Author Topic: craftsman 3.7 compression  (Read 188 times)

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Offline The Ripper

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craftsman 3.7 compression
« on: June 24, 2019, 10:30:59 am »
I've replaced crank seals, installed new rings, did not port exhaust port but did modify the carb plastic manifold extensively and widened intake port. Removed cylinder base gasket, milled cylinder flange .020 to achieve a .020 squish and still my compression is 125 psi a gain of 25 psi over stock. From what i've searched stock is near 150 plus, so what have I've over looked in rebuilding this saw. If I apply a light film of motor oil to coat rings compression shoots to 160psi, which would be normal I assume improving ring seal. Any suggesting
to what I'm missing or should I just run the saw like it is?  Also the rings are thin style

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Offline Efisher26

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Re: craftsman 3.7 compression
« Reply #1 on: June 24, 2019, 10:33:52 am »
They run great anything over 125psi, your saying your rings  havenít seated yet?


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Offline The Ripper

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Re: craftsman 3.7 compression
« Reply #2 on: June 24, 2019, 10:35:55 am »
The the saw has an hour run time.

Offline Cut4fun .

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Re: craftsman 3.7 compression
« Reply #3 on: June 24, 2019, 10:47:19 am »
Do you have the thin ring 3700 or the thick ring 3800 version of the 3.7?

Roger posted this

The Poulan 3800 will run on average around 125 psi on the compression.
The Poulan 3700 will run on average around 150 psi on the compression.
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Offline The Ripper

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Re: craftsman 3.7 compression
« Reply #4 on: June 24, 2019, 11:08:02 am »
Thin rings. bought six sets and they seemed to have good tension.
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Offline Cut4fun .

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Re: craftsman 3.7 compression
« Reply #5 on: June 24, 2019, 11:17:25 am »
WAG what type of compression gauge you using? Does it have the needle at bottom where it goes in cylinder?

 

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Offline The Ripper

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Re: craftsman 3.7 compression
« Reply #6 on: June 24, 2019, 12:41:32 pm »
yes it's a Lisle. The black craftsman I bought from you and partner R16 i have register 150 plus and my 3.7 125 psi. you can tell the difference just by pulling. Now when I apply oil film to the 3.7 the difference is night and day. When I first pulled this saw apart the piston, ring grooves and bore was in mint shape but one of the three is keeping it from building pressure or poulan at the time machined the compression chamber different to lower compression for craftsman, just guessing here.
 
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Offline Al Smith

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Re: craftsman 3.7 compression
« Reply #7 on: July 07, 2019, 08:31:53 am »
Having re ringed several using an old cylinder it could take a lot of run time before the rings seat .Not to worry eventually they will seat in .As a rule cast iron rings seat faster than chrome alloy or tool steel .
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Offline The Ripper

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Re: craftsman 3.7 compression
« Reply #8 on: July 18, 2019, 10:39:45 am »
Runs real good now,like you said gotta give the rings time to break in
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