Can a 1974 75hp block be used to rebuild my 1989 85hp block?

[tater76]

Like the title says, will the block from a 74 75hp swap with a 89 85hp? I have an 89 85hp Force with a hole in the block, but it is in excellent condidtion besides that I also picked up a 74 75hp Chrysler today for $100 bucks. My thought is to swap guts into the 75 case, but I can't find any info on the probability of this working. I know earlier years of both hp's will but no idea on this. Oh, and anyone that needs parts for the 75hp just pm me, as I will be parting this one out and would love to give fellow Iboaters a helping hand.

 

[Frank Acampora]

Re: Can a 1974 75hp block be used to rebuild my 1989 85hp block?

She will bolt up and pistons crank etc will swap. BUT---The 75 is ported more conservatively and will still only deliver 75 HP. Since the crank and pistons are the same size, there is no percentage in swapping in the newer ones UNLESS the older ones are three ring circus--I mean pistons.

Additionally, the top of the 75 is higher than the top of the 85 by about 1/8 inch. It has (4) 3/8 bolts holding the alternator stator while the 85 has (5) 1/4 X 20 allen head cap screws. You will need to very accurately drill and tap the block.

I have done it the other way, retrofitting an older ignition to a newer block. It is probably do-able to put on the newer ignition but you may need to get creative. At the very least, you will need to fabricate a spacer for under the stator carrier to locate the top bearing and keep the crank from rising that 1/8 inch. You will most likely need to reposition the top seal too.

NOW---If you want to get really creative, measure the distance down from the head to the exhaust and bypass ports on each engine. Port the 75 to the same dimensions as the 85 and it will now deliver 85. While you are doing that, might as well square the ports and gain a little extra HP

 

[tater76]

Re: Can a 1974 75hp block be used to rebuild my 1989 85hp block?

That's awesome! thanks Frank. I just had a feeling about buying this 75 today. So in your opinion is swapping the newer electronics over and making spacers worth my time? or should I just try to find a Force 85 block? I just checked compression on the 75 and it was 120 across all three.

 

[Frank Acampora]

Re: Can a 1974 75hp block be used to rebuild my 1989 85hp block?

Porting takes a lot of time--at LEAST one hour per cylinder unless you have access to a Bridgeport and end mills. Even then, you have set-up time. AND, that old 75 was rated at the block. You may be disappointed if you don't port it.

If you want a 75 with newer electronics, then give it a try. A spacer and re-drilling the top should not take that much time. It won't permanently affect the block. And what's the worst that can happen? The engine doesn't run as well as you like and you part it out or convert back to distributor. No harm, no foul.

Personally, if I had the time and was so inclined, I would try to convert to newer electronics and meanwhile look for a 1980s or later Chrysler or Force block. (Chrysler changed to Prestolite ignition around 1982) However, I have several really good running distributor ignition engines. Why bother converting when they run so well?

 

[tater76]

Re: Can a 1974 75hp block be used to rebuild my 1989 85hp block?

I think I will try the swap for now, and Just for kicks I will make a pitorial diary of my step by step process. I love this kind of challenge, and it should help to show all of us Force and Chrysler owners that we need to think outside of the box sometimes Here are some pics of the two patients.