Machining a 3.8 Bellhousing to Accept a 4.2 Gearbox - Dimension Question

Going through the archives, I see several owners (@64etype, @Robert_and_Darlene_S, @abowie, @MichaelPMoore, perhaps others) have fitted a 4.2 gearbox to a 3.8 bellhousing/flywheel combo. I need to do the same thing.

Is there a recommended size for the bore-out of the 3.8 bellhousing? (I don’t have a 4.2 bellhousing for comparison.) Or should I simply measure the front bearing dimensions of my gearbox and cut to fit? How snug does the fit need to be?



I did it to my car - I measured the width and depth of the bearing, added .002 to the dimensions, and cut the bell housing using a vertical Bridgeport. You must make sure, before cutting the bell housing that the tool you are using to cut it is absolutely centered. Also you should upgrade the clutch to a modern, lighter weight diaphragm clutch setup.

Also, as you probably know, you should replace the front seal.

I have a couple of spare 4.2 bell housings in the shed and will take some measurements tomorrow. Legend has it that some late 3.8 flywheels had both clutch bolt patterns. My '64 had to be drilled for the 9" B&B diaphragm… Dimensions of the transmission input hole is fairly critical as the front bearing race is “captured” by the bell housing. I’m a fan of Terry’s Jaguar for seals, in particular.

The easiest way is to use the 4.2 flywheel, clutch and bell housing. That all bolts directly to the 3.8 block which is how my car was set up. 3.8 motor and 4.2 from the back of the crank to the driveshaft. This means you use a 4.2 starter too because of the flywheel change.

Lois - Thank you for describing how you determined how to do the job.

Eric - Appreciate your offer to look at the dimensions on a 4.2 bell. My 3.8 bell has only a single set of clutch bolt holes.

John - I agree, it would be tidier to go “all” 4.2, but here I am with 3.8 components and a 4.2 gearbox that the previous owner installed incorrectly. The shortest/least expensive path from “a” to “b” seems to be machining my existing bellhousing vs. replacing the bellhousing and flywheel with 4.2 versions.


How did he install it incorrectly??


I tried to get the dimensions, but couldn’t get consistent readings in the thousandths range with my cheap and rudimentary measuring tools. Apologies.

Eric…want to sell a 4.2 bell housing?

ANY machine shop can easily re-machine the bell housing. Just take them the bell housing and the bearing. It takes well under an hour to do, even on a manual machine.

I’ve been following the thread since I have a 3.8 car that I wanted to get a 5-speed for, but am now considering using a 4.2 4-speed and bell housing that I already have from my 4.2 car that now has a 5-speed. I thought there is a difference in flywheel weights between the 4.2 and 3.8. How would a heavier 4.2 flywheel affect the driving experience on a 3.8 car? While it’s been many years since I’ve driven the 3.8 car, it seems like it reved nicer than the 4.2.

The heavier flywheel will make it a bit more lethargic: any good machine shop can lighten the flywheel. It would need to be rebalanced.