Stihl West German O24 AVS / Upgrade to 44.7 Bore

[Paul Fithian]

I have 44.7 oem P&C if interested.

PM sent, please advise details on these

 

[drf256]

You have to go through your saw and figure out why it’s ruining top ends before you do anything else, or you’ll be in the same boat with the new top end.

That NWP looks like worn through plating. Seems to not be uncommon with AM top ends.

I’ll take a look at the PM now.

 

[Woodpecker]

Asking for Nic.

In that case used tranny fluid at 33.666667:1

 

[cus_deluxe]

In that case used tranny fluid at 33.666667:1

you are the tranny expert…

 

[Woodpecker]

you are the tranny expert…

I thought Ed was the tranny man

 

[cus_deluxe]

I thought Ed was the tranny man

hes into rods and cranks mostly i heard….right @redline4 ?

 

[redline4]

I thought Ed was the tranny man

hes into rods and cranks mostly i heard….right @redline4 ?

I only work on straight transmissions.

 

[Paul Fithian]

I trimmed the ragged melted front edge of the top cover and cleaned it up a bit.


Instead of trimming the two rear top cylinder fins, anyone else try trimming the inside of the top cover like this guy did?

 

[drf256]

I trimmed the ragged melted front edge of the top cover and cleaned it up a bit.
View attachment 355635 View attachment 355636 View attachment 355637

Instead of trimming the two rear top cylinder fins, anyone else try trimming the inside of the top cover like this guy did?

View attachment 355638

Looks great.

Don’t ruin that cover. If you’re gonna go that way, I can swap you out a good set of oem 026 covers. The 024 covers look much better IMHO. 026 covers allow for the larger airfilter as well, but unless you’re porting the saw you won’t need it.

In the above video that guy wasted a lot of time on that jug. As they say, can’t make a silk purse out of a sow’s ear. Nothing beats an oem jug.

 

[drf256]

Found this old thread.

https://opeforum.com/threads/my-buddy-daves-ms260-and-an-am-vs-oem-jug-comparison.2606/

Went on for a bit, but in the first few pages one can easily see the transfers on OEM vs AM.

 

[Paul Fithian]

Brief update, Meteor kit has been returned. Following Doc’s prescription for this upgrade, more info in a week or so.

I greatly appreciate everyone’s help, adhering to one of my beliefs:

Understand what you don’t know, get assistance from those that do.

 

[Paul Fithian]

Received Doc’s Mahle piston & cylinder. Much higher quality than the NWP 42 mm P/C I removed:


Measured squish without base gasket:


Next step is to shave down the rear fins. New OEM piston pin bearing ordered.

 

[drf256]

Hope u didn’t toss the paper towel that was in the jug. There was an oem pin bearing in there.

 

[Paul Fithian]

I did, but retrieved it. Thanks!

 

[Paul Fithian]

Milled the 2 rear fins so the original cover fits:

 

[Paul Fithian]

I had planned to share a compression test picture in this post, hoping for 200 PSI +. I was feeling pretty good about the assembly and how the Motoseal went on, thin coat painted with an acid brush.


Discovered a new problem after the cylinder was torqued down and I rotated the flywheel, could not get to BDC.

It appears the 44.7 mm piston OD interferes with the crankcase ID. This may be unique to my saw, however not sure if anyone else has tried to fit a 44.7 P/C on an 024 AVS.


There is some mismatch at the bore ID in my crankcase halves:


After some consultation with Doc, plan is to relieve case a bit with a die grinder and burr at the 11-1 and 5-7 o'clock positions to enable piston/crankcase clearance. Any other suggestions?

 

[redline4]

I had planned to share a compression test picture in this post, hoping for 200 PSI +. I was feeling pretty good about the assembly and how the Motoseal went on, thin coat painted with an acid brush.
View attachment 357540

Discovered a new problem after the cylinder was torqued down and I rotated the flywheel, could not get to BDC.

It appears the 44.7 mm piston OD interferes with the crankcase ID. This may be unique to my saw, however not sure if anyone else has tried to fit a 44.7 P/C on an 024 AVS.
View attachment 357541

There is some mismatch at the bore ID in my crankcase halves:
View attachment 357542

After some consultation with Doc, plan is to relieve case a bit with a die grinder and burr at the 11-1 and 5-7 o'clock positions to enable piston/crankcase clearance. Any other suggestions?

I've seen that on a couple of my Stihls.

I'd take a burr to it too.

 

[Paul Fithian]

Thinking a bit more on how to attack the crankcase with a die grinder/burr and not mess it up.

Planning to remove the fuel tank and brake bail arm to enable better access.

I can make up a steel plate with 3 holes:
- Two small ones to bolt it to the crankcase flange
- One large 1 .875" diameter (47.62 mm) as a guide for the burr. This would give ~ 1.5 mm/side clearance between the bottom of the piston OD and the case ID

I can mount the plate and put a depth stop on the mill to prevent going below the desired depth. The plate will prevent the burr moving beyond the desired circumference.

Would 1.5 mm/side clearance be enough to prevent piston contact at high revs? Or should I go more? A 2" (50.8 mm) hole in the center of the plate would provide ~ 3 mm (0.125") clearance per side.

 

[Shane¹]

Thinking a bit more on how to attack the crankcase with a die grinder/burr and not mess it up.

Planning to remove the fuel tank and brake bail arm to enable better access.

I can make up a steel plate with 3 holes:
- Two small ones to bolt it to the crankcase flange
- One large 1 .875" diameter (47.62 mm) as a guide for the burr. This would give ~ 1.5 mm/side clearance between the bottom of the piston OD and the case ID

I can mount the plate and put a depth stop on the mill to prevent going below the desired depth. The plate will prevent the burr moving beyond the desired circumference.

Would 1.5 mm/side clearance be enough to prevent piston contact at high revs? Or should I go more? A 2" (50.8 mm) hole in the center of the plate would provide ~ 3 mm (0.125") clearance per side.

What was the clearance with the other piston? Check that and make it similar it didn't hit anything with that one

 

[Paul Fithian]

Good point, Shane. We can do that without measuring, assuming the 44.7 mm piston just barely contacts the ID of the crankcase now:

44.7 - 42 = 2.7 mm/2 :: 1.35 mm per side clearance with the OEM 42 mm jug to the crankcase.​

So using a 1.875" metal plate template would be the right size, enabling 1.5 mm (0.060") clearance per side.

Common belief that at least 0.020" squish is needed to ensure the top of the piston does not contact the squish band, right?

So 0.060" clearance per side should be more than enough, right?