Chainsaw Repair
Husqvarna - Stihl - Poulan - Jonsered - Dolmar chainsaws and more => Husqvarna => Topic started by: alsled on December 25, 2013, 09:21:00 am
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I recently picked up a jug and piston and bottom end off a 55. Proceeded to put jug/ piston on a 51, with no base gasket it had a squish of .045. Seemed large so I put Jug/ piston on another 51 bottom end I had sitting around, the squish was .035 with no base gasket. For the hell of it I put the jug and piston on the 55 bottom end that the jug cam with, .030 squish.
So what parts are so out of wack that they can't maintain a reasonable tolerance? Just struck me as some poor QC.
In a endevor to get the squish at .022, I filled the case for 2.5 hours very carefully. Will be interesting when I fire it up.
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I have a thought, SawTroll could probably confirm. I seem to remember that saws designed for third world countries sometimes had thick base gaskets in order to cope with the crap fuel they have. Perhaps Husqvarna did something in the way of design e.g. deck height or rod length to cope with the fuel but I doubt it.
Did you measure the crank run out to see if perhaps the crank was twisted? Niko could probably confirm but I still think (as mentioned before) that squish tolerances on stock saws can be any where between .030-.050.
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I didn't measure crank run out, plus it would have to be .015 at that point. The saw with the biggest squish was running fine, .015 runout would most likely mean some blown up bearings with stuff out of wack that bad.
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The crank can be twisted and cause run out like that and the bearings still be good. The cranks will twist because of a sudden stoppage. They twist at the crank pin and can be re-aligned
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In a endevor to get the squish at .022, I filled the case for 2.5 hours very carefully. Will be interesting when I fire it up.
Are you saying you filed down the case instead of the cylinder ?
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Yes the case, it was a closed port jug. I dont know how I could file this?
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660,
to get .015 that would be .030 total runout, this saw ran fine, at .030 wouldn't there be some destruction going on very quickly. I know on my snowmobile engines once a crank hits about .004 total runout it blows apart
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A crank with much run out at all will turn hard because the inner bearing races have to **** with the crankshaft ends.
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660,
The .045 and .035 bottom ends spin free as can be.
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I would be finding another crank or else get that one straightened.
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Yes the case, it was a closed port jug. I dont know how I could file this?
Redneck Idea: If no lathe etc. Take thick plexiglass or something else similar and make hole where cylinder O goes in. Then attach different emery cloth sheets around hole to take down base by hand. Make sure sanding base area is level and flat.
Make sure to measure all 4 corners and keep all even all the way around once you are removing base to where you want it.
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^^^^^^^^^ :)
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3/4 scrap piece of cabinet grade plywood would probably work also, can also staple emery to wood.
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It's not my saw nor am I being picky about the methods but it's normal practice to carve the cylinder base down instead of filing on the case .The fact that it's a closed port transfer port designed engine has nothing to do with it ,so are most Stihls and Husqvarnas .
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Al,
Its all good, I never thought of doing the hole in the plexi, nor do I have access to a lathe, so i didn't think I could file the jug down with the protruding portion of the jug.
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To tell you the truth, I thought of cutting the rod and welding it back together, just a little longer.
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Never understood why there is so much talk about squish measurement...
If you change hight on cylinder the gain you get is from port timing, rather than higher compression.
I'd rather have bigger volume so I can get in more fuel.
To me it is better to get more fuel above piston that can create power.
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Magnus,
Don't knock it till ya try it, it works!!!! :)
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WOW Alan, that is some pretty radical thinking! Cutting and welding the rod, would not have worked out too well. I have seen it done by a few people 'just to see' before having a custom rod made. The welded rods did not last very long and when they came apart they trashed everything in their path!
Well good luck with it but just in case something goes wrong, why don't you try the filed cases with another cylinder, that way you won't ruin that pretty OEM closed port cylinder! Then if all proves out you can swap for the CP jug.
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And when you lower the jug or put a taller piston on you are not changing the volume between the top of the ports antop of head. To lessen the volume in this area, you would have to bring the head down, which is very hard to do as the jug and head is all one piece.
Compressing fuel air mixture to just a point before detonation will yield the most power.
Kinda the same as when you have a tired saw with low compression due to worn rings, re ring it and leave everything stock and it runs better again, reason, compression.
I was concerned a while back about some of these high compression ratios on a saw with 93 octane fuel, i think the inefficientcy of a chainsaw muffler design vs a real tuned pipe on lets say a dirt bike or snowmobile is the reason. I have built 800cc snowmobile engines that made 185 hp and over 100 ftlbs of torque. 125 cc dirtbikes were putting out over 30 hp. Chainsaw engines with the muffler can't come close.
Getting back to compression, the 185 hp engine could only have 120lbs of compression, due to when running the pipe design sucked more fuel and air in, we don't have the luxury of tuned pipes on a work saw, so squish the hell out of it!!!
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I got the squis down to where I want it, no rod cutting or welding needed, but still would be fun to try.
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Alan, good info/discussion here on compression!
http://chainsawrepair.createaforum.com/ported-saws/compression-tester/
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It runs,
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Good on ya, bud! Tell us more! Did you achieve what you were expecting?
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It feels good, nice little work saw, it has its place in what it has to do when called upon. Minimal running, let it cool down, its at 165 psi.
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No pressure or vacuum leaks after the filing? How did you keep the filings out of the bottom end?
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No leaks, stuff bottom end with oily rag. Then wash bottom end many times with gasoline and blow it out, add oil to all bearings before start up.
I was using a piece of a cutting tool for a flat edge and shining a light behind it while it was sitting on the case, along with checking squish every so often.
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Well, no matter how unorthodox a method it seems to have worked out for you. You had great luck with the C/P off e-Bay so I guess your luck is still holding. Perhaps you should think about a lottery ticket!
I would like to see that saw run one day and the method you used to deck the case!
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I need to ask because I've been wondering since you hand filed the case. How did you make sure the filed case was keep square all the way around? Any pics of the filed case?
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No pics of the filled case. Not really sure if the case is 100% square, I kept putting the jug on shinning lights and checking squish at 4 points.
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You mention 'a piece of a cutting tool' was it large enough to cover the entire mating surface between the jug and the case? Perhaps you could provide a picture of it. IIRC there used to be a large surface grinder I believe made by Blanchard that was used to precision surface grind things like the mating surfaces for hydraulic pumps for large machinery, is that the type of cutter you were using?
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I used a file with heavy grit emery cloth on it.
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The cutting tool was 1/4 square cutting tool for a lathe, I used it just to lay on the case, shine a light behind it and look for gaps.
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Cool I like Redneck Ideas. ;)
I still would have went the other way I mentioned before doing the case. Just glad it worked out for you. 8)
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FWIW the Blanchard company made all kinds of precision grinders both surface type and centerless.I have worked on several that side ground connecting rods for auto engines .
I've heard of grinding aluminum but never seen it done .Aluminum is a material that can be precision machined to almost a mirror finish .Fact being I have a collection of synthetic diamond tooling once used to machine automobile pistons .Some as a matter of fact are real industrial diamond but not gem quailty made be DeBeers .
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Normally, you can not use a grinding wheel on aluminum for the wheel will load up and pack with the aluminum and in extreme situations, the wheel will explode.
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use wd 40 when grinding or cutting al, problem solved.
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Perhaps in your situation but not in an industrial environment .Generally speaking it's a water soluable coolant like "trim CE ".