a good time to do this. email me directly.
J Bubela

Thanks Andy for the sizes I can now machine the bushes to the correct sizes.

Incidentally when I used to make special reamers etc for the Aircraft Industry, the standard taper for a reamer was 0.0005" per inch so I'm guessing that BMC also used a similar standard for their reamers as well.
Martyn Wilks

Hi, Martyn. Yes, I'm confident that the kingpin journal diameters are 5/8"<>11/16" (drum brakes);
and 5/8"<>25/32" (disc brake). Although it must be said that I don't have a drum brake kingpin to confirm the measurement, I've taken those readings from information found on the 'net.

And, yes, a reamer taper of approx 0.0005" (0.0127mm) does roughly tally with my 'best approximate' reamer measurements (manufacturer unknown).

It goes without saying that the new bushes will have to be reamed once they have been pressed into the stub axle, having ensured that the grease access holes are aligned.
Andy Hock

This thread was discussed between 05/11/2015 and 09/11/2015

MG Midget and Sprite Technical index

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