Dolmar 123

[ppj690]

I was given a Dolmar 123 over a year ago which I was told was a non runner.
Having had a quick inspection through the ex port etc I put a comp tester on it to reveal it only had 60 psi of compression even though piston and cylinder looked really good.
Stripped it today to find that the ring has broken on both sides and wasnt visable from either port. (trying to upload pics but cant seem to)

Anyway, is it worth fixing? The rest of the saw is in good shape. Are there any aftermarket top end kits worth putting on or should I throw it in the bin?!

Thanks again in advance for any help...

 

[ppj690]

 

[ppj690]

Seem to remember reading that a 309 uses the same jug and piston. Anyone know if that’s true?!

 

[holeycow]

Chainsawr may have parts..

Ebay also.

 

[ppj690]

So this arrived today... 309 cutoff saw that’s had a hard life by the looks of it!

 

[ppj690]

Underneath all the crap it’s actually in really good condition. Very different to the 123 I will be building with its P&C.

 

[ppj690]

The flywheel on the saw (123 that’s having the P&C off the 309) has had a whack and lost 4 fins off the flywheel.
Due to the fact that the coil on the 309 is very different and looks like a PITA to change can I run it until I find another flywheel to replace it with?!
The coil on the 309 has an integrated HT lead that goes through a grommet in the case, anyone had any experience in removing these just in case I try it out?

 

[ppj690]

Built the saw up last night. LOTS of compression and runs pretty well apart from a tapping noise!
Initially thought it was the wrist pin or needle bearing but pulled the muffler to reveal quite a lot of play in the piston rings when rocking the flywheel... piston and cylinder came off the 309 and hardly look used?
Any thoughts? Or does this need a new piston and rings?!

 

[ppj690]

Anyone know how much movement there should be in the rings if any.?!? The top pic shows how much of a gap there is. This equates to quite a lot of movement when you rock the flywheel...

 

[ppj690]

Anyone?! before I tear it apart again...!

 

[psuiewalsh]

Since it was a concrete saw it could be piston slap due to excessive wear.

 

[Sepia]

Did you check the squish when you assembled it?

 

[ppj690]

No, just bolted new cylinder down with some loctite to seal.

 

[Sepia]

No, just bolted new cylinder down with some loctite to seal.

Better check - could be the piston lightly tapping the top of the cylinder.

 

[ppj690]

Better check - could be the piston lightly tapping the top of the cylinder.

Will check now.. and get back to you!

 

[ppj690]

Looked down the plug hole and found two small witness marks at the front edge of the top of the piston even though clearances look pretty good. Run out of solder wire so can’t measure at the moment. When pulling over by hand there is no noise and it’s nice and smooth but I guess when under load the front of the piston may be touch the top?!

 

[Sepia]

Looked down the plug hole and found two small witness marks at the front edge of the top of the piston even though clearances look pretty good. Run out of solder wire so can’t measure at the moment. When pulling over by hand there is no noise and it’s nice and smooth but I guess when under load the front of the piston may be touch the top?!

Yes when revving the squish clearance will tighten somewhat when compared to pulling it over by hand. Also, as psuiewalsh mentioned above, the piston may be worn and that would allow it to rock slightly in the bore, which would further decrease squish clearance when the piston is canted at a slight angle in the bore.

If you have something that is 0.020" thick like a metal strip or wire, stick a length of it in the plug hole so that it contacts the cylinder wall and rotate the piston to top-dead-center and see if there is contact/resistance. Check all around the bore this way.

 

[ppj690]

Yes when revving the squish clearance will tighten somewhat when compared to pulling it over by hand. Also, as psuiewalsh mentioned above, the piston may be worn and that would allow it to rock slightly in the bore, which would further decrease squish clearance when the piston is canted at a slight angle in the bore.

If you have something that is 0.020" thick like a metal strip or wire, stick a length of it in the plug hole so that it contacts the cylinder wall and rotate the piston to top-dead-center and see if there is contact/resistance. Check all around the bore this way.

Fantastic! Will do.
Many thanks, much appreciated!

 

[ppj690]

Just mocked it up without a base gasket and bolted cylinder down to test squish...
Started with a piece of solder and kept getting bigger until I had a cable tie in the plug hole! Thickness is 0.040.. (with no base gasket) is this right!?

 

[Sepia]

Well if it's .040" you shouldn't have to worry about piston contact with the top of the cylinder. I think I saw a pic in this thread showing a very tiny combustion chamber, so .040" may be what your saw needs. On the other hand, you will have to determine what caused the marks on the top edge of the piston. Any chance there may be some debris inside the saw? Are there corresponding marks in the combustion chamber?