I am building a chevy 292 straight 6. I have 2 transmissions and I don't know which to use.. I have a sm420 and the np435, what are the pros and cons of these two transmissions?
The SM 420 is a tough trans. It don't have a provision for a rear mount. You have to have a bellhousing that mounts to a rear crossmember. the newer trans ive had trouble with them jumping out of 2nd gear.
the sm 420 is a truck box with a straight-cut non-syncho (IIRC) granny (real low) 1st gear....most often driven by starting out in 2nd gear. If in good shape, its strong as heck, but I myself wouldn't put it in a conventional street-driven car....only a 3/4 t and up truck or an offroad 4x4...
I like a granny 4 speed in a truck even a 1/2 ton. the low gear makes taking off and creeping around in tight places easier. The slow reverse makes backing up much better especially when hooked to a trailer.
Granny 4 speeds are also great when you are building a house. You can tell your 12 year old to load up a bunch of bricks or trash and move it where ever you need it and unload it. Flat out screaming the thing won't go fast enough to get him in trouble. Of course if he does hit something, it is gone.
The bearing retainer is smaller on the 420 vs the 465. There is no backup light switch on the 420, unless it is a 1967 version, or the military one. There used to be a kit available for the 465's popping out of 2nd gear issue, it was a two piece ring that could be installed through the pto opening. The 420 has the lowest low gear ratio available, and is a little bit shorter in overall length than the 465. I also think all 420's have the two lower mounting bolts running from inside the bellhousing into the trans. What are ya building anyways?
I've had a few of these and I'm looking forward to another SM420 or SM465. I've had them in anything from a C10 to a C30. I've towed stock cars on trailers and the steep 1st and reverse were good in tight pits and backing up a driveway. They are a natural behind a 292 in an old truck.
someone mentioned a SM465. The OP mentioned the NP435, which is a different transmission--usually used in Fords and Dodges, but Chevy use them too? Make sure the 435 has the Chevy bolt pattern, if you intend to use it. If the SM420 is not worn out, like most of them are, then it's a decent trans for a truck with a 292. If you plan to use the truck as a truck. If you are going to use it like a car, get a car transmission.
My current daily using beater work truck is a 66 GMC 1/2 ton with a factory SM420 4 speed and a dana 44 with 336 rear gears and tall 16 inch tires on the rear.. And I use it hard. It will run 75 MPH and not be all wound up. and I can pull heavy loads because of the granny low. and doesn't use a lot of gas. If it was a stock three speed the clutch wouldn't last very long. Mine is a 250 six and when loaded I have to downshift on the grades. With a 292 you could run even taller rear gears than mine and still pull heavy loads and not need to downshift a much . Better than a overdrive if you ask me.
Like Old Wolf said. I had an OT 4x4 with a NP435 and put huge tires on it. Most of the time I would take off in first and it would cruise at high way speeds with ease. Excellent combo. Bones
I know the first gear on the granny 4 speed isn't syncro. However if you double clutch you can easily shift into second or downshift to granny low without clashing gears.
Years ago I had a Dodge D350 with the NP435 and have nothing bad to say about it. It shifted very smoothly and never had issues of any kind. I'd recommend it in a heartbeat. I believe the sm420 has a lower granny gear than the NP435 and seems to have a good reputation so I'd say base your choice on which gearing suits you better.
A little info on the NP435. http://www.novak-adapt.com/knowledge/transmissions/manual/np435 This discusses adapting a Ford transmission to a GM bellhousing. https://www.novak-adapt.com/catalog/adapters/engine-to-transmission/gm/ford-to-gm/
I realize that you're asking about the sm420 and np435 transmissions but I put a T5 out of an S10 behind a 250 inline, in my '63 C10, and loved it!
I saw some NP435s used with 396s in the 70s. Don't know for how long or if it was only used with big blocks. Too bad you can't use an NP445, all synchro with a granny low [about 4.5 IIRC] but a 3rd gear that was good for about 80mph. Made it a lot better for p***ing than other truck 4 speeds of the era. They made a 435 with the same ratios as a 445, but it was only used in medium trucks.
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