Puttin a SM 420 transmission behind a 1969 gmc 292 2wd what driveshaft to use??

[restorationboy14]

Well the title says it all guys. Im was told a c-30 driveshaft would work but id like an exact fit. Its a fleetside bed too. So what other transmissions would work?? Thanks a ton guys!!!!!
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[bigyellow]

if i were you i'd find a good overdrive transmission to swap in, and have the driveshaft you have cut and balanced at a driveline shop. i swapped a t5 to my 250 and not a single regret getting rid of the sm420. the only thing about the sm420 is you can't break it and if you haul heavy the granny is nice.

[restorationboy14]

This truck is a heavy hauler. Thats why i need to keep the 420. So what would ypu reccomend for a driveshaft?? I would like to avoid custom making one but if i have to its a last resort. Thanks!!!
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[bigyellow]

i'd go digging through junknyards honestly. measure from the center of the length you'll need and start crawling under some truck to measure. i'd start with 70s and 80s chevy trucks because i believe its the same u-joint.

['68OrangeSunshine]

Are you sure you have a 420? Their last year was '67.The SM465 came into production in 1968. They are very similar -- both bombproof, but the 420 is some few inches shorter. Both have the Granny gear. On the 420, Reverse is toward the driver, on the 465 Reverse is on the other side toward the passenger. Gearing on the 420 is slightly lower in 1-2-and 3rd gears. 4th is 1:1.
A PO may have put in an SM 420, but it's not usual for a '69.
A straight drive shaft should work in a C/10 application. What length is your truck?

[restorationboy14]

Im replacing my Saginaw 3 speed with the 420 my truck is a fleet side so it has the 8 ft box on. So will a driveshaft from a C-30 work??

[bigyellow]

if yours is a longbed you have the two piece drive shaft. put the transmission in and bolt the rear half of the driveshaft you have in and measure center of u joint on the trans to the center of the u joint on the rear driveshaft. any difference should probably be made up with the slip yoke.

oh and sm420 granny gear is lower than the sm465, also the sm420 doesn't have a synchroed first

['68OrangeSunshine]

The 465 doesn't have Synchro in 1st either.

[restorationboy14]

Originally Posted by bigyellow

if yours is a longbed you have the two piece drive shaft. put the transmission in and bolt the rear half of the driveshaft you have in and measure center of u joint on the trans to the center of the u joint on the rear driveshaft. any difference should probably be made up with the slip yoke.

oh and sm420 granny gear is lower than the sm465, also the sm420 doesn't have a synchroed first

Thanks ! And yup the granny gear ratio is 7.05 now that low haha

[restorationboy14]

Originally Posted by '68OrangeSunshine

The 465 doesn't have Synchro in 1st either.

The reason why I picked the 420 is because it will bolt up to my 292 bellhousing. Thanks for the reply!!

[restorationboy14]

Ok so the c-30 drive shaft won't work? If not where Springer abstinence drive shaft and how much do the run?? Thanks! - Sam

[restorationboy14]

Originally Posted by restorationboy14

Ok so the c-30 drive shaft won't work? If not where Springer abstinence drive shaft and how much do the run?? Thanks! - Sam

Stupid auto correct lol cant stand that phone sometimes. Meant to say if the c-30 drive shaft wont work where do I get one that does?

['68OrangeSunshine]

Originally Posted by restorationboy14

The reason why I picked the 420 is because it will bolt up to my 292 bellhousing. Thanks for the reply!!

Good reason. And by all means play with what you got. My '68 292 came with the larger diameter bellhousing for the 465. [Same as on SBCs.]
Anyway good luck.

[Shyguy]

I hope you got the bell housing with the SM-420 since the hole in the bell housing for a SM-420 is the correct size to re-enforce the front of the SM-420 tranny.

I think the bell housing hole for a SM-420 is smaller than a bell housing for later manual transmissions.

['68OrangeSunshine]

Originally Posted by Shyguy

I hope you got the bell housing with the SM-420 since the hole in the bell housing for a SM-420 is the correct size to re-enforce the front of the SM-420 tranny.

I think the bell housing hole for a SM-420 is smaller than a bell housing for later manual transmissions.

You are correct, Sir. The earlier bellhousing with cast-in markings: [GM11] p/n:3815891 has an interior diameter of 4-9/16" and works with the 3-speed Saginaw and the Muncie SM420.
The later bellhousing: [GM5] p/n:3925505, has an I.D. of 5-1/8" and works with the SM465, and perhaps other later transmissions.

1968, I think, is possibly the start of the timeline for 3925505, with 3815891 being used thru the '67 model year run. Exceptions being Canada and GMC where they tended to use up the older parts until changing out.

[restorationboy14]

I looked at the bellhousing and it checked out to fit the SM420. Ok so what do I need to use for a driveshaft???? Im on a deadline to get a drive shaft. What will work??? Thanks!!!