For Sale is a new cylinder jug for a Pro Mac 610 or Eager Beaver 3.7 Chainsaw. 

Notice this has the larger exhaust port that was the upgrade on the later Super 610 models so it would breath like a 650. 

McCulloch improved the transfer and exhaust ports for the Super 610 in the late 1980s to increase the power 30% as advertised over the standard 610.  This was so the Super 610 would perform like the 650 without a compression release.  This could be done due to the engineers at McCulloch developing the "Q" shaped exhaust port, you can see when you look inside, the exhaust port has a very narrow slit that goes toward the combustion chamber, this allows the compression to be released at low rpms when the operator pulls the chord but allows the engine to build very high compression during running operation eliminating the need for a compression release button while maintaining the high performance benefits of high combustion pressures.

This  fits:
McCulloch Pro Mac 610
McCulloch Eager Beaver 3.7

 McCulloch Pro Mac 650 or 655 - This does not have a compression release but it is 60cc same bore and stroke as the 650 saw.

It comes with a matching bottom, the bottom needs the oil pump mount holes drilled or you can reuse your original bottom.

Takes a standard 47mm or 1.85" diameter piston P/N 94130 or P/N 94131, This is NOT included but there are several on eBay if yous search these part numbers.

This may fit many more McCulloch saws, so please ask if you have any questions about your saw.