What size rearend?
 

I have an 1/2 ton 81 LWB with a 305cid and TH350C. What size rearend do I have? I am looking to buy some 3:73's for pulling and need to now the housing size.

Re: What size rearend?
 

My 80 C10 305 + TH350C has 2.56 but who knows.

There's another post referencing gear sizes, might wanna check it out, too.

Re: What size rearend?
 

Easiest way to tell is to count the bolts at the rear cover. This will tell you what bolt size you have. It's probably a 10 bolt but it may be a 12 bolt. You also need to pop the cover off the differential to find out what carrier size you have unless you're wanting to install a new one. Count the teeth on the ring gear and the pinion. Divide smaller number into larger number and this will give you a gear ratio on what you have in there now. You will need this to find out what carrier you have. "Usually" the lower numerical numbers like 2.56, 3.08, 3.42 have a Series 3 carrier and a larger number gear ratio have a series 4 carrier. You can resue the carrier as long as you know what size it is and get the correct gear ratio for the carrier. If you want a 3.73 and you have a series 3 carrier you may have to get a thicker set from the manufacturer but need to contact them to be safe.

Re: What size rearend?
 

:agree: 79Big10 hit the nail on the head. I would say to probably have a 3.08. Not really sure what year they switched to the 10 bolt. But an 81 is around that time. Take a look on the rear end housing for a stamped number or a tag if you don't want to yank the cover off.

Re: What size rearend?
 

Brandon is correct. I think a 3 series carrier will go up to 3.73 on a 10 bolt, at least it did on my 85.

Re: What size rearend?
 

I am looking for gears on Summit. It is a ten bolt. Would it be an 8.5?

Re: What size rearend?
 

Yes, it's a 8.5.

Re: What size rearend?
 

Look at Rand'ys Ring and Pinion website for more info.

Re: What size rearend?
 

Thanks everyone