Chainsaw Repair

Someone told you wrong?  The original carb handles the big bore kit just fine. You need to open the needle valves to let in more fuel.

A likely problem with your 365 spec is that it has a crankcase air leak is the reason the cylinder seized up? Either crankshaft seals, intake boot, or other source? You need to do a crankcase pressure check?

I can mail you a service manual for the 365 Special? The crankcase pressure test is described in the manual. It is not like the cylinder pressure test.

The 066/660 Stihl come up for sale all the time. Look on Craig's list or the swap sections of the forums? It might take you a year to find the one you want?

An aftermarket "BB" top end doesn't necessarily increase the power of a 365 - it depends on the exact kit, and what carb is on the 365.

Anyway, if it blew up, there likely is something else wrong with the saw, like an air leak  - or with the top end, like the rings catching the ports.

but how do I do a crankcase pressure check ive never had to do this so I don't know how! is it different from a regular cylinder pressure test?

A pressure/vacuum test is easy to perform if you have the tools. It differs from a compression test in that it tests the entire saw for leaks in the case, cylinder to case joint, intake tract, exhaust tract, decompression valve and both the crankcase seals all at once. If you are thinking of a compression test where you screw a hose into the spark plug hole, this will not do because it does not test the entire saw. Two stroke engines cannot leak air from any of the above areas or the engine will self destruct!

The pressure/vacuum test is a timed leak-down pass/fail situation. HVA specifies that you put 80kPa of pressure into the motor, wait 30 seconds. If at the end of the 30 seconds the remaining pressure in the saw is above 60kPa you are good to go. The vacuum test is the same sort of thing, you place the saw under a certain amount of vacuum, wait a specified time and check the indicated remaining vacuum in the saw, if above a certain number you are good to go. Can't recall the vacuum numbers or time limit but they are in the Workshop Manual 660Magnum sent you.

The first thing you must do is to seal off the pulse tube, exhaust, intake and spark plug holes. You can use an old bicycle inner tube and make blocking plates for the exhaust and intake tract. You then connect you gauge and pump to the pulse tube. I happen to use this device to do the tests it will do both pressure and vacuum with a single tool. http://www.ebay.com/itm/MIGHTY-VAC-PRESSURE-VACUUM-TESTER-TEST-BRAKE-BLEEDER-DIAGNOSTIC-ENGINE-TOOL-KIT-/251216512533?pt=US_Hand_Tools&hash=item3a7dabc615 but you can find others that will do the job at your local auto parts store you may even be able to borrow one from the auto parts store.

Failing to do these tests will result in the same thing happening to your saw. OBTW I quoted HVA numbers here because you have a Husqvarna saw but the process is the same and required for ALL brands!