The SM420 has proven to be one of the best transmissions ever built. Its watershed design made it a greatly respected predecessor to following designs.
We offer nearly all parts at the component level, and our master rebuild kit, which includes the most thorough collection of parts offered anywhere.
Our master rebuild kit has been assembled for our customer's rebuilds as well as our own. It is the most complete kit we know of an it features all the typical consumable and wear item components.
Note that the 1st-to-2nd gear synchronizer ring is not included in the master kit like for other transmissions, as GM manufactured the synchro into the 1st sliding gear. Because most drivers start out in 2nd gear and do not regularly shift from 1st into 2nd, this synchro ring usually outlasts the others. However, Novak can install a new synchro ring into your first gear if you would like to send it to us. Cost is $125. and we require about four business days to turn it around. Please inquire if interested.
It is very common for our customers to do top-level rebuild work, using good methods and by following our included instruction guide.
We suggest the rebuilder have access to a set of snap ring pliers, and a small arbor press, in addition to standard shop tools.
Many parts for the SM420 are available new and used and affordably so; however, gears and input shafts can be very difficult to source, primarily due to having been out of production for several decades. Please refer to the parts list below to determine availability. Contact us.
All asterixed (*) items are already included in our master rebuild kit, above.
All items marked with a cross (†) are unavailable or nearly impossible to source1. Expansion Plug
The early SM420 transmissions (1947-1953) feature a necked-down retainer sleeve bore and screw-back threads to push oil rearward back into the transmission during operation; akin to a pump. This is in lieu of a neoprene seal as used in modern transmissions. This type of system is revealed by the presence of a dripper hole at 6 o'clock in the retainer tube, whose role it is to drip any escaped oil down into the bellhousing instead of towards the clutch disc. This early method works only tolerably well. Parking downhill with the engine switched off can be especially problematic.
We've developed a clean way to fit the SM420 with a front seal. We grind a precision cylindrical seal journal into a customer's existing shaft. We then provide our billet steel SM420 front retainer and a neoprene seal. This has the benefit of providing an excellent seal at the front of the transmission, without sacrificing the flushing and cooling action of gear oil to the main input bearing. It also replaces the brittle, cast iron front retainer with a far more durable version.