304 to 258 conversion with a TH400???

have 1973 jeep commando with a 304/th400/dana 20 combo and I want to swap out the 304 for a 81 258 6cyl... I know I need to swap out the flexplate.. but various searches on the web have indicated possibly changing the torque convertor as well as needing a spacerplate??.. i would like to find out if a swap from a 304 to 258 requires anything nbeyond the flexplate and if it requires a spacer plate what size etc??

 

The engine/tranny adapter from the 304 will work on the 258. There's a spacer between the crank and the flexpate, it's the same as the 304. Torque convertors are all the same. Engine frame mounts are different.

 

thanks! I got the 258 motor and frame mounts from a the cj7 it came with I'm hoping to use those.. I hear one side was the same on one was different? are these bolt on or is welding required?.. it looks like I just need to get the correct flywheel..

I see different ones listed for 70's vs early 80's do you know where the serial #'s are on a 258 and how ot decode the year so I can get the correct flywheel?

thanks much!
Scott

 

Fortunately the mounts just bolt to the frame. I believe you're right about only one side being different but I can't remember which side, you'll be able to tell once you compare them.
I know there's a web site out there somewhere that has a break down of the engine numbers but I can't remember where I saw it. Try a Google search, should be out there some where.
Before you get too far, the flex plate is going to be a bit of a problem I think and I just checked my parts manuals to verify this. Up until 1980 Jeep was still using 400 turbo trannies. After that they went to the Chrysler 727, 999 etc. So you are unlikely to find a flexplate for a 400 that will bolt to the 81 258 if it is in fact out of an 81. Flexplate to crank bolt pattern is different. The engine serial numer is on the left/drivers side of the block where the head meets the block about mid way back.

 

Just a thought, maybe someone here knows or I can get off my butt and go check this out, but did AMC maybe NOT changed the crank bolt pattern but had their later model flexplates for the Chrysler tranny made to fit the earlier crank bolt pattern? That would be cool if they did. If I have time I'll check my engines in the shop. I have a 73 304 and an 88 360, I'll post back if I find anything out.

 

I got a JP article somewhere on the entire history and changes on the 258 I'll dig it up and see if it mentions the crank bolt pattern. and post if I find out.. I guess worse case I'll have to find a 70's shortblock and use it but Thanks for giving me the head up to figure it out before I yank out my running 304 expecting to swap the 258 in a weekend

Scott

 

The crankshaft bolt patterns are the same for the 258/232 engines. Just make sure you get a flexplate(flywheel) for a 258 with a th400. It should not have a counter wheght on it, if it dose its for a V8. The frame mounts should be the same allso. On my '72 Commando, I dropped a 232 in place of the 360 the PO had in it, he kepted the 360.I'm running a 232/727/D300. All I had to do is get the right flexplate for that combo.
I hope this helps and good luck on your project.

 

I just heard back from a guy who tells me the same thing, the bolt pattern should be the same, so the 81 will work. Now just a matter of finding the flexplate. I know a guy that used to have one but not sure if he still has it and I haven't heard from him for a while.

 

Passenger side MM is different from I6 to V8 or visa versa.

 

Did we cover this? I know that the pre-80 V8s have a spacer in the end of the crank that supports the TH400 convertor. To use one of these V8s with the later TF transmissions, you have to remove the spacer. Not sure about the I6s, but I assume they are the same. You may have to add the spacer to the '81 258 to fit the TH400.

The spacer is a really thin sleeve, shaped sort-of like a cam bearing. Looking at the 304 you may miss it if you weren't looking for it.

 

thanks for all the help guys, new flexplates for either pre 81 or after 81 are available new from adealer on ebay for about $50 I'm sure your local napa has them as well for anybody else interested in doing a conversion

 

Would that be this place? Pretty good prices.
http://stores.ebay.com/SPECIALTYENGINEPARTS

 

Also, be aware that your gas lines run from the left side of the engine on a 304 I believe, and they do run from the right on a 258. So, you'll need to re-route your gas lines as well.