Best Rear End for 409
- Thread starter RAG 65
- Start date
Depends on your intentions with your car, I have a factory 65 12 bolt posi with 3.07 gears in my 65 409 rag with a m20 muncie works great but I don't race it around and only thing done to my rear has been a oil change.
12 bolt would be the no brainer.
Yes.
This is actually word-for-word what my comment was gonna be!
Not all 65's came with 12 bolts as some of the low horse cars got 3 link 10 bolts and I have owned a handful of them with 10 bolts and about 50 of the 65's through the years as I parted alot of them out when I was younger and foolish
12 bolt will rob less HP 
Greg Reimer
Well Known Member
I had a '65 Impala convertible with a 327/250 horse and a 3 speed on the column, that car had a 12 bolt. I had it almost 20 years and never had an ounce of guff from that rear end.I put a 340 horse 409 and a 4 speed in it for a lot of that time.
Last edited:
Have a friend with an off topic GM product that finally broke his 8.5 10-bolt and replaced it with a bolt-in 9” and the car lost .20 at the dragstrip even with ratios being exactly the same. Today’s Fun Fact: that 9” low pinion centerline (in relation to the ring gear) is a horsepower sponge and was developed for FoMoCo products to have an unobtrusive driveshaft tunnel in the passenger compartment. The lower the pinion CL, the more tooth contact & obvious strength but there’s gonna be a HP price. In a perfect world, the early Olds/Pontiac should have been developed more by the aftermarket and that probably would have happened if it stayed in production cars longer.
If Chevy had used them in their big cars, they would have been the rear that the aftermarket flocked to because the numbers of them out there would equaled or exceeded the numbers of 9"s, I know they were more expensive but economy of scale would have offset some of that.
425/409ER
Well Known Member
12 bolt is a no brainer in a 65-70 Full size car. I have had several with 12 bolts in them and never broke any of them. Sure you have to replace a bearing every now and then or rebuild the posi but that's just maintenance. I have a 63 full size car and in another month or so it's getting a 12 bolt as the factory rear end in those car will break just looking at it, and ours is a factory leaker. I'm tired of trying to replace seals on it ever couple of years. Heck the good think is 12 bolts for the big cars are fairly cheap compared to a Chevelle/Camaro one.
So all of this info leads me to a question or two. How much horsepower (approximately) does it take to turn a 12-bolt, or a 9-inch or any other rear end? My third members, once broken in, seem to turn relatively freely. I was surprised to read Mark's post that a car lost .2 just by switching to a Ford gear, that would seem like a fair amount of HP was involved. I have heard this before, but I have never seen it quantified and I have to idea what it takes to turn a rear gear.