1988 Jeep YJ SM420 conversion

Hi all,

Just got some extra cash and found what seems like a great deal on an SM420. It comes with the full conversion to convert an 1988 yj 4.2 from the ba10/crap to the SM420. It is $800 and comes with:

- SM420 to bolt to the 21 spline np231
- bellhousing AND advance adapter to bolt to 258
- all clutch components and slave cylinder
- both front and rear driveshafts

Seems like a great deal. It was taken out of a completely running conversion from what I understand. I wanted to ask y'alls opinion about a few things though.

1. It is set up to accept a 21 spline input from the np231. I know this isn't as strong or desireable as the 23 spline, but for mild wheeling/beach driving this should be a non issue, plus either way the trans is a huge upgrade form the ba10/5, correct?

2. In the long run I will prob need a stronger carrier and lower gears, seeing as mine has the stock 3.07's, but again for mild wheeling and beach driving will trans gearing help compensate for the high gears in the diffs until I can save up enough money?

3. I won't lose any high end torque/top speeds because both trans have a highest gear ratio of 1:1, regardless of 4/5 speed, right? Only difference is higher revs between gears and lower fuel economy. As far as wheeling, the 4 speed is probably better due to less shifting?

4. Has anyone else done this conversion and have any tips/insight? Is there anything I'm overlooking that I will need besides fabbing up or modifying the crossmember and running lines?

Thanks guys!!!

I think I found most of the info here:

http://www.novak-adapt.com/cat…transfer_case/kit_421.htm

Looks like I'll need a new pilot bushing. Also I can't determine if their transmission mount allows direct bolt to factory crossmember.

Lastly, it says like four times that 21 spline will need to be converted to 23 spline. The guy is down in lower delaware and owns a small jeep business he said, and sounded like he knew what he was talking about and assured the 21 spline will work.

Thanks for the info! I'll try to take lots of pics throughout the process so that if it isn't too bad, maybe other people will want to try it too seeing as it has the 7.07:1 ratio for 1st gear.

Okay so we picked everything up. I was mistaken. It DID come with the pilot bushing, but not with the slave cylinder. Anyone know which one I should use? I was thinking trying to find a GM one from an s10 with a 420? Or should I look for something else?