Stihl 660 MOD

[henrass]

Hey. So lately there has been talk of a simple yet effectice mod that can have some power gain on a stock saw.
The old 'base gasket delete', closing the 'squeesh gap'.
Has anybody done this on a 660, and how reliable would it be, if it is a thing?

 

[J D]

Been done plenty & is perfectly reliable if done correctly. First step is to measure your squish... can take it from there

 

[Bob Hedgecutter]

Hey. So lately there has been talk of a simple yet effectice mod that can have some power gain on a stock saw.
The old 'base gasket delete', closing the 'squeesh gap'.
Has anybody done this on a 660, and how reliable would it be, if it is a thing?

Squeesh is Australian, we work on squish this side of the ditch.

 

[Sierra_rider]

If you measure your current squish at .045" or more w/the base gasket, you're golden to do the delete. I typically like to see around .025" squish on a 90cc saw and the stock gasket is about .020" thick. Some motoseal ads a few thousandths, but not much.

If you have the stock 660 muffler, either muffler mod it or get an 066 dual port cover. The stock single port mufflers are very restrictive.

They also like timing advance...basically you file off about .020" off the flywheel key. Be warned about that method, it also makes them harder to start. I've got a round-top 066(pretty much the same as a 660) that I ported, machined, and advanced the timing on. Between the added compression and timing, it has a stiff pull and requires a strong pull to start it...no between the leg or ground starts with it.

 

[The Shooters Apprentice]

Doing the base gasket delete will changing your timing numbers.

 

[Dennisthemenace]

Hey. So lately there has been talk of a simple yet effectice mod that can have some power gain on a stock saw.
The old 'base gasket delete', closing the 'squeesh gap'.
Has anybody done this on a 660, and how reliable would it be, if it is a thing?

As mentioned elsewhere make sure you have the dual port muffler and maybe open up the baffle. Some delete it.
I recently measured squish on my favourite, ported, 660 at 32/33 thou, gasket at 18 thou.Deleted original gkt. Cut a paper gasket out of a loctite catalogue and with a thin bit of sealer this reduced squish to 23/24 thou. 3 tanks in and running fine but it has new rings not quite run in I reckon

 

[Dennisthemenace]

Doing the base gasket delete will changing your timing numbers.

Yes, something to think about but it is a simple mod as asked about by the OP and easily reversed if no good. As is a little bit of timing advance

 

[henrass]

Thank you for all the replies!!
I saw Tinman on youtube has a cool vidoe on it explaining everything quite nicely also.

 

[The Shooters Apprentice]

Thank you for all the replies!!
I saw Tinman on youtube has a cool vidoe on it explaining everything quite nicely also.

Tinman puts out some good content. Very knowledgeable.

 

[J D]

Doing the base gasket delete will changing your timing numbers.

Squish from factory can vary a bit (& in turn port timing will vary slightly too).
The gains made by setting a more ideal squish will invariably outweigh any losses caused by the fractional change in port timing.

 

[HansFranz]

I saw Tinman on youtube has a cool vidoe on it explaining everything quite nicely also.

Thanks for the tip on Tinman, just watched his video on base gasket delete, very informative.

 

[Sierra_rider]

Squish from factory can vary a bit (& in turn port timing will vary slightly too).
The gains made by setting a more ideal squish will invariably outweigh any losses caused by the fractional change in port timing.

Yep, timing numbers may only change by 1 degree from doing a gasket delete. In the case of the 660, the timing change might even be towards the right direction...660s are infamous for having varying timing numbers from the factory, they often have very tall exhaust ports...and lowering of the port is good for them.

 

[henrass]

So the saw is all rebuilt. FACTORY SQUISH measure 29.5 thou, so I did not delete the base gasket, I was thinking of doing double coke can gasket (would have given me 25 thou), but not sure if that would be reliable for a saw that has to put food on the table.
Polised both ports and opened the baffle inside the muffler. (Have to keep spark arrestor for forestry code of compliance). Just gotta clean up tge tuning in the wood and we will be golden!
Thanks for all the help!! Definately may do another build in the future

 

[ballisticdoughnut]

Add a WCS universal exhaust port to the flywheel side of the muffler. Those saws are seriously choked up from the factory.

 

[Bob Hedgecutter]

So the saw is all rebuilt. FACTORY SQUISH measure 29.5 thou, so I did not delete the base gasket, I was thinking of doing double coke can gasket (would have given me 25 thou), but not sure if that would be reliable for a saw that has to put food on the table.
Polised both ports and opened the baffle inside the muffler. (Have to keep spark arrestor for forestry code of compliance). Just gotta clean up tge tuning in the wood and we will be golden!
Thanks for all the help!! Definately may do another build in the future

Has the saw got the dual port muffler most of them had from new over here? They make a difference and you often find the closed front replaced on saws built from parts.

 

[Bob Hedgecutter]

Add a WCS universal exhaust port to the flywheel side of the muffler. Those saws are seriously choked up from the factory.

WCS stuff is not as easily accessed way over our side of the Pacific.
Easier to find (here) and fit the Bark Box fronts to the muffler- but an expensive option for us.
Also note the retaining of spark arrestors- hard to get a modded non screened muffler past the forest police here.