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MG Midget and Sprite Technical - diff backlash

just got a new to me diff and chcking the backlash before fitting , it is etched on the back of the ring 007 then in a circle it has a +9 .backlash is .18mm which is 7.08thou which i take as correct but what does the +9 mean
mark heyworth

Interesting
Was thinking pinion shim adjustment, but 9thou would be a lot they would normally be in the -5 - +5 range so just don't know
Then ,I've only ever had one A series diff in and it had a very short life
Is there anything etched on the end of the pinion itself-?
William Revit

end of pinion has a 0 on it ,looking at it it could be 007 and +3

mark heyworth

Hi Mark,

I run spridget style diffs in my road TC and race TC. I have converted about 45 of them to suit TCs because I used to do it for other people too. I sourced diffs from Morris Minor and Major, Wolseley etc because they are the same, but ratios range from 5.3 to 3.7.

I am not an expert in the field but always figured that the backlash and other setting figures are a "means to an end". I believe the end goal is to get a good mesh, which can be checked with a "blue marking". As a result, I didn't use the numbers. (Should I admit I didn't understand them?) I fiddled with pinion and crown-wheel settings until I got the best blue marking achievable. I used advice from the Jaguar Mk2 Workshop Manual (pp H20 & 21) which had a great diagram of good and bad markings and said which way to move what, to achieve the good. I am sure the etched and stamped numbers would speed up the process for those who understand them. I am sure the experts check with blue anyway.

I have attached a photo of what I think is a reasonably good marking. Central on the teeth but slightly towards the "toe" end. Note that diffs which have done a couple of hundred thousand miles might not be capable of a setting which produces a perfect marking on both sides. I have found that old diffs seem to produce markings which are narrower on drive (like image) and broader on coast.

P.S. If anyone needs spider gears for spridget diffs, I have a box full because they are discarded during the conversion to fit TCs


R L Schapel

Hi Mark,

I run spridget style diffs in my road TC and race TC. I have converted about 45 of them to suit TCs because I used to do it for other people too. I sourced diffs from Morris Minor and Major, Wolseley etc because they are the same, but ratios range from 5.3 to 3.7.

I am not an expert in the field but always figured that the backlash and other setting figures are a "means to an end". I believe the end goal is to get a good mesh, which can be checked with a "blue marking". As a result, I didn't use the numbers (because I didn't understand them). I fiddled with pinion and crown-wheel settings until I got the best blue marking achievable. I used advice from the Jaguar Mk2 Workshop Manual (pp H20 & 21) which had a great diagram of good and bad markings and said which way to move what, to achieve the good. I am sure the etched and stamped numbers would speed up the process for those who understand them. I am sure the experts check with blue anyway.

I have attached a photo of what I think is a reasonably good marking. Central on the teeth but slightly towards the "toe" end. Note that diffs which have done a couple of hundred thousand miles might not be capable of a setting which produces a perfect marking on both sides. I have found that old diffs seem to produce markings which are narrower on drive (like image) and broader on coast.

P.S. If anyone needs spider gears for spridget diffs, I have a box full because they are discarded during the conversion to fit TCs


R L Schapel

Mark
Had a good think about this, and have come up with the following--
As your pinion has a '0' on it the etching on the crown wheel can't be for the pinion height
007 etching is the backlash that was measured by the manufacturer when the c/w-pinion had been lapped in on the lapping machine-
After it's lapped, backlash is measured and the 007 figure is what it would have been and set up to new
I feel the +3 figure is a shim packing figure used to get the correct offset of the crownwheel to get the correct backlash--I would imagine that this figure runs in a range similar to the pinion shim like -5-+5
SO when the set was being lapped to get the best marking at the spec'd backlash they would have moved the pinion in-out and the crownwheel L_R to get the best marking for the set
So when your set was lapped it was marked pinion 0 c/wheel +3 as that is where they moved them to get good markings
Pinion marked 0 would be fitted to a zero height shim as measured in the housing using a dummy pinion
Crownwheel marked +3 means you would need a 3thou thinner shim on the thrust side to get the crownwheel in the 0 position
-----------
But as Bob says above the tooth marking is the main object of all of it
and these etched on markings are only there to assist shim selection for the initial setup which must be checked after and adjusted if required for a good pattern
---
Bob, instead of bearing blue,I use orange ochre mixed with a touch of oil into a paste similar consistancy to b/blue, it leaves a magic bright mark that is real easy to see
Might be worth a try-------or not
William Revit

A plus figure means that shims must be removed from the left to the right, after the initial calculations have been done. In your case, 9 thou would be removed from the left hand side and fitted to the right hand.
The numbers are normally in a square frame.
Dave Barrow

they seem to look ok

mark heyworth

Dave--that'd be 3- Mark made a mistake with the 9

Mark-
To me the mark is a little too far towards the toe-meaning it needs a touch more backlash by moving the crownwheel accross probably 2-3 thou---BUT--You have to be real carefull moving 2nd hand markings around as the thing has probably been like that all it's life and worn into that position and if moved it 'could' be noisy--bit of a catch 22
If it were a race car I'd set it like your pic as the extra load of full throttle moves the mark outwards a tiddle

If it's been running ok like that I'd be tempted to leave well enough alone

willy
William Revit

Don't know how I repeated myself! ... Don't know how I repeated myself! Sorry. Sorry.

I agree with Willy. Your blue marking appears a bit towards the toe. Do you know if it made any noise? I would love to know how far from the "ideal" a mark must be before a diff gets noisy.

I have attached the (Jaguar Mk2) pages I use, in case they are useful to you. I only tend to look at pages 21 and 22 (circled bit).

I have read that several images can be sent in the one post but I don't trust myself to try .... so will send in "installments". Most useful first.

Thanks Willy. I will try that orange ochre ... sounds good!

Bob


R L Schapel

Second image. The circled bit is what I use.

Bob


R L Schapel

Third image. Rarely refer to this, but it has some interesting detail if needed.

Bob


R L Schapel

Bob
Mr Jag tries to confuse us a bit there calling the crownwheel the drive gear when really it's the driven gear---I had to read that twice before it hit me
Must be getting old---------

Your comment-
' I would love to know how far from the "ideal" a mark must be before a diff gets noisy. '
I know you have done plenty of diffs so please take this as a comment only-

Having done several B/W sets in Falcons,Commodores and Nissans-
If the mark is dead centre on the tooth, some ratios, particularly 3.08 for some reason will have a slight period whine around 85-90 Kph and all will have a quiet sing on medium throttle around 115Kph just where you don't want it.
Around a 3thou side bearing shim change to reduce backlash and get a mark towards the toe a tiddle similar to your earlier pic is enough to shut them right up but that 3.08 can still have the 85-90 little noise but at a lower speed and won't go away whatever you do- you can put in another c/w and pinion the exact same and get exactly the same mark and it will be quiet as----very frustrating
Going the other way with too much toe contact similar to Mark's pic doesn't really make a whine noise but some will make an audible growling noise like teeth contacting each other- on some diffs but at a lower 45-50kph speed
Not saying change yours Mark, if it's hasn't been making a noise
If it ain't broke don't fix it

I like doing 9" Ford diffs, the pinion is in a bolt in carrier, so it's a matter of setting the pinion preload ,,then ajusting pinion height is carried out externally by shims between the main and pinion housings
Crownwheel side bearing adjustment is by threaded retainers ,no need to pull anything apart, just move the pinion in and out with the bolts and select a shim for the gap and screw the crownwheel anywhere----they should all be like it
Cheers
William Revit

I guess i will have to fit it and see, backlash is correct to the engraving on back of crown wheel which is what i was checking before i fitted it, so we will see if it makes any noise .The person who sold it to me said he had had no problems so fingers crossed.
mark heyworth

Worthwhile checking the sun gear washers, the tufnol ones used on later diffs have a tendency to wear away and disappear.
David Billington

Thanks Willy, That is great feedback. I keep learning stuff on this forum. Trouble is I am getting old and forgetting half of it!
Bob
R L Schapel

Bob
I used to think (honest,I did when I had long hair)that my memory cells were in my hair, but when I lost my hair I was surprised to find I was still going ok.
Now ,I think memory cells are right on the top of our heads and as we get older and shorter it's difficult to recall things off the top of your head as it's lower and not in the correct place for memory

lol
willy
William Revit

R L Schapel
Please email about spider gears.
John
J Bubela

Have emailed John. If anyone wants spider gears for Spridgets let me know. I have a box full to select from. The sets have the small dished thrust washers but not the large flat thrust washers because the latter are needed in the MG TC conversions I have done. I plan to put them on Ebay because Moss etc. list them as "not available". I will keep a few "selected" sets for friends and associates on this forum.

Bob Schapel, South Australia.
R L Schapel

This thread was discussed between 15/08/2018 and 31/08/2018

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