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MG Midget and Sprite Technical - diff cross pin

does anyone know what the small ling cross pin is made from? all of mine are very worn will a HSS drill ground to size be too brittle?

oh and which way do the cutouts face on the large pin face?

d cusworth

What is a "ling" pin?

If it is the cross pin:
why not buy one?

only 15GBP, and no guesswork

(and, I would think that a drill bit would be WAY too brittle for a driveline application)

On the other hand, if you are referring to the little spring pin that retains the cross pin, that is another matter, and annoying one too, because not only do the MG supply houses not stock them, but I also found that it was an unusual size not stocked by any of my local hardware stores.

I bought a pack of 500 from Granger (a large supply house in the US of industrial supplies), for $15US, used one, kept 9 more for the rest of my life (just in case) and gave the other 490 to a fella in St.Louis who supplies parts for a lot of LBC folks.

Later, I'll look up what size it was, and post back here.

Norm Kerr

OK, the little spring pin is 5/32 x 1"

A pack of 500 of them from Grainger is $18.

a nail is way too soft, and a drill bit might be too brittle, so I recommend using a proper spring pin (very tough material). Honestly, the load in that pin might not be much, but if it ever came out that would quickly lead so something that could wipe out your differential.

Regarding your question about the two flats on the cross pin, I am sorry to say that in spite of my detailed notes I took when researching, and then rebuilding my own diff, I never did mention them, or see reference to them. Their purpose is to get oil into the bearing surfaces of the planet gears, so maybe it doesn't matter which way it faces? The more I think about it, the more it seems that there may be a "prevailing" side for engine torque, and facing away from that would be nice, if we could figure out which way that is...

Norm "not as much help as I was hoping..." Kerr

Norm Kerr

Norm. Finger / keyboard problems. Locking pin was what I meant. What is a spring pin? A rolled pin?
d cusworth

The one I have on the bench at the moment and one that failed on me years ago were both solid pins and if I had to chose between a spring pin and a spiral pin (sometimes called a coiled spring pin) I would chose the latter. The choice of a HSS drill shank may not be that bad of an idea as the plain shank sections of drills are not generally hard but somewhat softer than the cutting edge so the chuck can grip the shank, easily tested with a file. Regarding the shank material of a HSS drill I have seen it speculated that it may not be HSS but I don't know for sure, certainly they are soft which is why when a drill catches it'll raise a burr on the shank where the drill chuck jaws have slipped.

D Cusworth, the one I have in the diff on the bench looks good so I could pop it in the post, let me know your email address if interested.
David Billington

I'm surprised Norm found they were 5/32 inch - when you want to remove the burrs to drive the pin out during disassembly, the book says to use a 1/8th drill. And yes it's important to use the right material - if the locking pin shears then the crosspin slides out and will either wreck the pinion or punch a hole through the diff casing, and sometimes both.
David Smith

What do you think about those selection kits available in halfords ? So to confirm, roll pins and spring pins are the same?


Dan Cusworth

The usual thing with a rolled pin ( i.e. the one made of a roll of spring steel with a slit down the length) is that it is inserted as a slight oversize. As it is tapped into position it compresses slightly and this springiness is what provides the interference fit that prevents it coming out. I cannot see that a solid pin is going to do that, although I suppose one could be put in with locktight.

There are 2 types of 'spring pin':

The rolled pin, as Guy says is a spiral roll of spring steel that is virtually solid in appearance. They are known as 'roll pin' or 'rolled pin' and a common trade name is a 'Spirol' pin.

The second type is a single piece of spring steel rolled into a circle, with a split down its length. Often know as a 'hollow spring pin' or 'spring pin' and the most common trade name is 'Seloc' pin.

The roll pin is much stronger in shear than the spring pin and both are slightly oversize so that they compress when fitted to stay in place.

What is the original one fitted in the diff? Roll or spring pin?


I would not use a solid pin of any type for this application as it will not stay in place reliably.

There is a selection of hollow spring pins item on eBay (110674150473), which includes some 4.00mm dia, which is virtually the same as 5/32" (4mm is 0.001" larger). Lengths are 5/8", 1" and 1 1/2". Buy-it-Now of 4.99 and free postage!

Richard Wale


If you end up getting an assortment pack of roll pins they are usually divided up into size lots
Use a drill bit or a set of verniers if you have them to measure the bore of the hole and select the size required (hopefully labeled) from your kit
I'm pretty sure Norm has the size right at 5/32, the pin will be bigger than this even though it is marked 5/32 it will in fact be closer to 3/16 and will compress down when you hammer it in. If you measure it up you don't really need an assortment just get one the correct size
An old racer's trick is to then select a smaller pin to hammer into the hole left in the bigger pin just as an extra bit of insurance

I can recall a diff that had a solid pin but for the life of me I can't remember what it was out of. At overhaul time the old pin had to be bashed out in one direction only as the hole was slightly tapered. The new replacement pins were long tapered items and to fit them they drove in till tight then cut off flush and then driven in with a pin punch till just below the surface and then the edge of the hole was tagged with a centre punch to make sure they stayed put.

Cheers Willy
William Revit

Again, it pays dividends to know what was there originally - which can be found in the workshop manual. It states Roll pin - and they 'look' solid to a cursory glance. It also states to peen over the ends of the housing after insertion to prevent it coming out.
David Smith

The one I was meaning to describe was the second one you list. The Roll or Rolled pin. Perhaps it was my reference to spring steel that was wrong. I don't think I have ever seen a spiral one.

Willy, I recognise your description of a pin that only goes in (and out!) from one side. I've come across those as well although it was a roll pin that was inserted - in fact I am not sure that it wasn't actually in this midget diff application!

I've just removed the pin from the diff on the bench and it is solid and 0.156" OD x 1.130" long. The pin in the NOS 3.9 diff I have is also solid as I can see the end of it in situ. All I've encountered had the end of the hole staked to retain them as David Smith mentions.
David Billington

all the diffs I have stripped over the last few years mainly old 4.55 with the fill hole in the housing, have been solid pins the one i removed is pictured. all of by diffs have had simmilar wear.

I have been to halfords and got some seloc type pins. found one which is a good fit. I will take William's advice and fit a smaller pin inside to beef up the CSA.


Dan Cusworth

OK, we've worked out the roll pin for him, does anyone have an answer regarding the orientation of the flats on the cross pin?

Norm Kerr

The cross pin I have on the bench has the flats in the same plain as the locking pin hole so the flats can either face the crown wheel or away from it, I really don't think it'll make any difference to the splash lubrication.
David Billington

I fitted the shaft with cutouts facing away from cw.

here's a piccie of the "doubled up" pins as per William's idea. its a bit blurred

thanks all for your help


Dan Cusworth

Curiouser and curiouser! Taking David S's advice 'to know what was there originally', I have just looked at my BMC manual for the Minor dated 1966 and it says to:

'Tap out the dowel pin locating the differential pinion shaft. The diameter is 1/8" and it must be tapped out from the crownwheel side .....'.

It also talks about cleaning out the peening with a 1/8" drill, and making sure that after fitting the new pin to 'peen over the entry hole'.

I wonder whether wear enlarges the hole, creating the need for a 5/32" pin? Or did BMC increase the pin size in later years?

Richard Wale


I've just checked the diff on the bench and the hole at the crown wheel end is 1/8" and at the other is 5/32" so there is a step in the bore in the crown wheel side which can be felt and is I would expect intentional to provide a stop for the pin so it is driven in to the step and staked in on one side only to retain it.
David Billington

...or maybe the manual has been wrong for fifty years!
Norm is right, I just ferreted out my stock of diff rebuild parts and measured a new 6K631 bought from Moss in 2000, and it's 5/32 inch.

David Smith


The manual may not be wrong, it depends on the interpretation of the wording. I think you do indeed need an 1/8" punch or a bit smaller to drive the pin out but cleaning up the peening with an 1/8" drill may be of questionable benefit. My interpretation of what Richard posted from the manual doesn't rule out the pin being 5/32" but does indicate that a 1/8" tool is required to drive it out which makes sense to me having looked at the diff on my bench.
David Billington

David B,

Ahh, that sounds right, the hole is stepped to stop the pin going too far and beyond the cross pin. The smaller part of the hole will be 1/8", thus needing a 1/8" maximum diameter punch or drift to knock the existing pin out, but as you say, the pin itself is 5/32" or usefully 4.00mm, which makes them a bit more available, at least in the UK!

Richard Wale


That is my feeling on it and placing the locking pin against the diff housing shows that the locking pin extends at least 1/4" beyond the cross pin bore so the cross pin is fully supported by the 5/32" pin by the look of it. It would be interesting indeed to have access to the original drawings for these parts and material specs but I think much has been discarded or lost over the years.
David Billington

Solid here, too.


Dave O'Neill 2

I presume this is solid, you can see how much I drilled out before realising it tap out from the opposite end you drill. This seems wrong, that you peen over the end it comes out of.

See how worn it was.

Barry West

OK, now I realize that the original pin was a solid dowel (learning from the above posts, and their photos of OEM parts), I am now worried that I wrongly assumed mine was a spring pin and having used a spring steel "rolled" pin will get me into trouble.

So I did some checking and found the following:

The Grainger 5CY83 I used is a "slot" spring pin ("C" shaped in section), with a rated tensile strength 200ksi.

There also exists a coiled spring pin (Grainger P/N 5EB34, sometimes called a rolled pin) which is slightly harder, 220ksi, and being coiled, minimizes stress concentration that could be caused by the edges of the slotted spring pin above.

Based on the posts from folks above, a solid "dowel" pin is what BMC used, and although I could not find them available in 5/32", someone, somewhere must make them. As pointed out by Richard above, 4mm ought to make this easier to find. The ones sold by Grainger (in other sizes) were 310ksi, or 50% stronger than what I used.

Which brings me to the key questions: what is enough for this application, and what tensile strength/hardness did BMC use?

The differential housing ought to carry all of the drive line loading that the cross pin sees from engine torque and wheel inputs. I suspect that the biggest issue for this little pin is "chafing" and wear caused by the cross pin moving around.

Can someone with one of those OEM pins measure the Rockwell hardness for us and find out?

For comparison, the tensile strength of a grade 8 bolt is only 150ksi (RC33). The spring steel used in the above pins is 200 ~ 220ksi. The RC60 hardness dowel pin that Grainger sells (but in the wrong size) is 310ksi.

If the OEM pin was RC60, then there might be a pretty good argument to take my (freshly rebuilt) diff apart and replace that pin, because the amount of wear in those pins is pretty high by the time folks do get around to rebuilding them.

On the other hand, if the main concern IS chaffing / vibration / rubbing around type wear, then having used a spring pin might be substantially better than a solid pin, regardless of the hardness/strength, by the virtue of its inherent resistance to moving around...

Maybe, as Guy pointed out, blue lock-tite could be used with a solid pin to prevent it moving around (and chafing/wearing). However, if the OEM pin was softer than RC43 (my pin's hardness) then I'll just leave mine alone!

Norm Kerr

oh, I just found the 1 x 5/32" dowel pin on Grainger's web site: 2MB49
Rockwell C 60 hardness (311ksi tensile strength).
Double shear strength = 5000 lbs.
$8.35 / 10pcs.

Don't know why I couldn't find it before...

Norm Kerr

I had one of these pins fail. The cross-pin then worked its way out and caused a LOT of damage.

It was a S/H 3.9 diff that I had just fitted to the racer. After that, I always fitted a new cross-pin and dowel when installing a replacement diff.
Dave O'Neill 2

I had one fail also, I lifted off over the brow of a hill and on the other side when applying the throttle again no drive, didn't hear a thing. Got recovered by the RAC and later found the locking pin sheared and the cross pin had taken a major part of one pinion tooth off so was scrap. This is apparently not that uncommon when one of the rear wheels is spun up causing severe use of the differential which can lead to partial seizure of the planet gears on the cross pin which leads to the locking pin failing.
David Billington

Mine failed on the first lap of qualifying at Assen.

Fortunately, I managed to find a 4.2 diff for the race, although it was a bit 'short' for Assen.
Dave O'Neill 2

The car is driven by the planet pinion shaft bearing in the diff case holes.
Any slop here will load the retaining cross pin and beat it to death.
Failure is either from this over time OR the planets seizing on the shaft during wheelspin - common on cars that get stuck in mud & snow a lot.
The cross pin is not hard, as evidenced by the wear and drilling illustrated. Maybe about the equivalent of a grade 5 bolt at most, but I think less.
The cross pin needs to be fairly soft to deform to grab the hole in the case, and in the planet shaft if it is a tight fit.
A hard pin will likely either fracture itself, if it winds up taking the drive due to a worn case;
or, fracture the planet shaft if it is a tight fit in the shaft.
Hard pins are NEVER used in a drive fit with hard parts!
The pins were originally solid, and these occasionally fell out because of case wear;
but, we started getting the roll pins from the dealers (factory) at some point long gone.
These have much less tendency to fall out, but again worn cases cause the spring pins to crack lengthwise if they are beat upon.
We used to double pin them as Willy said.
Or, I've seen them with roll pins having split pins stuck through and coming out the other side (bent over) as insurance.

The obvious fix is to Loctite the pinion shaft into the case, and just use the springish cross pin as a locater/safety.
All the more so with sloppy cases, which might call for thicker Loctite.

And it might not hurt to grind another 2 flats opposite the existing ones to improve lubrication under wheelspin abuse. As I stated above, all the load is in the direction(s) of rotation of the cage, not on the sides.
Worth remembering that under constant rotation there will effectively be NO lubricant in the center of the cage, as it all gets thrown out until the thing stops turning. That's why the pinions seize when folks spin the wheels long after it is clear that it ain't a'gonna move!

FR Millmore

This thread was discussed between 30/04/2011 and 06/05/2011

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