British car suspension threads.(MG, Morris etc)

[cardiffrob]

Hello.

Does anyone happen to know what pattern of ACME-esque thread was used to cut the threads on suspension trunnions for Morris/MG/Wolseley etc during the 50s? The Minor seems to be about 10tpi and 3/4 OD but that was me measuring it in the dark. The thread is a flat-top like a lead screw.

Failing that, what threads should I try to compare it to?

Cheers

Rob

 

[RonA]

Is it an ACME type thread or just a standard thread with the top of the thread skimmed off to allow oil to get all the way to every thread? Worked on my fathers morris minor traveller & replaced the trunnions at least twice in its lifetime because they started to seize up after a while, despite lubricating at the recommended interval. In extreme cases they failed altogether & the whole wheel fell off!!
Had a look in my Zeus book & 3/4 UNC is 10TPI. Is the one in the photo also a LH thread?
RonA

 

[cardiffrob]

Hard to know. I only have old ones and the threads are all pretty much knadgered.

Edit...0.715 OD and 11tpi, as close as I can measure it.

 

[Cobbler]

I don't know if there are still any about Rob, but back in prehistoric times there was a BMC die as a service tool to rethread & undersize trunnions were available to fit if I recall correctly. I've not heard of them for donkeys years, but they used to be in the official manual.

Edit ... I haven't had a Morris Minor for 45 years, so I may be out of date!

 

[cardiffrob]

Thanks. I had thought of that but haven't found any. Actually, I need to drill and tap for OVER-size by going larger to make heavy duty versions of something. Hard to explain.

It can't be UNC due t the vertical sides to the threads.

The flat 90 degree profile must be a nightmare to machine!

 

[zzr1200]

Unfortunately there's various versions of the square type ACME threads, there's even a possibility that MG, Morris, Wolseley may have even had their own versions of a thread, BSA motorcycles liked their own threads at times.

You need some thread gauges, calipers and to borrow a copy of the "Machinery's Handbook"

 

[CompoSimmonite]

It's British Standard Cycle thread on A35, A40 and Midget. Almost the same as another type so often that is use but then you get premature wear due to the angle being different. Not sure what was used on your trunnion but might be the same.

 

[Keith 66]

As far as i know the original ones had a normal thread profile not a squared off top. The replacement ones have this & effectively come pre knackered. When i was rebuilding my Moggie truck i found there was considerable variation in the sizes. I had one pair of old stock good king pins & took them to the Morris centre that used to be down wharf road in Chelmsford, I went through a whole crate of trunnions before i found two pairs that were a good fit. Most of the king pin & trunnion sets i saw were so sloppy i doubt they would have passed an mot.
The trunnions have a slot broached in them to allow passage of grease. Use a good moly grease & grease them religously & often! If you do not the bottom one will eventually strip & the undercart retracts violently into the wheelarch usually on a busy corner or roundabout thereby spoiling your whole day!

 

[cardiffrob]

We've had Minors for over 35 years but have never had one down on an elbow, as they say.

The squared off tip to the threads might make sense if that permitted the flow of grease round the edges. if so, .75 x 11tpi would be UNC, although the side profile looks to be straight rather than a more normal 55-60 degree sort of shape. I'll have a play tomorrow and see what else I can find in the sheds.

Thanks, guys. Always helpful here on this site.

 

[wristpin]

The trunnions have a slot broached in them to allow passage of grease. Use a good moly grease & grease them religously & often! If you do not the bottom one will eventually strip & the undercart retracts violently into the wheelarch usually on a busy corner or roundabout thereby spoiling your whole day!

Saw that happen, but should it be grease or SAE90 oil, as was the case for Triumph Heralds?

 

[dannyp]

By the sounds of it they want an autogreaser loaded up with ep 80/90

 

[gordon stephenson]

Same as the Triumph Herald, I believe the original lubrication for them was a thick but fluid grease,

 

[Cobbler]

Wouldn't be that difficult to rig something up off the oil pressure system like Morgan did, assuming the trunions were OK being oil fed. I would guess that regular oil lubrication would be better than irregular grease lubrication.

 

[Keith 66]

On the Moggy i have never heard of oiling them all the manuals say grease, think oil would just leak out everywhere, the rubber seals are not that effective.

 

[Olderisbetter]

The morris site has lots of info, maybe some common to alot of BMC,
https://www.mmoc.org.uk/Messageboard/viewtopic.php?f=4&t=51709

 

[cardiffrob]

Big book of "Machinery's Handbook" on it's way. Meanwhile, where might I look for someone to make a one-off tap for a square profile thread?

 

[rtbcomp]

My Riley had a similar system, recommends grease every 1000 miles.

 

[cardiffrob]

The seals that keep the grease/Oil in are rather pathetic so I imagine that grease will outlast oil. It always seems to be the bottom trunnion that dies when water rots the threads away.

 

[Olderisbetter]

I have the same problem on my moggy, The grease never gets all way round so i am going to do the extra grease point trick on it when i do the front end, i think certain ones had 2 grease nipples at some point.

 

[cardiffrob]

Evidence is starting to suggest that original parts have a square buttress thread whilst more modern repro parts use an angled thread pattern, still as yet unidentified.

http://mgaguru.com/mgtech/suspensn/fs200.htm