transmission swaps

[Lionelgee]

Originally Posted by 20387774

Have any of you CMP owners tried to swap your transmissions for one with synchromesh, like the GM SM420 transmission? How easy was your conversion? Were you happy with it?

John

G'day John,

I found this website about the SM420: http://www.novak-adapt.com/knowledge/sm420.htm

Also suggest you go to stovebolt.com which is one of the best Chevrolet restoration websites and go to their Tech Tips.

Kind Regards
Lionel

[Bob Carriere]

HI John

The tranny swap can be done.... the big problem is that the 420 is much longer and you will have to spend $$$ to get a custom made very short drive shaft to link up to the Transfer case..... or move the engine forward or move the T-case and cross member backward. The original CMP 4 speed is incredibly short by todays standards.

Some additional problems maybe encountered when you install the emergency brake lever... it will probably bolt right on but be in a different location that will not match the floor opening.

Which CMP model are you working on..... C15a would be easier than the C60 because of the different system for engaging the front axle which bolts on the tranny for the C60....

Bottom line anything can be made to fit if greased with enough $$$$$

I had original plans to install the 4 speed that came with my 261...fully synchro.... would have been a dream...... gave up and went back to the CMP tranny to avoid all the headaches...

Boob

[cletrac (RIP)]

I don't see why anybody would want to change them over. There's something neat about driving one of those old gear crunchers with their telltale noise and all.

[Yeo.NT]

I'd say if you are going to the trouble of changing the transmission go with a possibly easy route. Change everything (ie Engine, Transmission and Transfer case) I know of a 30cwt that runs a chevy 350 with a TH400.

[Matt Baker]

Gday,

I have a CCKW five speed gearbox fitted to my C15 radio van and it works great + it gives me a higher cruising speed which i haven't worked out yet due to no speedo drive, mind you it keeps up with jeeps ok.

[Phil Waterman]

This topic comes up from time to time, but one thing I always wonder about is the matching the ratios of the transmission and axles to the peak torque of the engine.

While lowering the engine RPM in 4th gear over the road for the original 216 engine is kinder to the engine, without and increase in power don't really think any of the bigger CMPs are going to travel faster.

Has anybody matched up the ratios of the different synchronize candidates?

I saved the transmissions and bell housings from the 235 and 261 cubic inch Chevy engines I have. Both are synchronized transmissions 4 speeds.

Briefly considered going the full engine and transmission transplants into the C60L and C60S but the list of modifications concerned me as some of them require modifications to the trucks which I didn't want to make.

As mentioned the bell housings for the synchronized is different, engine mounts different, clutch fork location different, and of course the short shaft would get even shorter.

Matt with the CCKW in your C15 Radio Van which engine are you running? I didn't think the CCKW transmissions were synchronize so the advantage in that swap is the 5th gear?

Cheers Phil

[cletrac (RIP)]

Any of those older trannys will be 1 to 1 ratio in high gear. You have to get well into the 1970s to get a five over. Ratios in the lower gears wouldn't make much difference with everything that slow speed. If you use a non Canadian military bell housing the clutch fork sticks out on the wrong side so clutch linkage would be a nightmare. Those old straight cut trannys will last forever so I can't see any reason to make the swap.

[maple_leaf_eh]

Originally Posted by Matt Baker

Gday,

I have a CCKW five speed gearbox fitted to my C15 radio van and it works great + it gives me a higher cruising speed which i haven't worked out yet due to no speedo drive, mind you it keeps up with jeeps ok.

My sweetie gave me a dashboard mounted GPS reciever for Christmas. It gives a speed across the ground, which shows that my car speedometer is underreporting by a few km at 110kph.

[Lynn Eades]

I drive two Toyotas (not at the same time) and a Holden. All read faster than the gps.