Re: need to know
Although the block is the same and everything is interchangeable, The 70 can not be easily converted. That is, unless you want to disassemble the engine.
The carbs on the 70 have a smaller 1 3/16 diameter venturi while the carbs on the 85 MAY have the 1 5/16 diameter venturis. Some 85s did have the smaller carbs. HOWEVER: Switching the 70 to the larger venturi carbs will only gain a couple of horsepower.
The reason is that the exhaust ports on the 70 are quite a bit smaller than the ports on the 85. That's where the real power is gained.
Thus: If you disassemble the engine and enlarge the ports on the 70 you will gain significant power. If you square the ports in addition to enlarging them, you can boost the power to over 90 HP. This takes a good bit of work with a Dremel and carbide bit--figure about 6-8 hours to do the porting correctly.
My suggestion: Since the 85 has a bad piston and you must disassemble it to repair it anyway, why not keep the 70 as your running engine while you play with the 85. Boring the cylinder only costs about 65 bucks and a new oversized piston can be had for 125 or so, depending upon where you buy it and what you buy. So, you can see that repairing the 85 is really not too expensive. Only one or two special tools are needed and that is only to disassemble the piston and rod. If you are a backyard mechanic, and if you buy a Clymers manual, you will have no trouble disassembling, repairing, and reassembling. AND since you also have a running engine, you have the luxury of taking your time.
If you are at all interested in repairing the 85, and are at all interested in porting it, then Google The Chrysler Crew. Join then scroll down to the big question mark, questions and answers. Scroll to the top left corner and click on photos. Look for the album titled Frank's Boats. In there you will see an example of porting a three cylinder block.
