I'm just wondering what these things mean. I posted here because i hear it more usually when it comes to classics.

Are they by any chance the same thing?

I thought a 12-bolt rear meant a rear end with 12 bolts, where a 'rear end' means the rear differential. I guess the rear differential cover has 12 bolts? But why would it be such a point of advertising if it was just a bolt count on the rear end cover...?


12-bolt main i have no idea. Is it an engine thing?



....anyone know what the 65 bonnie had ?

The 421 had a 4-bolt main block. That the seems to be what most cars have(including our 3.8 engines)

The 12-bolt Rear Diffs had better guts in them to be able to handle to added power from the 421 engines. Some people would replace a 6 cylinder engine with a big block and blow apart their standard 6 bolt rear ends

12 bolts refers to the number of bolts that hold the ring gear to the carrier.
It is inside the differential, so you can't really see it with the cover on.
The cover is distinctive from a 10 Bolt cover. I don't have pictures handy to show you the difference.

Thanks guys for the good responses. I now know what a 12 bolt rear end means thanks to you two. However, when you say a 4-bolt main block, where are these 4 bolts? I.e. what do they hold together?

normally there is a code on the rear diff thats states what it is
well atleast DANA axels are like that

A four bolt main.. is the things that go over the crankshaft.. you know what I mean? Well, that means there are four bolts holding that block on. There are two kinds. Two Bolt Main, and Four Bolt main. Four being the better.


-justin

what is meant by 2-bolt main and 4-bolt main, are how many bolts are holding on the crankshaft bearings. most are 2 bolt, ie each cap holding the bearings on the crankshaft gets 2 bolts.

http://www.buickstreet.com/images/455stage3-4-tn.jpg
notice the 4 bolts holding each main bearing cap? most are 2-bolt, except the high performance engines like the 325a trophy 389, and super-duty 421'* and 455'* i belive ever pontiac engine is drilled to accept 4-bolt mains, its just a matter of finding the caps.

the GM 12-bolt differential does indeed have a 12-bolt cover on it, and it'* and easy way to tell the difference.

But, be cautious, some mid-60s Oldsmobiles had a 12-bolt diff cover and 10-bolt carriers.

Were i work, we sell Differentials all day long, customers call non stop about them, and the inner workings of them
Bolts refer to how many bolts are on the rear cover, not how many bolts are holding the crown gear to the carrier...
We also go by crown gear diameter..
aka
GM 10bolt 8.5" rear diff...most common GM diff pretty much..