I did a lube change on my rear diff at the weekend (Dana 35) and I'm not too happy about the condition of a couple of the bolts and their holes.
Am I right in thinking that the XJ is a mixture of imperial and metric but I'll need imperial taps to tidy the diff cover threads up?
Cheers.

 

Probably the best thing you could do is get the best looking bolt and match it's threads to a tap. The bolts and holes just might need cleaned out. I know a few of mine were stuffed with dirt and crud! Nothing a can of brake cleaner and a pick didn't fix!

 

Correct.

Don't know what year your XJ is, but '97-'99 XJ Dana 35 cover fasteners are .312-18 x .56". I didn't look in any other years catalogs.

 

And for the laymen in the group....

5/16-18 x 5/8"
pretty standard cover bolt size.

 

Actually, 5/8" is too long (.6250). The actual catalog spec is .562" long.

9/16" is .5625.

 

Fair enough, but unless you buy a more pricey and oddball OEM flange head bolt ... A washer should be used. So 5/8" minus the washer will equal approx 9/16"... Also works for the front different and 8.25 and D44 as well. I just wanted to add the standard sizing as most folks don't do decimals well.
I could just imagine some poor person at the hardware store trying to locate a .312" (.3125") fastener ... Although I may just try that one day for the giggle factor.
In no way am I meaning to argue here, just add clarity is all.

 

9/16" length bolts are made from Unobtainium, and like you say, if you don't get them from the dealer, you ain't getting them.

My preference would be to buy, and use, 5/6-18 x 1/2" flange head bolts. The .062" diff between the OEM .562 and .500 doesn't amount to a pinch of $hit, especially if using RTV gasket maker as the seal. .062 equates to a little over one full turn on a 18 TPI fastener.

I'm lucky enough to have two hardware stores near me that carry Hillman hardware, which includes the flange head bolts. Fastenal also carries flange head bolts.

Hillman flange head bolts are SAE Grade 8.

I'm not a big fan of using standard head bolts with washers mixed in with factory flange head bolts.