372xp build, need some guidance

[subhunter]

I have decided to try my hand once again at porting. I am going to use a "used" cylinder and a full circle 266 piston. I know I am going to raise the lower transfers as I did on the 365's jug as well as widen the ports.

Now where the "guidance" comes in to play.I need to know,if some one will, how much am I going to need to machine off the crown on the 266 slug? Also,if any, to machine of the jug.

I am thinking that to start I should match the two pistons heights from wrist pin to crown, the 266 piston seems shorter so is that meaning turn some off the jug as well?

Keep in mind that I have did some porting on a few of my saws and so far they have held up nicely but this will be the first saw for me to really attempt to mod in this manner.Any of you guys helpful advice is always welcome!

 

[mdavlee]

If the 266 piston is shorter then you will need to machine the jug down to set the squish. Do you have the piston yet?

 

[subhunter]

I got the piston from a 266 I got a few weeks ago. Yesterday I pulled the jug on it and seen the full circle piston and it was in decent shape, so I thought to myself. Why not get in on the TN GTG 372 build?

NOW If someone would kindly point me in the right direction. I have a guy that has been doing machine work for 30 years who is going to be turning the piston for me.

 

[Blowncrewcab]

Proper squish with a Flat Top is a better Gain than too big of squish & a Pop-up. Do a Pop-Up if you can but don't worry too much if you can't as long as you get "good tight squish"

 

[subhunter]

I am going to go and see the man and really show him what need to be done. First I am going to have him take some off the crown of the 266 piston. Then I am hoping to get some numbers. I was hoping someone would share the numbers they have used in the past for the 266 piston.
Edit: I also have a almost new 266 pisiton that isnt a full circle. would that one be a better match or the full circle?

 

[mdavlee]

The one with the widest skirt would be the one I would use. Put it together and check the squish before you have anything machined. If you do a popup you may end up dropping ther jug too far and having too much intake duration.

 

[subhunter]

The one with the widest skirt would be the one I would use. Put it together and check the squish before you have anything machined. If you do a popup you may end up dropping ther jug too far and having too much intake duration.

I was thinking that the circle one would be best. So you say check squish with stock piston and the machine from there?

 

[mdavlee]

Check the squish for whichever piston you want to use. You don't want a huge intake duration that will make it a little laggy.

 

[subhunter]

squish with the stock piston is .027. that was the biggest number. smallest was .0263 above exhaust. I may have been going to jump the gun a bit as I was just going to have the pistons crown turned down to match the stock one.

The 266 piston will not clear to make a rotation, it hits before TDC. This being the reason thnking to match the stock pistons height from wrist pin to crown.

 

[Roanoker494]

Would it be better to turn the 266 piston to match the 372, which means you are going to end up with the same squish as stock, or turn the 266 piston to set the squish you want to have? If you want a .019 squish then turn the 266 piston until it is .008 taller than the stock piston.

 

[subhunter]

Would it be better to turn the 266 piston to match the 372, which means you are going to end up with the same squish as stock, or turn the 266 piston to set the squish you want to have? If you want a .019 squish then turn the 266 piston until it is .008 taller than the stock piston.

Thats what I was thinking to do but the 266 pistons skirts are also a bit shorter. I have made a thick gasket to try the 266 piston with now. We will see in a minute

 

[Roanoker494]

Thats what I was thinking to do but the 266 pistons skirts are also a bit shorter. I have made a thick gasket to try the 266 piston with now. We will see in a minute

The skirt length did not even cross my mind until you mentioned it. Wish I knew more about porting but I am just really throwing ideas out there.

 

[subhunter]

With a cork gasket I just made {.060 thick} the squish was .012.

 

[Roanoker494]

With a cork gasket I just made {.060 thick} the squish was .012.

Wow, that is close. Looking through other porting threads it seems that .018 is the closest you want to go, through there was one builder that went .017 on a big Stihl.

I believe you were correct on having to do some milling on the piston.

 

[subhunter]

Does anyone know if the 365 and 372 both use the same flywheel? The 372 is limited coil but 365 is unlimited.I can just swap coils if they use same FW.

 

[Roanoker494]

Does anyone know if the 365 and 372 both use the same flywheel? The 372 is limited coil but 365 is unlimited.I can just swap coils if they use same FW.

That one I can help you on. The 365 coil is a direct bolt-on part for the 372.

 

[tlandrum]

with the original piston in the jug you need to see how much skirt you have at tdc sticking below your ports. that will tell you how much the jug could be dropped with the new piston skirt length. you need to machine the pop up onto the piston and then mill jug to set squish and on that saw id go to .020 -.022 for a work saw.

 

[mdavlee]

Shoot for .020" for squish or a tad over. How much is the crown to ring groove on the piston you want to machine down? You don't really want to go under .100". You need to put it together with a timing wheel and see what the intake will be with the piston in there. You could even do it with the taller one and the big gasket. About .010" is around 1°. That will give you an idea of what it will be when the cylinder drops down from the popup.

 

[AUSSIE1]

What's the dimensions of both pistons?

Ring land to crown.

Pin to crown.

Piston length.

Skirt width.

If your measurement from the bottom of the piston to the pin centre is smaller, this will increase your inlet timing.

Take off the piston for a popup the difference between the 372 piston minus what's needed for a .020 squish (you can go as low .018 so aim for .020), then do a reassemble less rings and degree the saw to see where your at.

You can use a rule in the barrel and read of the port edge then transfer that measurement to the piston plus approx .025 thou to see where you are for skirt length/port coverage.

 

[subhunter]

I finally got some spare time this evening to work on the 372. I made a .050 base gasket making my squish on the 266's piston .021. The timing numbers were EX= 100 in=85 transfer 126.
What do these numbers look like to the saw experts?