Discussion in 'Chainsaw' started by NickC, Oct 31, 2019.
Gotta have meat with yer taters in our hard woods.
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...
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?
Yes, got most things. Seen pics of someone using a plate in a 4 jaw tapped to match mounting holes, then long bolts put through.
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
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.
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.
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.
Threw the wheel at the saw in the video above, 91/115/74, with 16.5 thou squish.
91! wow. What’s comp like?
Compression is right at 187. Turned out pretty good for something I winged completely.
Thanks for taking the time to do that, raising the exhaust that much and retaining good compression I guess you cut the band etc.
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?
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
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.
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.
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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