BUSY BOLTERS Are you one?  The Shop Area
continues to pull in the most views on the Stovebolt. In August alone there were over 22,000 views in those 13 forums.
| | Click on image for the lowdown. 
====
| | | Forums66 Topics126,777 Posts1,039,265 Members48,100 | | Most Online2,175
Jul 21st, 2025 | | | | Joined: Jul 2019 Posts: 2 Moderated | | Moderated Joined: Jul 2019 Posts: 2 | I've been searching the forums/web looking for concrete details on swapping out my crash box trans in my bone stock 41 GMC 3/4 ton for a newer synchronized gearbox. Probably an SM420, but I'm looking for whatever is the most straight forward swap. I can't seem to find any definitive answers on the following:
Will an SM420 bolt to my existing GMC bellhousing w/o modification? What part number should I look for?
Do I need to change out the clutch?
Do I need to modify the cab floor?
Do I need to move any crossmembers?
Do I need to change/shorten the driveshaft?
Do I need to change the ring and pinion? (no idea what ratio I currently have)
I don't care about highway speeds (it tops out at 45mph and that's PLENTY fast), I just want to increase drivability around town. Has anyone done this exact swap? Is it the same as a chev 216/235 to SM420 swap except no torque tube? I'm not finding the answers I need. Hoping someone can help me out with clear details and links to information.
Thanks!
1941 GMC 3/4 Ton
| | | | | Joined: Apr 2005 Posts: 242 'Bolter | | 'Bolter Joined: Apr 2005 Posts: 242 | Call me I may be able to help 1-262-392-2989 | | | | | Joined: Jun 2011 Posts: 5,096 Crusing in the Passing Lane | | Crusing in the Passing Lane Joined: Jun 2011 Posts: 5,096 | Seems to me the early boxes do not have the bottom right bolt offset like on 420’s, 465’s, etc. you could inspect and verify. If bolts are not alike, one can drill and tap an appropriate hole in the bell housing that matches the trans, or get a newer bell housing with the more recent trans. bolt pattern. 420’s have 5.875” bearing retainer diameters, what is your diameter?
While in there, clutches are inexpensive compared to the labor, so it would be a good time to replace the pilot bearing, throw out bearing along with the clutch.
Do you have a park brake on the tail shaft of your present trans? 420’s and 465’s come with and without, get the matching one to what you have.
Or: learn to drive a non-synchro box, your grandfather did!
Ed
'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.
| | | | | Joined: Feb 2020 Posts: 45 'Bolter | | 'Bolter Joined: Feb 2020 Posts: 45 | Your present bell housing will not work with the SM420. The bell housing that works for the 420 Does not have the 3 bolt mounting area for the brake and clutch pedals. Hooking up to the torque tube is another problem, unless you replace the rear end and driveshaft. I am installing a T-5 transmission (using an adapter to my original bell) and rear end this winter. no more crash box for me.
Good luck, Joe
just retired and finally making headway on my project truck. 1940 GMC AC152.
| | | | | Joined: Nov 2011 Posts: 1,608 'Bolter | | 'Bolter Joined: Nov 2011 Posts: 1,608 | If you have an other wise bone stock truck, I would suggest that you learn how to double clutch...its really not that difficult with a bit of practice. | | |
|
|
