SM 420 to 258cid

I read on Novak's site that you can mate a SM420 to the 258 engine fairly easily. Has anyone done this conversion? If so, what kind of unexpected trouble did you have?

 

I expect it's not so common, because you can adapt the Ford T-18 or NP435 much more easily. You're pretty much swimming against the tide with a GM transmission.

There are many old posts here about the T-18 swap if that interests you.

 

The guy that went into business with me has an '89 YJ, 4.2L and SM420. He is using the Advance Adatpor kit, but hasn't had any problems. The adapter is about 6"s long. His only complaint is no overdrive when he's on the highway since he drives his Jeep everywhere he can.

 

I would go the T-18 route if possible. The shifting mechanism is much better than the 465 or 420.

 

It doesn't sound much harder than adapting a ford t 18.

http://www.novak-adapt.com/knowledge/420_to_amc.htm

With novak's adapter the total length, tranny + adaptor is only 13.9". No adapter is needed on the front. I am looking for the shortest posible combo so I can install a Kune-V in front of the dana 300.

 

The main advantage of the T-18 is you can put one together for a Texas pattern transfer case entirely from junkyard parts. If you're going to adapt to a NP pattern with a Novak or AA kit, you have a lot more latitude in picking the transmission.

 

Regardless what tranny you go with the rear will have to be adapted so that is a non sequitor. The advantage with the T-18 or NP 435 is the front can be bolted up with NO machining except the pilot bushing and these are available from many sources including Novak, AA, Parts-Mike, R&P, etc. The SM-420 has the lowest first gear of any common manual transmission (7.02-1) so that is a plus. It was also made in 3 versions that I know of and can be difficult to find parts for in some areas of the country and tend to be noisy. Remember, hasn't been manufactured since 1967 so it's not like they are on every street corner. They are a pretty compact strong transmission though. Nickmil

 

Nick, I understand what you're saying about parts availbility, but once you have a good one should you really need parts assuming you're not putting a crazy amount of power in front of it? I have one on the side of the house I'm about to go through, but the more I read about problems finding parts I'm considering looking for a 465. I understand those are more readily available.

 

If you've got a good one and want the lower first gear then I'd go for it. The down side is down the road if anything wears or breaks you "might" have difficulty finding parts. SM-465's are much more prevalent as they were used up into the '90's to my knowledge so parts are much easier to come by. The SM-465 is larger and doesn't have quite as low a first gear, 6.5 or so first gear, but is much quieter. I wouldn't use a 420 in my own Jeep just because of the noise and parts availability that can be an issue but lots of people use them and are very happy with them. Nickmil