I recently bought a 1957 GMC truck with a 235 I6 (4 speed) I took it out for the first time today and was having issues figuring out the shift pattern and when to shift gears. I believe the pattern is as follows : Reverse= far left and down. 1st= left and up. 2=left and down, 3= right and up, 4= right and down. Does this seem right? From 1st to 2 was rough but I had to give it lots of gas when I transitioned into second. Shifting from 2 to 3 I almost stalled. I never did try 4th. ( Ps there is no tach, I’m just shifting based on sound) Downshifting from 3 to 2 wouldn’t work. I even tried double clutching and I couldn’t get it to sync( it would grind gears). From 2 to 1 also wouldn’t work but I believe 1st isn’t synchronized so it will only slip into 1st when I’m completely stopped. This is my first classic and I wasn’t expecting this standard transmission to be so different than what I’m used to. Any help on what I’m doing wrong would be greatly appreciated! Thanks a bunch Sent from my iPhone using The H.A.M.B. mobile app
#1 - DON'T start in 1st gear, start in 2nd. First gear is so low it practically useless unless you're dragging a 10,000# sled up a steep grade. #2 - Don't rush the shifting process, the ratio split is so wide that the harder you try, the harder it is to shift. The same goes for downshifting. #3 - That trans is not performance oriented in any way, shape, or form. It was designed for durability. #4 - Drive it like an old farmer and things should go alot smoother for you. Hope this helps.
In addition to the four excellent points above, keep in mind the transmission is over 60 years old, and it's plumb wore out.... You might have to rev it higher than you think, those engines make a lot of noise when they rev up. And if it's really a 235, it's not the original engine. GMC had their own 6 cylinders, 235 is a Chevy engine.
It’s a canadian 9300 so I believe it’s the same as a 150 Sent from my iPhone using The H.A.M.B. mobile app
that explains the 235. is it running ok? brakes adjusted correctly? does the clutch feel like it is releasing ok? does it roll freely?
Thanks Squirrel. I had it appraised and was told it is a 1959 235 so it could very well be from a Chevy. But very interesting, i didn’t know Gmc had their own 6 cylinders. I noticed that when I was reving high, the oil pressure went up to around 45-50psi. Do you know if this is normal? Sent from my iPhone using The H.A.M.B. mobile app
yes, that's normal. Might want to add you location to your profile, so we can tell where you are. And of course, you have to post pictures of the truck and the engine, so we can see what you're talking about (really, we just love pictures)
Look on the engine block on the driver's side ... 5 freeze plugs, it's a GMC, less than 5, it's a Chevrolet ... It's really hard to wear out an SM 420 trans, so make sure it's full of gear lube & take your time. It should shift from 3rd back to 2nd quite easily. 50 lbs. oil pressure would be just right for either engine in good shape.
The "Canadian" Pontiac's, that were based on the Tri-Five Chevrolet chassis and wheelbase, came with 261's, not the 235. Maybe your truck has a 261? You can always hope! And like has already been said, for the most part, just ignore first gear. I am Butch/56sedandelivery.
does the shifter feel sloppy? there is a pin/pins in the "tower" that wear out and make it hard to find a gear. shifter should only move a little when in any gear. if you can move the shifter in a circle while it is in gear, the pin/pins need to be changed. it will shift much better.
The top cover is one aspect that should be looked at and easy to fix as mentioned above. My brother had a SM420 in a GMC that was street raced and it shifted well. Once in a while he would wind it up in 1st and let it go. That had a 4.56 rear. My '65 C30 had around a 5.14 or 5.38 rear in it and it was wound up by 55 mph.
I've had at least three in running trucks with an unknown history and they shifted and sounded just fine. Matter of luck I suppose.When in good shape they can be shifted faster than the later 465 trans...The 420 is still very popular with off road guys and is extremly rugged..
It 'should' be a 261 in it if it's a Canadian built GMC but that could have been changed out over the years.
The 420s that I've had that were worn out were rather difficult to downshift into the gears that have synchronizers, so what the OP describes sounds rather normal to me. Even with a trans in good condition, you have to take your time shifting it, and double clutching for downshifting is a good plan...a better plan is to not downshift until you're stopped, or moving very slowly. Drive to avoid having to use the transmission to slow you down. It's not a sports car, it's an old truck.
I grew up with those granny low 4 speeds. there is one in my 66 GMC 1/2 ton. and it has 336 rear gears. I like it really well. The extra low first and reverse are great when taking off pulling a heavy load. and the slow reverse makes backing up a trailer much easier. I can shift 2 thru 4th up or down without touching the clutch. I simply match engine RPM with gear speed. Its called floating the gears. Double clutch and use the gas pedal correctly and you can shift from second into granny low without being dead stopped. My wife has her daddys 66 chevy 3/4 ton that he bought new . Debbie also don't hardly use the clutch and can downshift into granny low without stopping. . its a 292 six and a SM 420 trans . 456 rear gears. she commonly hauled 6000 pounds of feed in the bed of it.
Interesting!! I was told it’s a 235 but would it be stamped somewhere on what size it is so I can double check? Or is there a visual way to distinguish between a 235 and 261? Sent from my iPhone using The H.A.M.B. mobile app
I didn’t notice anything out of the ordinary but ’ll try this out next time I get it out! Everything else you mentioned in your previous post is good... breaks, clutch etc. Sent from my iPhone using The H.A.M.B. mobile app
Look behind the distributor. If it's a GMC, the first 3 numbers are the displacement. If it's a Chev, suffix letters on serial will tell. A 261 in a 3/4 ton would be a rarity indeed, but anything is possible with these trucks.
Canadian built GMC 1/2 and 3/4 ton trucks were identical mechanically to the Canadian made Chevy branded trucks, and the base engine was the Chevy 235 engine. Back in the 60s, I did see GMCs with the American Pontiac V8 engine, but they all were equipped with the hydra-matic transmission. Taking into account that this next tip would not help much with a worn out transmission, if the transmission is not leaking, you could upgrade to a synthetic gear lube and that will help make shifting smoother. I have done this with several older manual transmissions and have experienced excellent results. Bob
The Canadian Pontiac cars used the 261 cu in motor, starting in 1955, but I can't find anything to show if they were used in the GMC trucks. The Canadian GMC trucks were manufactured in Oshawa Ontario, but some speciality Chevy and GMC models did come from the US. As an example, the trucks ordered with the hydra-matics, and my Chevy Cameo, which was built in Flint. Bob
Two out of the last three 56-7 GMC's I stripped, a shortbox and a 3/4 ton longbox and a 61 GMC all had the 261. All were Canadian built.
Just drive it like an old farmer. Get it into top gear and when you get to an intersection, push the clutch in, roll around the corner and slip the clutch till you get back up to speed.
Ive never seen a GMC Truck that has the vin stamped on the frame. on a 55 chevy truck the vin is stamped on the frame between the steering sector and emergency brake linkage. I wonder if Canadian GMC,s have the vin stamped on the frame?
I drove a used up tow truck that had a bungee cord to keep it in 4th gear. On the 3-4 shift, I just let up on the gas and the tension of the bungee put it in 4th by itself.
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