390 case question?

I worked on most of the 390's now that I think about it does the 390 crank fit in the 250 case? I rebuilt them but never changed the cases. Are all the late 70's cases the same?
Will the 390 crank fit in the 83/84 250 case?

the primary kick cases come in 2 flavors, 250 and big bore. the difference is an extra web in the crankcase. you could remove the web if you wanted and you would have a big bore case. i would imagine its the same for the earlier motors

I think the early 250 case is the same as the early 390 case. This would let the 390 crank fit in the newer 82/84 250 case "if" the 390 crank is the same with outside to outside on the counter weights. The ignition shaft needs to be changed to the later shaft so the smaller motoplat ignition can be used.

No it will not fit.The 390 has a diameter that is too large for the 74-77 cases

I want to use the newer 82/83+ case for the 390 crank and cylinder. The balancers appear to be the diameter of the 250 case rather than the 430 case.

Sorry to be jumping around but the different options are worth looking into. I have two 430 cases right now. I could end up with a 430 and a 500 but I would like to have a 390 too. Just thinking what will fit in a 80 to 84 AC frame.

I have already had the 390 engine mounted in a WN frame before I pulled it down to fix the internal issues. It goes in with 79/80 exhaust and would only need to fab front engine mounts unless the ones off a 1980 390 would work. You use the rear engine mounts from the 390 as they locate on the swingarm pivot

Go to, www.husqvarna-parts.com

They list frame and engine case numbers. You will find the early 250 cases are the same year different number as the 390 cases. This tells us what fits what and interchanges.
I ran into this year's ago with my three 390's.

The '78, '79, '80, '81 of the late 70's cases should fit the 390 crank. I believe there was one fits all cases at this time but match the 250 case year to the 390 cylinder year the stud location , base gasket and transfer ports changed through these years. The newer '82 250 case has a double wall we can remove to fit the 430/500 crank shaft.

Darn I love these bikes.

Bill

I measured the ID of my 390 crankwell and that of the 84 250WR and found there is only .050" difference(250 case smaller). If you bored out the 250 well it would be thin. I would suggest if you really want to do this would be this; Knock out the web to make it 430 capable, then press on a custom sleeve for the 390 crank. Lower end volume is key here

I don't have any 390 stuff. I would check the length of the crank pin. My auto stuff has pork chop shape crank wheels and narrower cases than standard shift 250--400-500 mentioned above. 360 has pork chop shaped crank as I recall. 420 auto crank cavity is much smaller than 430 auto.

How about turning down the crank then replacing some steel with heavier material in the right spots to bring it into balance?

Will the 390 crank fit in the newer 250case? Where is it .050" off on the crank diameter?
The last 390/420 cylinder has the same stud location as the newer case. You need to open up the cylinder transfer ports. The 390 crank with the larger transfer ports plus if we port the cylinder to the husky specs she should be a wicked ride.
I have an extra 430 case with a 430 crank. And I have a complete 430 engine. But I'd like to build a 390/420cr.

The 390 crank in the larger 430 case would have less pressure in the intake through the transfer ports right?

Why do you ask? I have a good 420 cylinder and the head has a compression release.

Just some food for thought. I had built some old 283 Chevy v8's that ran better than the 327/350's. The smaller pistons with the shorter stroke ran better. It reved quicker and higher.

It is the diameter of the cavity the crank cheeks . The crank OD of the 390 crank is .050" too large for the 82> 250 crankcases. If you remove the web the cavity will be too large for the 390 crank hence the suggestion to sleeve it down. It has been done here before(sleeving the crank cavity). My 390 crank has the same stub ends that my 82 > engines have and the thicknes over the crank cheeks are identical within .5mm

Frank: If you turn down the crank cheeks the con rod big end will now protrude above the OD of the crank and you would need to cut the same kind of clearance groove that Andy did to make the 300WR on the 250 engine

That works better on the brittish twins. There may be more complications like weakening the hole the pin goes in, just a thought.