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Bridgeless transfer ports on Stihl 026 / 260 oem

NickC

NickC

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Thanks.

So far as I can measure these jugs have already got a smaller combustion chamber, to check that theory I'll do a dry run comparison (with wet rings) with this jug against the new old stock Mahle to compare ballpark compression.

Then figure out some sort of fixture for the lathe to cut the band...
 
NickC

NickC

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So... Just checked the three cylinders, doing the same every time with a drop of oil spread on the rings and a few spins to get the oil around, put the tester in and give it six firm pulls.

Stock Mahle Stihl - 165 psi
Stock Brazil Stihl - 164 psi
- 0.01" ish base cut Brazil Stihl - 181 psi

So, my thinking the compression may be up stock is wrong, the cylinder 'ceiling' is lower but the compression chamber must be a bit bigger.

I know that this test isn't perfect but I've read that a wet compression test is a good indication of what you'll get when the saw is run in? It looks like I've got a decent gain. I'd imagine that cutting the band and base by the same amount would give good gains. I'll have a think about a fixture to hold it.
 
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huskihl

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So... Just checked the three cylinders, doing the same every time with a drop of oil spread on the rings and a few spins to get the oil around, put the tester in and give it six firm pulls.

Stock Mahle Stihl - 165 psi
Stock Brazil Stihl - 164 psi
- 0.01" ish base cut Brazil Stihl - 181 psi

So, my thinking the compression may be up stock is wrong, the cylinder 'ceiling' is lower but the compression chamber must be a bit bigger.

I know that this test isn't perfect but I've read that a wet compression test is a good indication of what you'll get when the saw is run in? It looks like I've got a decent gain. I'd imagine that cutting the band and base by the same amount would give good gains. I'll have a think about a fixture to hold it.
Do you have a 4 jaw independent chuck for your lathe?
 
Agent smith

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Has anyone got any good timing numbers for an 026?

I'm going to have a go at one of these Brazil pots, got my depth gauge working and found that the outer ring is a bit higher by 0.25mm (less squish) but the roof is about 0.25mm lower and a close match to the piston crown. There is very little step compared to the Mahle.

I've cut the base to get the squish down, will fettle the ports and having sourced a right angle handpiece for a Foredom I fancy having a proper go at it......
I dont know about on those new cylinders you got there but on the old style cylinders after you cut the base/squish 98/118/77 works pretty well
 
Yotaismygame

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Fixtures are easy to make. Guy On eBay sells all the aluminum you’ll ever need. Drill some holes, cut down some all thread and you’re good to go. getting the jug square is the difficult part when you first do it.

6B1998D5-194D-4BD9-B049-A75DFCF0A2BF.jpeg
 
Ketchup

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I’d have to check my notes. I believe I take .050-.055 out of the band, cut the dome flat, and cut the base to get .018” squish
Yep. You’ll make much better compression with a flat squish band and piston. It also gives you room to set a more agressive blowdown. I didn’t cut as much out of the band to keep intake higher.

The advantage of closed transfers is that the runner can be routed away from the piston to better direct flow when it exits. Open transfers can work fine, but nobody tries to rout them far from the piston. To just hog out the separator will greatly increase case compression and runner volume. You'll have a lazy engine that won't rev - you'll have low transfer velocity and poorer mixing & loop scavenging.
My thoughts exactly. Lacking stuffers, the only option is more intake. Widen the hell out of the intake, and put an 044 carb on it. Keep the Intake relatively high. You have to feed that bigger volume but you want the intake closing early so the case will gain transfer pressure.

I ported a closed transfer 026 about a year ago for and friend. Final numbers were 100/122/80. 0.019 squish. My notes say .040 out of the band, but I think they’re wrong. Exhaust and Intake widened to 1.010”. It’s not a race winner, but it pulls 20” 3/8, so he was happy.

I think good numbers for open port would be roughly 98/120/77. Flat piston/band, 0.018 squish. Holding high RPM will be more important in an open transfer, so the exhaust may want a touch more height.

Just my thoughts. Seems like you’re going to make a nice runner out of it.
 
NickC

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Thanks for the replies guys, I get all the points made. I don't want to go down the route of 044 carbs, this one at least is for a mate so I don't want to cut it around or make it finniky to run.

I did see a comment that you can work on the original carb but no details, I have a lathe, mill so can do most things.

Will get the band cutting fixture made, to cut the band flat on this jug will result in a significantly wider band than a Mahle for the same depth of cut - I think. Needing a similar cut on the piston which may take too much material from the dome. I'll have to look at that, but I'm thinking cut the band following the existing shape?
 
82F100SWB

82F100SWB

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I really didn't think I moved the exhaust up that much, width is a different story...
Band is cut, base is not, gasket delete only. Kind of wish I had of put the wheel on there before I started grinding now.
 
Ketchup

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I really didn't think I moved the exhaust up that much, width is a different story...
Band is cut, base is not, gasket delete only. Kind of wish I had of put the wheel on there before I started grinding now.
That sounds about right. My jug was 94 exhaust to start, so raising to 91 wouldn’t take much grinding
 
NickC

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Fixture made, just waiting on some long bolts.

This is what I mean about the difference in the two jugs, both have 0.02" squish at the edge. If I take a 0.04" cut the band on the Brazil jug will be at least twice as wide as the Mahle, checking with a depth gauge marked with black sharpie.

Good for compression, but I'm already + 15psi. What is more of a concern is the amount of thickness I need to take off the piston dome for that width of cut, what's a "safe" thickness of there is such a thing? I think it'll be ok, and I haven't accurately measured the thickness yet, currently looks like about 0.1" thick at the inside of where the band will be cut.

Perhaps try 0.02" and see.

I've got plenty of overlap on the piston skirt so have no concerns about free porting on a 0.04 cut.
 

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Yotaismygame

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Hmmm I don't really see a difference except for the one has already been sanded or cut on. I don't know about others but I never take bigger than .015 cut at a time when machining cylinders. I would do the .040 if not more. You'll end up loweing the exhaust about 5 degrees or so. So when you bring it back up you'll lose that compression.
 
NickC

NickC

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The jug chamber's are different shapes, that's why I'm getting + 15psi on the Brazil for the same squish.

I set a depth gauge to 0.04 over the squish, the black sharpie mark is where the depth gauge touches, on the Brazil jug it's twice as far from the jug wall so a wider squish band for the same cut so I'll have to cut further into the diameter of the piston to match it, as it's domed it may get a bit thin.

0.04 was mentioned, not sure if that's what I'll do straight off.
 
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