Stihl West German O24 AV Restore / Upgrade to Ported 44mm

[Paul Fithian]

Had a bit of fun, learned new things, and got to know some wonderful folks upgrading my vintage 024 AVS in this thread:
https://opeforum.com/threads/stihl-west-german-o24-avs-upgrade-to-44-7-bore.27703/

Decided to have another go at it to build a 16" saw with some leftovers from that upgrade. Received this dead 024 AV the other day from an eBay seller, built May 1986 in West Germany. Six months after the saw referenced above. Parts and fasteners are identical, for example slotted screws used in many places instead of Torx.


Cleaned up real nice:



Bearings and seals are tight, zero side play in the crank.

I had hoped to restore the Mahle cylinder it was built with using Randy's method, however it was gouged prettty bad, through the plating and into the aluminum. Squish measured 0.053" without a gasket.


I installed NWP 1121 020 1200 P/C (used previously on the 024 AVS) on the 024 AV without a base gasket. Plan is to run it a bit and maybe upgrade with an OEM piston/cylinder later, perhaps with some mild porting.

Further cleanup revealed the fuel tank has a crack in it, looking for a similar vintage tank and a set of 024 top/filter covers. Installing a new 194 carb as well.

 

[drf256]

Is it an AV or AVS? I believe there is a 2mm stroke difference, so not only will squish be excessive, cylinder timing will be all over the place.

I have a few of those AV’s. If you need a parts saw let me know.

Make sure you vacuum test it. Need to figure out why the original cylinder failed so you don’t ruin another.

With the shorter stroke, she might rev to the moon.

This was my converted AVS with an 044 carb.

 

[Paul Fithian]

Measured a few things and took some more pictures. Squish with the no base gasket NWP 1121 020 1200 P/C on the 024 AV is 0.046" This measured 0.020" on the 024 AVS without a base gasket. I ran that 024 AVS for over a year with this NWP P/C and a 0.040" base gasket, resuling in 0.060" assembled squish, right? Saw ran perfect and I cut a lot of wood with it using a 16" bar. Burn pattern inside the cylinder looked perfect also.



From the parts catalog PDF I have looked at, there is a different P/C between the AV and AVS. So I ran the AVS with the AV P/C from NWP:
AV: 1121 020 1200
AVS: 1121 020 1201

I have on my bench:
- As built 024 AV Mahle gouged cylinder and scored piston
- As built 024 AVS scored piston. I tossed the as built cylinder a year ago



Pistons appear to be the same height overall, maybe the wrist pin location is different relative to piston top? Any suggestions for measuring to validate any differences?

I'll put a timing wheel on this to see what it shows with the no base gasket NWP P/C.

 

[drf256]

The pistons are the same. The jug is the difference on the S.

There is a 2mm stroke difference on the AV vs AVS, so 1mm up and 1mm down. Should be a difference of .040 on the squish with the saws. It’s possible the case deck height is different on the saws.

Thank for taking the time to do the measurements. I still wanna try a 44mm jug on an 024AV.

 

[whitesnake]

Is it an AV or AVS? I believe there is a 2mm stroke difference, so not only will squish be excessive, cylinder timing will be all over the place.

I have a few of those AV’s. If you need a parts saw let me know.

Make sure you vacuum test it. Need to figure out why the original cylinder failed so you don’t ruin another.

With the shorter stroke, she might rev to the moon.

This was my converted AVS with an 044 carb.

View attachment 359102 View attachment 359103 View attachment 359104 View attachment 359105 View attachment 359106

That’s some crazy compression!

 

[Paul Fithian]

I should have timing numbers on this setup in the next few days.

Had a look at some numbers and have questions on measurements around distance from base to squish band on various cylinder options. I was able to measure the Mahle 024 AV 42mm cylinder this vintage saw was built with:


Table of data I have:


Several postings I've seen show a 44mm jug on an 026 has piston interference with the squish band without a base gasket, and a 44.7mm has ~ 0.020" clearance. Suggesting that the 44mm jug has a shorter base to squish dimension.

If an 026 44mm OEM jug measured 2.367" base to squish, it would have 0.020" squish on this 024, right?

 

[drf256]

I can only get you close on the numbers for an 026/260 with no gasket.

It is a difficult measurement because if the piston/chamber geometry. Both the tiny squishband and the piston crown are arced.

I cut the band and piston flat. For Meteor 026/260 slugs I take .045 off the piston crown which leaves a very slight rounded bevel at the piston edge. It leaves around .010 of crown thickness in the center of the slug. I was never comfortable with that amount, but I have never seen one fail, even with 275psi of compression and timing advance.

The height from the base to the band, with the band cut square and .015 squish and no gasket is 2.467 inch. With a Meteor 246 piston it’s 2.413 with no gasket. These are on an 026/260 case with stock crank.

It should be the same with an uncut chamber/piston at the band if the factory machining matches up.

It’s tough to measure accurately with the stock band because of the geometry of it. My depth micrometer has a rounded measuring tip which doesn’t enable me to ready accurately touch the band/cylinder interface. You can try to measure with a caliper flipped upside down, but it’s not as accurate.

 

[drf256]

Your base to band measure is a factor of stroke, conn rod length, case deck height and piston compression height. It should be relatively constant between the cases of the same model and be independent of the jug. The height is the height.

Your numbers between the AV vs AVS on the 42mm NWP don’t make sense to me. They should be .039 off, not .026 off. It’s possible that the 42mm piston and the 44.7 have different compression heights. I’ve seen meteor vs stock be as much as .008 different on a few models.

Using the thinnest solder possible (I use .6mm which is around .025) and spinning the saw over with a drill motor is what I find gives the most accurate readings of squish.

I usually wind up with 16-17 thou on a gasketless 44.7 stock jug, so it makes me wonder if you’re using too thick a solder to check it and you are getting some solder spring back.

 

[Paul Fithian]

Mystery continues . . . I measured squish again using some twisted 0.030" electrical solder with the NWP 024 P/C mounted on the 024 AV case without a gasket and got the same result.

From this thread: https://opeforum.com/threads/stihl-024-to-026-needs-some-tlc.3431/ WoodDoctor measured 0.038" squish when he mounted an 026 44mm P/C on an 024 AV with gasket. So this would give ~ zero squish without a base gasket, right?

Timing wheel should be here tomorrow to check values for the NWP 024 AV P/C currently mounted. Next step would be to mount a stock 026 P/C on this 024 AV and check squish / timing numbers to see what they show.

 

[drf256]

Mystery continues . . . I measured squish again using some twisted 0.030" electrical solder with the NWP 024 P/C mounted on the 024 AV case without a gasket and got the same result.

From this thread: https://opeforum.com/threads/stihl-024-to-026-needs-some-tlc.3431/ WoodDoctor measured 0.038" squish when he mounted an 026 44mm P/C on an 024 AV with gasket. So this would give ~ zero squish without a base gasket, right?

Timing wheel should be here tomorrow to check values for the NWP 024 AV P/C currently mounted. Next step would be to mount a stock 026 P/C on this 024 AV and check squish / timing numbers to see what they show.

There are different gaskets, even for same model at times. They vary in thickness and material.

The typical metal oem Stihl gasket is .017. Paper gaskets are usually .017 or .034. I say usually because they really are all over the place.

 

[Paul Fithian]

Finally had a chance to install an 026 Mahle 44mm cylinder with a new Meteor piston on this 024 AV case.

Squish with no base gaset was 0.036":


Set up a timing wheel:


and got these results, with comparison to the NWP 024 AV P/C installed on this case:


This should run well with the above port timing numbers, right? Any suggestions for improving these numbers for the Mahle/Meteor 44mm setup? Perhaps shave ~ 0.015" from the base to reduce squish?

 

[Agent Smith]

Nice doing business with ya on ebay Paul. Someone will chime in with some good advice soon I'm sure

 

[Paul Fithian]

Nice doing business with ya on ebay Paul.

Ditto! The Mahle 44mm cylinder you sent looks very nice!

 

[drf256]

Finally had a chance to install an 026 Mahle 44mm cylinder with a new Meteor piston on this 024 AV case.

Squish with no base gaset was 0.036":
View attachment 360558

Set up a timing wheel:
View attachment 360559

and got these results, with comparison to the NWP 024 AV P/C installed on this case:
View attachment 360561

This should run well with the above port timing numbers, right? Any suggestions for improving these numbers for the Mahle/Meteor 44mm setup? Perhaps shave ~ 0.015" from the base to reduce squish?

Interesting. Those are really good numbers for the exhaust. I’ve always wondered about this swap.

The shorter stroke is creating less exhaust duration with the same physical ex roof height.

You could definitely take 16 off the base and be at 20 squish. That would lower the exhaust roof even more (I bet 101) and give you more intake as well.

The only issue is your transfers. You’d really want to raise the uppers to around 120.

 

[Paul Fithian]

Thanks Doc! What got me thinking this would work is that the NWP P/C revealed different squish measurements 0n my 024 AVS case vs. this 024 AV. And that a 44mm on an 026 would have piston/squish band interference, but clearance when a 44.7mm was installed. All the references I found said you couldn't put a 44mm on an 024 AV due to the stroke difference with an 026 or 024 AVS, but I've never seen any documentation of squish or port timing of that.

As a learning experience in porting, I will sand the NWP 42mm base on a flat surface to obtain ~ 0.020" squish, then modify the ports with my new toys:


I'll run the saw like that once the porting work is done to see how it operates. More to follow.

The Mahle 44mm cylinder will be sent out to a professional to make sure the base is cut perpendicular to the bore, then I'll modify the ports accordingly.

And thanks again for everyones help here. I didn't know what squish was until a few months ago, and now I own a timing wheel and porting tools!

 

[Paul Fithian]

I set up the timing wheel with the NWP 42mm P/C and verified timing. I sanded the base a bit, didn’t want to invest further time in it as this is a practice jug for learning to port. Squish is 0.045” with no base gasket.

Here’s the ring at 120 degrees, barely at the top of the transfers.


Any suggestions on how to shape this port at 120 degrees? Widen it?

 

[drf256]

Your flywheel side upper is already opening earlier than 120. You also have huge bevels and open ports, which tend to be sluggish because there is more volume to them. Harder to get true direction out of them.

If you just want to practice cutting an upper then do just that. Raise them both to 118 and remove that huge bevel.

If you are looking to make that jug run well, I’m not sure what to tell you. That’s a sow’s ear buddy. The zebra striping of the plating is common in AM jugs. It’s from the cheap thin plating wearing through. I’ll see if I can get a pic.

If you just want to play, get a cheap 44mm jug with closed ports on eBay.

 

[drf256]

Here’s a hyway jug. Look familiar?

 

[Paul Fithian]

Agreed Doc, this NWP P/C is only being used to learn porting. For sure I won't run this one long term

Had a chance to do some grinding early this morning, how'd I do on my first port job? I was very careful not to touch the exhaust or intake at the cylinder wall. Not much work on the intake other than to take out sand cast finish that was on it.





Unless advised otherwise, I will build the saw up like this and run it for a while until I can install the Mahle 44mm that is out for base shaving.

I'll work on my grinding skills with the scorched closed port Mahle 42 mm that came with the saw.

 

[drf256]

Hard to draw on the pic, but try to square up your corners. Even with the smallest burrs, you’ll leave enough of a rounded edge to not tear up the rings.

Much better than my first attempts.