Broken crown gear bolts...

[74Twincam]

Hi all-
I was working to take my rear differential assembly apart, to fit new seals, TTR shafts, and generally clean it up, working to be, so I thought, ahead of any failures. Well, I didn't have broken output shafts, but after getting them out, and the casing apart, I found two of the bolts that fasten the crown wheel were broken, and the rest were only finger-tight!
Is this something that just "happens"? I would suspect the crown and pinion was changed in the past and never tightened properly, but is it possible for them to loosen up with use? I'm guessing I will need to really inspect (replace?) the gears closely for wear, given it was loose inside.
I was so frustrated after getting the damn axles out and everything apart to find those bolts, it was time to walk away and pour a scotch.

[benymazz]

I've seen a couple of posts about this I think. The one that stands out to me the most is what happened to Rohan (link to the thread below) which was of the same nature as yours but much more... catastrophic.

viewtopic.php?f=37&t=34302&start=

-Ben

[2cams70]

Is this something that just "happens"?

Yep. Quite common and can just happen in general use. Good insurance to always use red Loctite on these bolts and ensure they are correctly torqued. Many OEMS now specify Loctite for crownwheel bolts from the factory. Loctite was not generally available back when our cars were new.

[rgh0]

I've seen a couple of posts about this I think. The one that stands out to me the most is what happened to Rohan (link to the thread below) which was of the same nature as yours but much more... catastrophic.

viewtopic.php?f=37&t=34302&start=

-Ben

down at Phillip Island now between races rereading the write up I did on the diff failure here 5 years ago brought back memories. The diff is still performing prerfectly

vheers
Rohan

[74Twincam]

Thanks for the replies, and sincere thanks for the link- lots of details to pour through and decide if I’m going to rebuild myself or consult a specialist.
I seem to have been very lucky, as I drove the car briefly in this condition.

Thank you!!

[2cams70]

Thanks for the replies, and sincere thanks for the link- lots of details to pour through and decide if I’m going to rebuild myself or consult a specialist.

If the gear teeth are in good condition, the pinion is still in place and hasn't been removed and the diff was otherwise quiet prior to the incident you could just reassemble with new bolts. You'll need a large micrometer to measure cap spread and you'll also need some bearing blue to check the contact pattern after reassembly. You'll also need a tool to tighten the end caps with. I'm not sure where you could source this tool from nowadays however. I sourced mine from MKF engineering in the UK prior to the owner of the business becoming infamous! You could possibly improvise with something else

[elanner]

You'll need a large micrometer to measure cap spread and you'll also need some bearing blue to check the contact pattern after reassembly. You'll also need a tool to tighten the end caps with.

When I was last messing around with my diff (a long, boring story) I decided to check the cap spread and CWP contact pattern.

To tighten the end caps a simple bar with a couple of bolts works just fine. I've never seen a cap spread tool and the pictures don't fully show it. So I made the simplest possible platform for a dial test indicator magnetic base & cantilever arm (a rectangular piece of steel with a bend and hole in it). This worked very well.

I found the Rear Axle section of the 105E manual to be more comprehensive than the Elan manual.

Diff-Setup-105E-compressed.pdf

(370.68 KiB) Downloaded 434 times

Nick

[74Twincam]

Great references and pics-

I’m going to see if a local shop can help with the set up, but if not, I’ll carefully proceed on my own..!

I appreciate the help and resources,
Bill

[Davidb]

the differential bolts are identical to ARP flywheel bolts for the BMC 'A' series engine-i got mine very cheaply. Might need shortening a few thou.

[Davidb]

https://www.ebay.com/itm/ARP-FLYWHEEL-B ... SwVJFc5WcK

Despite the listing saying "1600cc A series" the part number is correct.