372/385-390 hybrid. Is it possible? Been done?

[P.M.P.]

That's some awesome info right there! I have most everything here to build such a hybrid.

Do it

 

[Stevetheboatguy]

Might also make a good stroker with a 385/390 crank and a xpw or 52mm am top end if the 385/390 top is to far off to work.

Someone needs to buid one. I need a 385/390 donor. I sold the last two I had before this idea crossed my mind.


Steven

 

[beaglebriar]

Maybe @awol could add a little more insight?? [emoji1696][emoji16]

 

[beaglebriar]

Might also make a good stroker with a 385/390 crank and a xpw or 52mm am top end if the 385/390 top is to far off to work.




Steven

Not a bad idea.

 

[dieselfitter]

I know of a guy with a 372 that has 38mm stroke. The crankshaft fits in an un-modified case.

 

[tree monkey]

I know of a guy with a 372 that has 38mm stroke. The crankshaft fits in an un-modified case.

and what do you know about saws? well?

 

[dieselfitter]

and what do you know about saws? well?

Only what i've read on the interwebs

 

[fossil]

Only what i've read on the interwebs

You're being rather modest.

 

[Mattyo]

Assuming you can use the 6003 bearing....what happens to the pto side seal / bushing? Seems almost easier to wallow out the bearing pocket and use the 390 pto bearing which has the integral seal. Assuming you can machine the pocket to an interference fit.

 

[XP_Slinger]

Maybe a different route to go could be 281 cylinder with a 272 piston. Pin is 12mm, and 272 uses 6202’s just like the 372. Lots of stuff to measure and wag out but could be another way to skin this. I’ve got most of the parts to try this, I’ll do some looking today. Only downer if you wanna call it that is you’ll only get 77cc going this route. Bigger bore but 2mm less stroke with the 372 crank.

Would be interesting too me to see how that closed port cylinder ran on the tighter crank case of the 372.

 

[beaglebriar]

I know of a guy with a 372 that has 38mm stroke. The crankshaft fits in an un-modified case.

Did you bore the 372 crank to accept a 395 rod?

 

[XP_Slinger]

Case needs to be opened up by 2mm to accept the skirt on the 281 cyl. Would also need to fill and drill for a mount here. And of course the transfer cut outs on the case would have to be dealt with by either filling them or building up on the base of the cylinder. None of which I have the equipment accomplish. I seriously considering talking to someone about some contracted machine work. Will feel like a cheater not doing everything myself though lol.

pin to crown height at 18mm on the 272 slug seems favorable for use with the short stroke 372 crank. 372 pin to crown is 16.5mm

 

[Mattyo]

my concern is more of how to do the crank seals, especially PTO side, assuming the larger crank is used, then the pto bushing won't work. to me this seems like the larger challenge.... beyond fitting a different top end.

I'm still trying to wrap my head around how to wallow out the bearing pockets. if I could accurately machine it, then you could use 390xp pto proprietary bearings on both sides ....

 

[jacob j.]

Just get a 26 x 17 seal for the flywheel side, you wouldn't need to run the 390 proprietary bearing on that side.

On the PTO side, a guy could machine the seal pocket out to 28mm and run a 28 x 17 seal, then machine a groove in the
stub shaft of the crank at the appropriate depth ala MS-661 and install a snap ring, and then run the 390 worm gear (that is
if the PTO stub shaft and shoulder on the 390 crank is a similar length as the 372 crank) - One possible scenario.

Or machine the PTO side of the case for the 390 proprietary bearing and run it on that side.

 

[Mattyo]

The 6003 bearing sure opens some possibilities. It's actually thinner than 6202s...so it'll fit more than flush in the pocket. And yes...much easier to machine the seal pocket appropriately.

Snap ring is an interesting idea as the clutch on a 372 tightens against the bushing... so...get it to tight to the snap ring .... hmm.

Lots to think of on the bottom end.

Thanks @jacob j.

 

[jacob j.]

The 6003 bearing sure opens some possibilities. It's actually thinner than 6202s...so it'll fit more than flush in the pocket. And yes...much easier to machine the seal pocket appropriately.

Snap ring is an interesting idea as the clutch on a 372 tightens against the bushing... so...get it to tight to the snap ring .... hmm.

Lots to think of on the bottom end.

Thanks @jacob j.

I wonder if it's possible to just machine the stub shafts on the 390 crank to match a 372 crank? I haven't compared the two cranks side by side.

 

[Mattyo]

Another intresting possibility. ..

 

[XP_Slinger]

my concern is more of how to do the crank seals, especially PTO side, assuming the larger crank is used, then the pto bushing won't work. to me this seems like the larger challenge.... beyond fitting a different top end.

I'm still trying to wrap my head around how to wallow out the bearing pockets. if I could accurately machine it, then you could use 390xp pto proprietary bearings on both sides ....

I was thinking more along the lines of using the 372 crank. But is the juice worth the squeeze is the question.

 

[Mattyo]

The 390 piston uses a 13mm pin. 372 12mm I think. Aside from the stroke issue... there are other issues

There's a lot to work out for sure. Would probably make most sense to use the 390 crank too but it's a pile of work I think. Lots of different little things to consider.


If i end up with some lathe time I'll see what I can come up with.

 

[huskyboy]

The 390 piston uses a 13mm pin. 372 12mm I think. Aside from the stroke issue... there are other issues

There's a lot to work out for sure. Would probably make most sense to use the 390 crank too but it's a pile of work I think. Lots of different little things to consider.


If i end up with some lathe time I'll see what I can come up with.

Maybe port a 390xp and slap a 372xp sticker on it? Would have to look closely to tell the difference at a gtg. Most average suckers (stihl guys) wouldn’t know the difference. Work smarter not harder. Lol