Can a 1974 Chrysler75hp block be used to rebuild my 1989 Force 85hp block? UPDATE!!!

[tater76]

Ok folks, so I said I would make a pitorial step by step of my ignition and component conversion, and here are the first few steps I have taken. If anyone remembers I wanted to swap the ignition components from my blown 89 85hp Force to a 75hp Chrysler powerhead. Why would I do this you ask?!! well.. I got a screaming deal on each motor, but the 89 was blown and the 75 had bad ignition components, so... being a tightwad I knew what I needed to do. However, I am at a turning point and need some guidance from some more experienced fabricators. Here are the pics.

I am at the point where I need to drill holes for the newer style stator plate/carrier. View attachment 104027View attachment 104028The question is, how perfect do I need to align the stator plate before drilling? if I am off a fraction of an inch compared to the 85 block will it make a huge difference?

Also, I ran into a few differences in the block that need to be noted if others want to try this. First, the 75hp block needs to have a few places relieved for the newer components.

View attachment 104029If you look close at the pics, you can see an indent in the black block just to the left of the shaft. This must be cut into the 75hp block too (a hacksaw works)

There is also a bolt mounting tab on the 75hp that must be removed for clearance.


Oh, and I decided to use the practically new starter from the 85hp on the 75hp, however, the mounting holes on the 75 are small and threaded. So... I had to drill them out to the same size as the 85.

While tearing down the 85 I found this View attachment 104024View attachment 104025 If you ever wanted to see a thrown rod this is it. If you look close you can see the indents in the piston, any guess what caused those? yep, you guessed it, bearings being pounded against the head into the piston, YIKES!!!

Believe it or not, the 85 only had 20 mins on a fresh rebuild!!! REMEMBER THE OIL IN THE GAS????

Stay tuned for the next installment

 

[tater76]

Re: Can a 1974 Chrysler75hp block be used to rebuild my 1989 Force 85hp block? UPDATE

Well, looks like some of my pics didn't make it, sorry. Here are pics of the indent in the block that needs to be relieved in the 75.

View attachment 104035

Here are the thrown rod pics too.

 

[tater76]

Re: Can a 1974 Chrysler75hp block be used to rebuild my 1989 Force 85hp block? UPDATE

Jeeeze! ok, so HERE are the pics of the indent in the block

Don't know what's with my pics, sorry guys I will get it fixed.

 

[lucas_chrysler_1]

Re: Can a 1974 Chrysler75hp block be used to rebuild my 1989 Force 85hp block? UPDATE

yes you can do what you are doing. ive bone it before. and as for the top bearing cage (where the stator sits) as long as it is close that is all that counts, because the timing can be adjusted. but you must realize that will still be a 75hp chrysler dressed in a forces clothes. but i find the chryslers have a better quality than forces (even though they are the same engines, different manufacturers) so it will work out well. the chrysler has smaller pistons(and is crank rated at 75 hp, the force is prop rated at 85hp), you might get a little more power if you changed the carbs over from the 85 onto the chrysler. but it will still be slower than the 85, but it will work. GOOD LUCK!

 

[tater76]

Re: Can a 1974 Chrysler75hp block be used to rebuild my 1989 Force 85hp block? UPDATE

Thank Lucas, thats what I thought folks might say about the bearing cage. However, the pistons and crank from these two particular engines are identical in size. The 85hp has larger ports than the 75hp creating the power difference. The carbs are a direct bolt on too, although I don't know if the jets are of a different size. I plan on swapping them also because I lucked out and they were just rebuilt!

My intentions with this motor were not to try to make a 75 to an 85, but to see if the ignition swap could be done. I originally was just going to find a clean 85hp block and stuff the good components in it and be done. That is when I stumbled upon the 75hp with bad ignition parts for a $100 bucks. Anyway, I will continue with the swap just to see if it can be done easily, at least until my 85 block shows up Thanks for the input Lucas

 

[lucas_chrysler_1]

Re: Can a 1974 Chrysler75hp block be used to rebuild my 1989 Force 85hp block? UPDATE

yes the jets are a different size, but not by much. and as for the piston sizes, who told you the chrysler was a 1974? do you have any pictures of the chrysler, and i can tell you what year it is or nook on the spark plug wires on the origonal distributer (if it is origonal) it will say the year in them. but a 1974 should have smaller pistons, they changed piston size (to your force size) years later (i forget what year exactly) but in the late 70's 78, or 79 i think but i could be wrong, but every thing else is the same. here is another quick way. is the lower unit the same as the(look the same but different color) if so the motor is 1980 +. if it is different it is in the 70's some where

 

[tater76]

Re: Can a 1974 Chrysler75hp block be used to rebuild my 1989 Force 85hp block? UPDATE

Through a little research on here and google, I found that the model 757HA was somewhere between 74 and 76. I say this with uncertainty though. The cowl was pretty trashed, and was hard to read as someone had painted over everything, but it did have the Magnapower emblem on the front.
You said that you had done a similar conversion, any advice on drilling for the tower, as the 75hp block does not have a hole on the upper block?

 

[lucas_chrysler_1]

Re: Can a 1974 Chrysler75hp block be used to rebuild my 1989 Force 85hp block? UPDATE

i just drilled through. lol. just mark the hole and drill it, theres nothing to hurt.

 

[tater76]

Re: Can a 1974 Chrysler75hp block be used to rebuild my 1989 Force 85hp block? UPDATE

Here are some pics to explain where the hole must be drilled in the 75hp (white block) for the tower.

 

[tater76]

Re: Can a 1974 Chrysler75hp block be used to rebuild my 1989 Force 85hp block? UPDATE

I made a few steps forward today, thought I would show some pics. Here is the 1/8" spacer I cut off of the original 75hp bearing cage. I removed the seal and used a hacksaw slowly and carefully to cut it off. However, all that I need is 1/16" thick, so to the sander I go. The 85hp bearing cage has a 1/16" drop into the bearing housing already, so all I need to make up is the 1/16" difference. I can also use the original seal with the spacer and cage all together.. You can also see the one cage assembly hole I drilled and tapped to locate everything.

 

[lucas_chrysler_1]

Re: Can a 1974 Chrysler75hp block be used to rebuild my 1989 Force 85hp block? UPDATE

YES good job with that. alot of it is just figuring it out as you go. but looking at the block for that 75 hp, it never been rebuilt, and looks pretty clean, which is a good thing. but ya for the tower just drill through, the plastic bushing wont fit, different thicknesses of block. but you are progressing well. have fun! but if pistons are the same size on both motors, and you put the 85 hp carbs on it, you will have an 85hp again

 

[Frank Acampora]

Re: Can a 1974 Chrysler75hp block be used to rebuild my 1989 Force 85hp block? UPDATE

Do not drill and tap all the way into the crankcase when you do drill for the five bolts that hold the stator mount. Remember: The crankcase needs to be a sealed chamber and 5 screws penetrating it at the top will defeat the seal of the stator mount. Drill just deep enough for the screws and use a bottom tap.

As for drilling to clear the tower: Make a jig that bolts to the distributor mount holes in the 85 (which you will need to tap 3/8-16) and align with the bushing hole. Now transfer to the 75 block and drill ---Simple!

 

[tater76]

Re: Can a 1974 Chrysler75hp block be used to rebuild my 1989 Force 85hp block? UPDATE

Its funny you mention that Frank, as I set up to begin drilling the mounting holes, I had a bad feeling. Luckily, I stepped away for a moment to revaluate the situation. I then took a drill bit and put it down the mounting holes on the 85. I took the depth measurement, and transfered it to my drill with a peice of masking tape as the depth marker. Sometimes I get ahead of my grand ideas without thinking things through :facepalm: Next step is fitting the spacer to the bearing cage with the press in seal. Any thoughts on sealant between the spacer and bearing cage?

 

[Frank Acampora]

Re: Can a 1974 Chrysler75hp block be used to rebuild my 1989 Force 85hp block? UPDATE

Good thinking--measure twice---cut once!

Black RTV silicone should be fine. However if you want to spend the 15 bucks, Permatex makes a red anaerobic sealant that is the recommended compound for sealing OMC engine block halves. I don't remember the number though. You would need to research it.

 

[tater76]

Re: Can a 1974 Chrysler75hp block be used to rebuild my 1989 Force 85hp block? UPDATE

Well, instead of working on the spacer, I tackled the hole for the tower. Here are the results. As you can see, I was able to retain the plastic bushing through carefull fitting.

Tomorrow, I finish the bearing cage mounting holes

 

[lucas_chrysler_1]

Re: Can a 1974 Chrysler75hp block be used to rebuild my 1989 Force 85hp block? UPDATE

Nice job, your progressing well. i wouldent recomend using anarobic sealer (seals with absents of air) you would want to use anerobic on perfectly flat machined sufraces that have no way of air getting to them (later force blocks have a machined surface for the case halfs, with no spagetti seal, that is where aneroric us used) but I would say use rtv. because it it not 100% perfect, good luck.

 

[tater76]

Re: Can a 1974 Chrysler75hp block be used to rebuild my 1989 Force 85hp block? UPDATE

Well, I had to postpone progress on the swap tonight. I had to go pick up a craigslist find, it's a 90hp force powerhead. The middle piston broke a ring, but everything else checked out. Best of all I picked it up for $50 bucks and made a new friend!! I will finish up some things on the 75 tomorrow and post pics Thanks for the input fellas.

 

[tater76]

Re: Can a 1974 Chrysler75hp block be used to rebuild my 1989 Force 85hp block? UPDATE

Sorry for the lack of updates fellas, here is the progress so far. I drilled and tapped the 5 holes to mount the bearing cage, finished the final fitting of the 1/16" spacer plate, sealed the spacer and pressed in the seal, mounted cage, mounted stator, mounted coil/cdi plate. Now, you can see the bare aluminum on the block behind the stator in one of the pics. This is where I inletted the block according to the later model. After fitting the flywheel, it looks like the indent was not needed Oh well. However, there were more mods that were needed upon connecting the tower to the trigger linkage. The height difference in the 75hp block made the tower a little too short to make connection with the trigger linkage. I pondered a bit, then decided that because I had two plastic trigger linkage parts, I could afford to alter one. I then filed down the black plastic end peice just where the attaching screw went into it so the tower arm would not bind (Hope You Can See It In Pics). Oh, and the rectifier mounting holes need to be either redrilled and tapped, or do like me and use two of the original screws, and open up the mounting holes on the rectifier a little. Then line up one of the original holes on the block, screw it down, now redrill a new hole for the other mounting screw, tapp, and you are in business. Once again see pics I hope to attach the powerhead to the leg tomorrow, then it is just a matter of a few steps to light it off. As always, I will report back with more pics.

 

[lucas_chrysler_1]

Re: Can a 1974 Chrysler75hp block be used to rebuild my 1989 Force 85hp block? UPDATE

looks nice. I like it! i see you put the 85 hp intakes and reeds for max hp. GREAT JOB!

 

[tater76]

Re: Can a 1974 Chrysler75hp block be used to rebuild my 1989 Force 85hp block? UPDATE

Yeah, I figured why not try to eek out as much potential out of this thing as possible I have a set of 90hp reeds that may be fun to try too The big thing tonight though is that the powerhead is sitting atop the leg!!!! woooohoooo!!! Tomorrow she gets attached to the boat, and if I'm lucky maybe a test run in the barrel. More pics to come I think this one deserves a name.... maybe....FrankenForce2 after my 55hp FrankenForce1 below?