Poor compression

[Alanth]

I have been given a 252RX brushcutter which has probably been run without oil.

The piston is scored on the exhaust side. I have replaced the piston ring (same as on a Husky 51 chainsaw) but the compression is only about 65psi and it doesn't start. On a similar working machine its about 125psi. I was hoping the new ring would hold the pressure up, isn't that what its for? It may not be worth spending £80 ish on a new piston assembly. Is there a typical minimum pressure for an engine to function?

Thanks for any ideas.

[Eddy_t]

I have been given a 252RX brushcutter which has probably been run without oil.

 

The piston is scored on the exhaust side. I have replaced the piston ring (same as on a Husky 51 chainsaw) but the compression is only about 65psi and it doesn't start. On a similar working machine its about 125psi. I was hoping the new ring would hold the pressure up, isn't that what its for? It may not be worth spending £80 ish on a new piston assembly. Is there a typical minimum pressure for an engine to function?

 

Thanks for any ideas.

 

 

You need around 150psi, that piston is FUBAR

 

As another question, have you cleaned the transfer out of the cylinder?

[Alanth]

The cylinder appears to have a liner and looked fairly OK. I could take it apart again and polish it up.

[Stubby]

Piston definitely is knackered . If you can salvage the pot then do so but don't just assume it was lack of oil . Could just as well been an air leak making it run lean , over revving and over heating . No point in sorting the pot and piston for it to nip up again .

[treequip]

You cant just chuck a new ring in.

 

Piston is well past its best and the and needs replacing. The bore will need careful inspection to determine if it can be re used, as a minimum it will need any transfer cleaning off and may also be fubar.

 

There is no guarantee that the damage is due to lack of oil so check for air leaks as well

 

Since it came for free, treat it to a new pot and piston for trouble free running

[spudulike]

What the rest said.....125psi plus compression on small engines to run with 150psi being typical but does depend on the engine type and design. You have probably trashed a new ring, a piston doesn't seize that badly without transferring aluminium to the bore of the cylinder!

[Alanth]

What's the best way of removing aluminium from the bore without damaging it?

[spudulike]

What's the best way of removing aluminium from the bore without damaging it?

 

Chemical - either Acid of strong alkali, both work. Hydrochloric Acid or Sodium Hydroxide do the job.

[Alanth]

Thanks, I happen to have both, but I won't use them at the same time

[Eddy_t]

Thanks, I happen to have both, but I won't use them at the same time

 

 

Well you can, but the results will be fairly boring and require more cleaning up.