372xp big bore kit..do i or dont i.

[wyk]

Both ring ends run thru the inlet only and don't come anywhere near any other port.

 

The piston skirt covers the inlet tract by 1.2mm or so so close but we shall see...

No the exhaust hasn't been lowered. just the rough casting removed and polished up. Raised by 1mm.

 

Like I say its my own saw so you have to push things a little to find out what does and doesn't work,

 

Lowering the exhaust doesn't hurt if the ring clears. It allows faster blow out, and is the reason many porters put a shelf in the exhaust itself. I think Randy has ported one of these, too. Prolly has a thread on it.

[wyk]

Lowering the exhaust doesn't hurt if the ring clears. It allows faster blow out, and is the reason many porters put a shelf in the exhaust itself. I think Randy has ported one of these, too. Prolly has a thread on it.

 

Found it, google: Building a 372XP Big Bore With 272XP Piston

[spudulike]

Get a vid up once you are back in the land of the living, make sure you do bevel those edges with a diamond file and I smooth them with 400grit paper so I have a nice bevel that will not give trouble in time.

 

If the 1.2mm is too small, when you rev the engine (assuming it runs), you will get a blowback of fuel/air vapour blowing back on to the air filter which may cause issues but hopefully not.

 

Good luck with the project - make sure you open the exhaust right up or the mods you have done will just make the saw run very rich and boggy:thumbdown:

 

I drill a number of large holes in the baffle plate and insert a decent size outlet in to the front of the muffler - it works for me:thumbup:

[wisecobandit]

Yeah ive bevelled the edges spud on the final clean up.

If the 1.2 mm clearance doesn't work then the stihl 064 piston gives an extra 5% coverage on the piston skirt is my theory.

 

Not decided what to do along the muffler line yet, I have the standard muffler or a cheap chinese replacement which has no baffle but still has the original size outlet on it.

TBH bending over porting has had me up half the night in pain so high as a kite on pills so allsorts of weird thoughts have been going thru my head regarding the muffler.

[wisecobandit]

Found it, google: Building a 372XP Big Bore With 272XP Piston

Interesting read. Seems hes unsure on the windowed piston theory.

Yeah the step in the port is meant to increase velocity, bike builders have done it for years but cant say ive ever noticed a difference. I guess it must have an advantage for ppl to still do it tho. I want to check out the theory at some stage of drilling a few minute holes under the piston rings to allow pressure to help push out the rings and in effect get a stronger seal.

[spudulike]

Interesting read. Seems hes unsure on the windowed piston theory.

Yeah the step in the port is meant to increase velocity, bike builders have done it for years but cant say ive ever noticed a difference. I guess it must have an advantage for ppl to still do it tho. I want to check out the theory at some stage of drilling a few minute holes under the piston rings to allow pressure to help push out the rings and in effect get a stronger seal.

 

IMO, those holes will drop your compression off from leakage past the rings!

 

Think those drugs are a bit heavy:lol:

[wisecobandit]

IMO, those holes will drop your compression off from leakage past the rings!

 

Think those drugs are a bit heavy:lol:

 

Lol probably but a lot of the 2 stroker boys done it on the racetrack to the production modified bikes back in the days of the Yamaha tz's and some of the karting lads also etc. Ive always had my reservations tho about trying it.

[spudulike]

Lol probably but a lot of the 2 stroker boys done it on the racetrack to the production modified bikes back in the days of the Yamaha tz's and some of the karting lads also etc. Ive always had my reservations tho about trying it.

 

Guess it will save weight

[wyk]

Guess it will save weight

 

I've never seen them make a whit of difference myself. I can see an argument where the skirts have been entirely removed to allow access from the windows, or on most cylinders where the bottom transfers are on the side VS the bottom like on the 346 and 372. But, if you ask me, on a 372 or 346, it only serves to lighten the piston. This is good for high rakers, short bars, or race chains, but for a work saw, I think you lose some torque in the trade unless you increase the compression significantly.

[wyk]

Yeah ive bevelled the edges spud on the final clean up.

If the 1.2 mm clearance doesn't work then the stihl 064 piston gives an extra 5% coverage on the piston skirt is my theory.

 

Not decided what to do along the muffler line yet, I have the standard muffler or a cheap chinese replacement which has no baffle but still has the original size outlet on it.

TBH bending over porting has had me up half the night in pain so high as a kite on pills so allsorts of weird thoughts have been going thru my head regarding the muffler.

 

On these ported 372 BB's, there is almost no limit to how far you can open up that muffler.