Welcome to our resource for MG Car Information.
MG Midget and Sprite Technical - proper bearing preload?
|wire wheel hubs, replacing them because of worn splines (150,000 miles), want to confirm what the proper bearing preload should be.|
I measured the new hubs, and compared them with the original hubs. The original ones were: one was 1.496 between bearings, the other one 1.504. The new ones are both 1.507.
It seems that there should be some preload on the hub bearings.
The spacer between the bearings is 1.500, so anything over 1.5 is going to be a smidgeon loose, and anything less will be preload.
I studied about ball bearings in the machinery's handbook, and searched on line. Also, of course, studied the archives here and on other web sites, but no one talked about how many 0.001" the preload should be on midget front hub ball bearings. Or, what the min/max should be.
I don't want to assume that the new hubs are "right". I want to know what they should be, and if needed, I will have them machined to be spot on, if anyone knows what that should be.
Does anyone out there in midget land know what the proper wheel hub bearing preload dimension should be, and what the limit is?
By the way, these are the original equipment, BRG bearings, 39LJT25 inner, 3MJT17 outer. Face machined, and with the large, 3mm radius.
Don't really know on a Midget but on the MGB the spec is .003' =/- .001 endfloat. This really surprised me when I looked it up as I have always set B's up at .001"/.002"preload and never had a problem. I think I'll stick to a bit of preload. In my opinion the main thing is to get the nut nice and tight to help stiffen un the stubaxle. Cheers Willy
thanks for that info. So to set the preload you measure the distance between where the two outer races are going to be pressed into the hub and then add shims as necessary to make that 0.003"+/-.001 greater than 1.5" (the length of the spacer that holds the inner races apart). (ref attachment)
All of the sources of bearing design that I referenced on the internet and in books this week commented on the importance of getting this right with angular contact ball bearings, but none of them said how much for a Midget!
PS: this image is from Tom's article on front wheel bearings!
(thanks Tom, I guess this makes one more point to be added to the story!)
I don't know anything about all of this, but the implications are scary. It's been years since I rebuilt the front suspension on my '74 midget and I can just vaguely recall having some kind of problem with the bearings. I may be riding on borrowed time.
Do the current popular suppliers carry the correct bearings now? If not, what are the part nos. and/or source for the correct bearings?
Is there a tech article anywhere for a step by step walk thru diagnosis of what I currently have and how to cure it if wrong?
I'm more than a little nervous!
|please read Tom's link that I referenced above. His article is the result of a whole lot of effort by several long time mini mechanics (I only added a bit at the end).|
The proper bearing part numbers are:
BRG bearings, 39LJT25 inner, 3MJT17 outer. Face machined, and with the large, 3mm radius.
The are available from:
They only make them in batches of 10, and are rather expensive. But, I would rather pay more and know I got the right thing, than take some discount price from Moss and "guess".
Note that BRG was bought by NSK on the early '90s, but apparently NSK does not make this exact bearing any more. Moss, et. al, sell an "equivalent" bearing, but it isn't quite.
By the way, there are lots of folks out there driving around with mystery bearings, and they don't seem to be having too many failures. In fact, one fella wrote that he'd been driving on loose bearings for years and never had any real problem with it, except for having to convince the MOT guy each year.
Anyway, we can each make up our own minds, I'll go with the best I can get because I only want to do it once on my own car.
I realized that I mis-read your post.
You said that MGB hubs use a clearance of 0.003, but you've been using an interference of 0.001 ~ 0.002.
Now I do have a decision to make. Can you give me a link to the MGB info you referenced? I sure wish I could find something like that in a Midget reference book somewhere.
|Fletcher R Millmore|
It is out of an old W/shop book I have------
Scientific Publications No70 - MG
Page 2 of the front suspension section.
Unfortanatley there is no mention of Midgets in this book Cheers Willy
In my opinion having worked in the industry, I would say that the ideal would be the hub dimension and the spacer length should be equal. However machining tolerances will always give a little leeway. I would say the spacer should be 0 to -.001" but +.001" would probably be OK. Whilst you dont want play slack is better than tight as any additional load will shorten bearing life. .001" of slack should not show any play as this will be taken up by the grease.
By the way the correct bearing is 34LJT25 not 39 although I think Guy on here had some 39s that worked OK.
|I wonder what the difference was between a "34" and a "39"?|
Maybe the cage material was changed from brass to resin as a cost savings, or something like that? Or, was there a dimensional difference...
All of this would be so much easier if we could just somehow get ahold of the drawings from BRG!
|Sorry Tom do not understand what drawings you refer to? and BRG being the original supplier/manufacturer, which they were not?|
If you want a copy of the original drawings I have them and can send you a copy unless Tom can supply you with a copy of the ones I sent to him.
|Bob Turbo Midget England|
I do not remember getting a copy of the original drawings. If I did they have been misplaced.
Would very much like a set. tom (at) smithtr6.com
|ooh! Me too, please!|
please send the drawings to:
normkerr123 (at) comcast (dot) net
|Sorry Tom but I thought what had happened when this subject first began about the bearing radius being a problem that David had asked me if he could forward these drawing to you. Naturally that was not a problem so I had assumed you had received them|
Anyway they should now be with you and it can be easily seen that the inner bearing 34LJT25 has 1mm radius' on most edges except the inner one of the stub axle face which is marked as 2mm minimum :)
|Robert (Bob) Midget Turbo|
|I may have missed them too since I had a lot of e-mails during that time relating to the subject.|
11/MJT17 and 39/LJT25 are the bearings sold in Unipart kit GHK1924 (with oil seal & split pin.
The kit is supplied by MGOC for Midget. Is it safe to assume that MGOC use this kit in their workshops at Swaveney?
This thread was discussed between 12/06/2010 and 24/06/2010
This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.