Ts410, sorry another

[openspaceman]

seeing as you have got the ring offer it up to the cylinder and square it in the bore with the piston and using feeler gauges check the gap in various places.

[adw]

Look at picture five, the shaded area on the inlet port is where the nicks sill lining has gone, classic dust ingress, has one of the rings passed the ring peg?

[Mike the builder]

Rings siting correctly in grooves. I’ll see if I can find some feeler gauges to test the rings. I have ordered new chinese cylinder, piston and rings and I’m going to check the compression when I fit those. I’m just interested to find out exactly what’s wrong regardless just for future. I’ll let you know how it goes, if yr interested of course. 

[spudulike]

A compression gauge for small engines will typically have an extremely light gauge schrader valve mounted in the part you screw in to the engine. The valve type is chosen because you can only spin the engine over for a few revolutions unlike a car engine that you can hold on the starter for a few seconds.

The valve being close to the engine stops the tube acting as part of the volume of the combustion chamber and lowering the compression.

Car type compression gauges can have bicycle type schrader valves in them and will give lower readings than small engine gauges.

The Gunson "HiGauge" is one that works although I modified mine to take Presta valves as I was destroying the schrader ones with modified engines making 200psi+

The rings.....put them in the cylinder, make sure they are nice and square, measure the gap and it should be 7-15 thou, much more and the compression will suffer badly.

The light part of the cylinder ADW mentions may well be the Nikasil plating worn through - try scratching with a craft knife and if it scratches on the light part, it is through to the aluminium.

If you have another of these machines, just pull them over slowly and compare them. 75 or even 105 psi is pretty low. An engine in good condition will make 170psi.

[Mike the builder]

Thx, I’ve only one cutter. The new cylinder and piston will be here in a few days. Be interesting to see the psi difference. By ring gap do you mean the gap between the two ends. 

[coppice cutter]

Compression testers, any of them, are OK for a handy pointer but can throw up misleading results sometimes.

 

Ultimately for a decent assessment of engine condition you need to use a leak-down tester which will give you a much more accurate idea of just how good or bad your piston to bore seal is.

[openspaceman]

29 minutes ago, Mike the builder said:

Thx, I’ve only one cutter. The new cylinder and piston will be here in a few days. Be interesting to see the psi difference. By ring gap do you mean the gap between the two ends. 

 

Yes, look at this one, the nearest ring  is badly worn as shown by the wide gap, the one underneath near the top of the cylinder is the new one with the smaller gap.

 

@adw is most likely right and the very hard but thin nikasil layer has worn through. This rarely happens with chainsaws but disc cutters work in a much dustier environment and I have no experience with them.

Edited October 10, 2021 by openspaceman

[bmp01]

To me the cylinder looks pretty good....I don't like the second to last picture of crankcase where the painted finish shows clean and dirty patches though. 

 

With reference to ADW's comment about picture 5 - picture is too dark to tell much but the light patch is just some reflection....I think. ... Look at picture no 3 its the same area and looks fine allbeit slightly blurry.

Mike, do you have any other 2 stroke eqpt you could pressure test with that guage, just for comparison, or borrow a mate's spanking new saw to test.

Here's a question,  if you run a finger nail down the piston skirt (from the ring groove to the bottom of the piston) is it polished smooth or can you just feel the ridges from the machining ?

 

[Mike the builder]

I’m going to check that ring clearance. I’ve an old stihl hedge cutter and that’s harder to turn over than the cutter, I think this is the answer. 

[spudulike]

21 hours ago, coppice cutter said:

Compression testers, any of them, are OK for a handy pointer but can throw up misleading results sometimes.

 

Ultimately for a decent assessment of engine condition you need to use a leak-down tester which will give you a much more accurate idea of just how good or bad your piston to bore seal is.

Sorry, a leak down test on a two stroke bears no indication of secondary compression (the compression between the piston and combustion chamber) and shows no indication of engine wear on the cylinder and piston.

The way you determine wear of these components is inspection, measurement and if the engine is not in bits, you measure the compression on the cold engine after one pull and many pulls with a good engine giving 1/2 of its maximum compression on the first pull. Gauging the drop of compression from cold to hot is a good indication of engine wear - expect around 20 psi drop.

The leak down test (pressure and vacuum) is used to check the integrity of all the gaskets, mating surfaces, seals and gaskets - a two stroke should not leak from these areas for a predetermined time as the only air entering the engine should be via the carb.

Not having a go, just stating fact, perhaps you are meaning four stroke leak down which is another matter!