Sierra diff mounting holes, advice required
b3ngy - 1/8/12 at 06:19 PM
I was re-fitting my prop shaft a couple of days ago and noticed that the diff was moving slightly in the mounts.
After abandoning re-fitting the propshaft and having a closer look at the diff, I realised it was pivoting about the the upper bolt (its mounted using
the 2 long m12 bolts) meaning there is play about the lower bolt. The play was mostly coming from the diff rather than the whole bolt moving....if
that makes sense...it looked like the mounting holes in the diff were oversize.
I noticed that there was a gap between the lower chassis mount and the diff and the bolt wasn't particularly tight.
I decided to remove the diff for further inspection as I knew something was worn. After taking some measurements I discovered that :-
- the bolts were approx 11.8mm dia. with no wear marks or steps
- the holes in the chassis structure measured anywhere between 12.3 - 12.5mm
- and the holes in the diff where anywhere between 12.84 - 12.97mm 
Has anyone else had an issue like this?? It looks like the mounting holes in the diff used to be threaded?? I'm not sure as I didn't build
the car originally and remove bits from the doner.
Obviously I want to try and correct this but what would be the best way???
I think opening the holes up to m14 would be the cheapest and easiest way of doing it but I need to use cap head screw (allen key) on the top mounts
and I'm not sure if that size head will fit in the diff casting??
Anyone got any other ideas/advice???
Ben
rusty nuts - 1/8/12 at 06:23 PM
Get all the b?olt holes helicoiled back to 12mm
AdrianH - 1/8/12 at 06:29 PM
What size diff is it, if 7 inch and you use two long M12's to mount the diff then there is no point in helicoiling the diff.
What you could do is ream out the holes in the diff mounting points and then bush them.
You just need to keep the through bolt centred in the hole.
If you fit the odd washer to the sides between the diff cradle and diff then you could have a few top hat bushes made or otherwise just bits of
tube.
Not sure if that makes sense or not.
Adrian
coozer - 1/8/12 at 07:24 PM
Get some high tensile threaded bar to go right through and tighten the bugger right up!
bonzoronnie - 1/8/12 at 07:31 PM
Replace the M12 bolts with suitable length 1/2" bolts ?
May need to ream the holes out to suit.
The long mounting points of the Sierra diff were not originally threaded , the 2 on the nose were, as were the 2 on the rear cover.
[Edited on 1/8/12 by bonzoronnie]
b3ngy - 1/8/12 at 08:10 PM
Thanks for the replies guys,
I did think about reaming the holes and bushing them (probably the best idea) but thats going to be a lengthy process compared to others.
not sure which route to take at the moment.....feel like my rebuild is going backwards
AdrianH - 1/8/12 at 08:16 PM
You have to consider the sierra diff casing is just aluminium, so not hard to wear, drill or ream out.
Leave the chassis structure as it is.
If M12 are 8.8 or better then fine.
If there is a gap between chassis and dif, make up with large washers, you need to have some clamping force.
Which ever way you go make sure it is tight afterwards
Adrian
[Edited on 1-8-12 by AdrianH]
b3ngy - 1/8/12 at 08:34 PM
thanks adrian
I like to do things properly and to me that would be to ream everything then fit bushes (thats how we would do it in the aviation world). This method
will take some more time and effort but will be worth doing.
Ben
snakebelly - 1/8/12 at 08:48 PM
Just done this as we had the same issue last year at the ring on our Avon ( now becoming a gta) bought some high tensile 1/2" from metal
supermarket and put a 1/2" unc thread at both ends, they needed a little persuasion with a nylon mallet but with them all the way Through the
diff was solid as a rock. Much happier now
indykid - 1/8/12 at 10:37 PM
An M12 clearance hole is 13mm.....
The bolt shouldn't be taking shear loads if they're torqued up properly, so reaming them to fit will only locate the diff better on
assembly. If you use high tensile rod/long bolts and torque it up properly, it won't move. Standard threaded bar is nothing like high tensile.
b3ngy - 2/8/12 at 06:47 AM
quote:
An M12 clearance hole is 13mm.....
Really???? that seems a bit on the large side, I would say clearance size is 12.2mm max.
Locating the diff better on assembly is what i'm trying to achieve.
daviep - 2/8/12 at 09:37 AM
Indeed a 12mm clearance hole should be 12.2mm
As per previous posters comments I would agree that there is very little to be gained from trying to re-size the holes / bolts in my opinion.
I would get some decent material of 8.8 or above and tighten the mounts properly.
Cheers
Davie
b3ngy - 2/8/12 at 12:48 PM
quote:
I would get some decent material of 8.8 or above and tighten the mounts properly.
I understand what your saying, the bolts are of 8.8 grade and I understand that the mounts need to be tightened correctly, but you can't ignore
the fact that all the holes are different sizes. Putting the original bolts in and just doing everything up super tight is gash work imho.
Im going to open the holes up and put some steel bushes in keeping everything m12 size.
thanks for all the advice.
Ben