The Stovebolt Site Forums The Shop Area Driveline tranny swap for SM420

Is it feasible to fit a 1954 SM420 to a 1940 bell housing....... are there any adapter plates made to fit the different bolt pattern....... eyeballing it seems that only one bolt on the RH side is further out.
It is essential we keep the 1940 bell housing as it has the right hand drive clutch for the CMP. A smooth shifting 4 speed synchro would be so nice....

Thanks

Bob C

Found my answer........ not the one I wanted but a proven solution.

Along the way I tagged Pre-Dave as an adapter plate for the H2O pump will be needed and always helpful as usual..

The problem arose when a rolling chassis with a running 54 235 with a SM420 surfaced and was considered for transplanting into a right hand drive Canadian CMP...with the clutch system on the right side of the bell housing.

.......the solution is to keep the existing 54 bell housing and to cut out a window in the right hand side and transfer everything over to the right side....... measure carefully, pre drill the four corners of the new window location then open up with a small 2 inch air driven cut off saw. Now it takes ....big family jewels.... to cut up a cast iron bell housing and not crack it all up..... and you should use a drill press to properly drill a new hole and thread it for the fulcrum ball that holds the fork........ one of the two perpetrators is now 82 and still driving his truck although currently floored with Covid and the other in N.H. is recovering from back surgery....but their conversions live on.... The rest is just bolting things back together.

I have a 1959 Chev bell housing, if I can find it in the barn..... that was once bolted to my 261....and if I can find it I may try my luck....no point having a SM 420 sitting on the floor of the garage.....

Another approach not so scary....... there was a bell housing made by GM for "certain" trucks built with a 261 and they had a right hand side clutch opening and operated hydraulically....... The time frame seems to indicate 1960 to maybe 1962....... and it is set up to use SM420 as a bolt on.......

Does any one have an idea of what model. year, special purpose, actual part number, etc. that would confirm the specific existence and narrow down a search.

I find Chev dealers typically uninterested BUT may be a secondary transmission shop rebuilding trannies would be better........ that is 60 years ago.... how time flies......

Thanks guys

Bob C

I don't know the part numbers but 1960 through 1962 235/261 6 cylinder trucks used a hydraulic clutch
with a right hand throughout arm. It would also have the correct mounting for the later transmissions.
There is a 60/62 bellhousing listed on the Orange county Craigs List but it is for a V-8.
I believe both would be rare.
Fred

Instead of doing all the whittling on the bell housing, how about using a hydraulic throwout bearing that bolts to the center bearing retainer, and finding a 48-53 216 or 235 truck bell housing with the SM 420 bolt pattern? There would probably be less monkey motion to adapting the existing clutch pedal to operate a master cylinder than doing all the metal butchery on the bell for mechanical linkage.
Jerry

Thanks for the tip Jerry.......

There are more old 48 to 53 trucks around than 60-62 with a six...... the sad part is that most of the old junk is getting melted but the further out of town you go the more likely to find old farm trucks... just on Farce book alone there are a few complete 1 or 2 tons rolling chassis available..... what we can't find around here with the salted roads is decent sheet metal!!!! I am not to in love with an hydraulic clutch and may try my hands on a few spare bell housing here at the barn....... according to at least on "inventor" it is less than 6 hours of work the big issue is locating the fulcrum point for the threaded ball but apparently the bell housing was cast with a blank boss, for the fulcrum ball on the right hand inside of the early bell housing.

May need to search for an actual part number if I buy online as I have enough boat anchors. Most of my reference books stop in early fifty.
PS....found a bunch casting and part ## numbers for 47 to 51......... and it is not a bell housing but a Housing, clutch......

Only time will tell what I wind up with......

I'm with Jerry, concentric slave cly / release bearing would be the way to go.

Yup...... but some fiddling around is needed no matter what side you choose....

The CMP is a right hand drive truck...... specs issued by British Engineers.... very limited room to work....cab is a semi COE with the pedal cluster bolted on top of the frame......
pedals in the cab are left for the clutch, throttle in the middle and brake on the right...... with so little room in the foot well that you can not drive safely with heavy toe capped safely boots.... I use a pair of fancy Italian leather boots....... stomp starter is a lever, shifter for the 4 speed crunch, two E brake handles ...one for the truck, one for the winch and a final lever on your far left for the 2 wheel, 4 wheel, hi/low and winch control all in one...... it helps if you are a typical WW2 soldier, less than 20 Y.O., 140 pounds and 5" 6

I am 6"2' 200+( my wife made me add the plus sign) pounds and I use a shoe horn to get in and out....... flat out at 45 mph.

But I love it!!!!!!

Gas pedal is between the the brake and clutch? What is the reasoning behind that?

Right hand drive Military vehicle.

Leave it to the Brits to make something weird.

Yes, I know it;s a RHD truck but why did they put the gas pedal in the center? Pedal placement was pretty well universally standardized by WWII, even on RHD vehicles.
Clutch-Brake-Accelerator

I hear all the Oooh and Aaahs!!!! BUT Only the Brits would know.......

You should see the throttle linkage rod that has to leave the central button shaped accelerator..... through floor..... go forward to the front of the engine..... then turns 90 degrees to the left and the shaft goes between the water thermo elbow and the top of the water pump......opnce on the left side the rod is attached to a long arm that goes down to be inline with the centrally located fulcrum pin that hold a bell crank with a rod going up to the carburetor....Carter W 1

Now when you complicate things by installing a water pump adapter plate to a 235 /261 ...sometimes you need to grind lightly the underside of the elbow to get a few mm of additional clearance.

One of the British spec was that the distance between the end of the steering wheel and the center line of the front axle was to be 13 inches. In the only joint effort that I am aware of...Ford and GM agreed to work together to come up with pilot model ( prototype model were not in the vocabulary at the time) After the initial 50 pilot models production started in the last few days of March 1940 but apparently they were all stamped April 40...... by Xmas 40 11,000 +- had been made and the British Expeditionary Force had been bumped out of continental Europe....leaving most of their new trucks to the German who salvaged them and fondly ran them for years........ everyone panicked and order for tens of thousands replacement trucks were ordered...... Ford and GM held their ground and agreed to produced the required numbers only if they were allowed to redesign the cab to the last model number 13...... they did....99% of the same power train was used but with a North American humanly designed cab that was more practical for our NA assembly plants... the rest is history and lots of treasures are still slowly wasting away in Brazil/Argentina.......

They are challenging tolocate, restore, service and drive ........but that seems to be the attraction.

Just a quick note to say that our search today of the barn( really a garage) the attic of the barn 3 sea cans and other assorted shelters was very successful. He** found some things I did not remember I had.

My greatest find was the 60-62 Right Hand Bell housing with the slave cylinder still attached with a steel/rubber line....... about 4 or 5 1950s Bell housing with the SM420 tranny bolt patterns...... 2 complete SM420 with assorted PTO and one SM420 with broken bolt ears but a spare SM420 already cleaned up, sandblasted and painted with POR15 ready to receive the guts of the damaged one..... various assorted RHD early bellhousing to fit to about half a dozen early crash boxes...... spent time counting the teeth of 137 flywheels and and 168 later 235/261 flywheel clutch packs....... a few 261 complete blocks and a few assorted 235/261 short blocks leaving me short a few heads. So now I need to find time to work on all of them. As long as the wife does not find out all I have I have a few years of toys to lay with..... time....where do you buy time. while need to search for more small parts like the different forks... they are not all the same and the fulcrum ball for the forks are all the same size balls but the threaded part that fits the bell housing various in thread size. Lots of parts to practice on. Thanks for all the tips and clues........ but I am left with a flywheel that has 146 teeth and can't remember what it came from....I even have a full early flywheel clutch pack for a flatty V8 Ford.... Carrying some of them back to the barn made me realize how heavy they can be...... normally would use the bucket on the tractor. Also reminded me of all the transfer case we have some two speed some one speed.

A very successful day indeed.....now if Spring can get here!!!!

As far as bolting a 420 to an older bellhousing with only the bottom right mounting bolt not lining up, mount the 420 with the 3 bolts that line up, mark the other bolt hole in the bellhousing on the face of the 420, disassemble, drill and tap, 1/2-13 into the 420, insert that mounting bolt from inside the bellhousing into the 420. Make sure the seating surface for the bolt head inside the bellhousing has a flat surface for the bolt head to seat on.

Ed

Changed my reply to ED as it may have offended ED and some of our members.

I sincerely apologize for offending anyone,to ED in particular and to the membership at large.


It seems that I was so happy with Ed's suggestion that I got carried away'. I am forever grateful for the fresh look/approach to our problem.

Once again it shows to me, the great value of the forum and its ability to reach far and wide for a different perspective.

We will be experimenting, measuring in put shaft and doing some precision drilling and keep you updated with our failures or success..

Thanks for the fresh look ED and the caution to clean up the chips from inside the tranny after drilling.


'Bob C

Well, of course the retainer must fit, the location of the new threaded hole in the case must encounter enough meat to share the load with the other three mounting bolts, splines count can be dealt with by disc. Obviously trans should be disassembled to allow the tap to cut threads all the way through, and allow the chips to be removed.

Ed

Experiment will be carried out tomorrow.....today we are going to gather the parts into the barn....and measure shafts etc tomorrow we set up to drill...... stay tuned......

First the bad news.....then the other bad news....... then the fall back position.

Spent all day lugging 3 different SM 420 around....... heavy little chunk of steel....so we reverted to using a two wheel lifting dolly...... then lugging in the different Bell Housing some 1940 for the 216 qand some for the 1954 with 235 and the one and ONLY right hand drive hydraulics to the work table.....and .....

Bad news....... the bolt pattern of the SM 420 ( all of them that I have) are totally different than what I has assumed....... I was under the impression that 3 of the tranny bolt holes wouldlline up and only one hole on the right hand side was too far out..... but the spread of the 4 holes are different just enough that they willnot fit and too close to weld and redrill.... I least not at my skill level....conclusion is there is no way for me to fit a SM420 onto the bellhousing that fits to a military 1940 bell housing...even if the 1940 bell housing bolts right up to a 261......and

More bad news..... the horn that goes around the input shaft of the SM420 is about a half inch larger than the horn that fits on the 4 speed crash box.... the military bell housing would need to be machined to the larger size and that would seriously weaken the boss that hold the fulcrum shift fork.......

Conclusion no 1....... you cannot safely fit a SM420 onto a 1940 right hand drive bell housing as it will not fit/bolt on....... and would be nearly impossible to redrill for a new pattern without seriously weakening the bell housing.

One further observation relating to possibly redrill the tranny case..... the right hand bolt, to match the 1940 bell houisng hole would be right into the idler shaft of the SM420....

OK for some good news....... the running 235 can be bolted to the and frame and use the 1954 bell housing with the SM420 but the rear motor mount are different from the solid military mount and the civilian rubber mount of 1954...... however.....fabricating newer adapter motor mount are far more feasible.....or swapping the rear engine cross member from the 54 frame to the CMP frame........ but that reduces greatly the value of the finished vehicle for collectors....... but it could work.....and that implies cutting a rght hand window into the 54 bell housing and relocating the fulcrum bolt to the right side....... needs ver precise drilling or better still machining a new 1 inch threaded hole with a shoulder to insure the fulcrum bolt screwed in with an hex key would be at exactly the same depth for the fork.....dicy and leave a rather weak section of bell housing casting......... good news but very iffy in my mind...... and that leaves the short joining drive shaft that links to the transfer case to be shortened about 1 inch.....

Conclusion no 2 ....... bolt the running 1954 engine with the flywheel and clutch pack to the existing 1940 right hand bell housing..... install the 4 speed crash box and get used to double clutching.....and the military motor mount remain a straight bolt in.....and short drive shaft fits.

Beware of parts that look similar and/or the same until you actually physically get down to measuring.......

Final gold plated conclusion...... that 1960-62 GM right hand drive bell housing that I purchased years ago on a lark cheap ...part # 3817816.... that bolts right up to my 261 and is designed to take the SM420 bolt pattern is now so highly prized and valuable that I may store it under my bed!!!!!!

Time consuming but a worth while learning session.

Did I miss an option........ feel free to jump in .......I may have over looked some other solution.......

Bob C.

Looks to me like putting it back together stock would be the best way to go.

Well a bell housing has been found from a car or 1/2 ton pickup that has the SM420 bolt pattern and larger 4 11/16 centre hole and square bolted side bell housing mounts like the 40 military.....but ..... it was attached to a 3 speed tranny with side sifting for a 3 on the tree......and meant to be used with the enclosed driveshaft....... tranny case is all one piece from ears to bolt on the bell housing to the flange for the tube U-joint.....
........ and when you flip it over....... the inside of the bell housing has a bulge on the right hand side that can be drilled for the 7/16 fulcrum bolt....and has the outline in the casting of the RH clutch window sealed with a wooden plug........ only thing left to do is drill 4 holes and make a right hand window for the fork....... have not seen more than pictures and measurements and is currently in transit....... so the original GM casting could have been be used for export.....

who would have figured ......will have pictures and casting number shortly....... what it came off from is only a verbal story..... the web can be wonderful sometimes...

Bob C.

PS..... confirmed the 3 speed tranny was from and only from a truck..... huge 3 speed ....side loader with the large center hole and SM420 bolt pattern....so 48 to 52 truck BH will fit a Canadian CMP truck.

Closing the loop...... Bellhousing has been received and fits....... has been modified to RH fork...... BH is from 47 to 52 or 53 on small trucks only..... small trucks had the bolt pattern of the 6 cyl.... 216-235-261 and the tranny is only 3 speed BUT has the bolt pattern of the SM420.....considering the work was all done by a young chap first time into RH trucks.....a very nice job.....

PS thanks for all the help, clues, suggestions, etc.

Bob C

Nice work!