dilemma - turbo crank in a 3.2 motronic?

brp914 · 01-21-2012, 03:03 PM

I'm building a 3.4 motronic engine - or so I thought. I have the case closed and cylinders mounted. I go to mount the flywheel and see there is no receptacle in the crank for the flywheel roll pin. Also, the 3 bolt holes for the pilot bearing are not there. I did not try to mount the flywheel. I may have been sold a turbo crank. Now what? Can a turbo crank be used in an otherwise motronic engine? Can a turbo crank be used here but just not with motronic? Or is this combination just not compatible?

Flat6pac · 01-21-2012, 06:37 PM

The crank youre trying to use is the 78/79 turbo with the small pressed on the flywheel pilot bearing. What you thought you were buying should be an 80 and later with the 3, 6mm allen head bolts for the pilot bearing, then you would be able to use the 84 to 86 flywheel.
Bruce

brp914 · 01-21-2012, 07:37 PM

Does this mean I need to get a turbo flywheel and pilot bearing or does this engine need to come apart? Turbo flywheel means no motronic. Will a turbo flywheel work with 915 clutch and gearbox?

Flat6pac · 01-22-2012, 10:07 AM

You need to get an 80 or later 3.3 or 3.2 crank for the motronic flywheel application inless you could drill the crank flywheel end to receive the 3 bolt pilot. I know the 3.0 80 to 83 crank can take the 78.79 flywheel but I dont know if the early crank can be drilled for the 3 bolt pilot bearing.
Bruce

You have a PM too

brp914 · 01-22-2012, 11:09 AM

Well, I test fitted the flywheel - no problem. The bolt holes line up and the raised circle in the center of the end of the crank exactly fits in the motronic flywheel. I then test fitted the sensors and they line up fine. It appears the only problem is the missing bolt holes for the pilot, unless there is some other issue I'm not aware of.

aws · 01-22-2012, 03:29 PM

brp914: When this happened to me quite a few years ago I was up against some time deadlines and couldn't find a used correct flywheel. As a result I very carefully aligned the pilot in the flywheel and tigged the flange of the pilot to the flywheel in 4 small places around the pilot to hold it on the flywheel. It has been running that way for over 12 years with no problems. ----- You would be better off finding a good used early flywheel.

aws

turbo nut · 01-22-2012, 05:16 PM

Mine is also tig weled in place in three places on the flywheel. Works fine no problems.

Flat6pac · 01-22-2012, 05:52 PM

That is basically what my suggestion was if we had talked.
Bruce

Henry Schmidt · Henry Schmidt's Garage

The end of the crank can be drilled to suit the correct flywheel.
The crank is case hardened so once you're through the first few .001" the drilling gets easier.
The dowel pin locates the flywheel to insure proper/consistent Motronic timing.

brp914 · 07:12 PM

Bruce, I called but you were out.

AWS - When you say correct early flywheel, do you mean a non-motronic turbo?

I see how it could lead to disaster, but attempting to drill and tap is out?

Henry Schmidt · Henry Schmidt's Garage

Quote:

Originally Posted by brp914

I see how it could lead to disaster, but attempting to drill and tap is out?

Why?

safe · safe's Garage

Why use a turbo crank?
It has the same stroke as the 3.2 crank, that I assume you already have??

aws · 01-23-2012, 04:54 PM

The early nine bolt flywheels used the early press in pilot bearing....at some point they changed over to the 3 bolt flange pilot---I am not sure what year--- maybe in 1984 when they started using the 74.4 crank in the Carrera. The 930 flywheels use the 240mm clutch--I am pretty sure you are using the 225mm clutch--- Henry (as usual) is right--you can drill and tap the crank for the 6mm bolts, but I understand your hesitation---I have broken enough taps off to be paranoid myself.
aws

Flat6pac · 01-23-2012, 05:42 PM

The problem isn t the crank but the need to properly fit the Carrera flywheel when the crank isnt made to use the larger pilot bearing needed for the Carrera flywheel.
Mig the bearing in place on the flywheel and finish the build.
Bruce

Henry Schmidt · Henry Schmidt's Garage

I've seen the welding method result in a cracked flywheel. If it cracks and the crack propagates, it is possible that the flywheel will explode resulting in a catastrophic failure.

brp914 · 01-23-2012, 09:37 PM

Well, I don't have a welder and transporting the engine is very impractical. I would think that welding would need to be done with the flywheel mounted so as not impact seating on the crank.

So I think I'll need to try drill/tap - gulp.

But possibly another way: make a plate to fit under the flywheel bolts. Maybe 2mm thick. Tap the 3 pilot bearing holes and put screws into the pilot thru the plate. Maybe also put loctite sleeve retainer on the circumference of the pilot for good measure. I could get longer 12.9 bolts to mount the flywheel from mcmaster carr, but they only have full threaded in this length, which they say are not as strong. I then looked at ARP. They have the right length, but they don't say if they are full threaded. Also, they rate theirs at 180,000psi - I don't know how to relate that to 12.9.

brp914 · 01-24-2012, 07:23 AM

Safe - this engine is being built from my parts stash. I have an undersize crank that I'll keep for emergency or sell. I was sold the turbo crank with the understanding I could use it with motronic. I toil in cubicle-land by day and did not know these details.