372 Pston swap/mod

[Yotaismygame]

Might want to double check those numbers. Personally I find using duration the most accurate or at least consistent way of measuring. An exhaust height of 90 is way up there. I've never ported a saw with that high of an exhaust. Any of the 372 cylinders Ive timed with a 268 040 pop up piston showed low 100s for exhaust after raising it, and transfers were all at 125. Good way to measure transfers is to stick a ring in the jug right at the transfer opening and then run the piston up to that point when measuring. Those transfers can be hard to see. Ill put a wheel on the jred I just posted and see what the exhaust height is at tonight.

 

[old guy]

Remember, I took 2 m/m off the top of that port as per Trapper Mike, he assured me it wasn't too much, how much did you take off your cyl.

 

[Yotaismygame]

I just did it based off compression. I’m surprised you have as much compression as you do. I tested the jred last night and it’s only making 145. I bet it will settle around 150 once it’s ran more. I’ll time the exhaust here soon.

 

[Yotaismygame]

That jred I did is at 101. I can’t imagine your saw is at 90! I’m also thinking this ring I’m using is junk.

 

[Yotaismygame]

If your exhaust height is 90 thats mean the duration is 180. I measured intake duration with this piston and its 167. I really cant imagine your exhaust is at 90. I could see 95 ish.
167 intake and 170 exhuast durations would make sense. Generally those two numbers need to be close to each other to make a good running saw. Even at 101 I feel like its plenty of exhaust.

 

[old guy]

This is my first go with a degree wheel, so I could be wrong.
I have another stock 372 & 2 more new OEM cyls., so I can do some study.

 

[old guy]

I tore it down again today and came up with the same reading, 90° atd ex. opens.
I bolted on a new cyl. & got a reading of 98-99.°

 

[Leafy]

I think you're doing it wrong. A factor cylinder shouldn't open that early. Are you actually finding top dead center right? I have a good eye and just see where the piston starts going down both ways and center up on the dwell. But if you don't have an eye for tenths you should use a dial indicator through the spark plug hole.

 

[old guy]

I used Masterminds method of hitting the piston stop from both ways, & then splitting the difference, this is also where the coil magnets hold the flywheel, I also double check this by sticking something in the plug hole & moving the piston.
I then set the ° wheel to TDC & turn it clockwise till I see lite coming thru the ex. port looking thru the plug hole, where am I going wrong.

 

[huskihl]

I used Masterminds method of hitting the piston stop from both ways, & then splitting the difference, this is also where the coil magnets hold the flywheel, I also double check this by sticking something in the plug hole & moving the piston.
I then set the ° wheel to TDC & turn it clockwise till I see lite coming thru the ex. port looking thru the plug hole, where am I going wrong.

99-100 9n a stock exhaust port sounds right to me. As long as you're getting the same reading on either side of the wheel your numbers should be right.

 

[Yotaismygame]

I’m not sure where you’re going wrong but I got 101 on a saw I thought I ground on too much. Just measure it in duration. No need to find tdc. Shine a light through the spark plug. Run the piston down until light starts to shine through the exhaust port. From here zero your wheel. Double check you didn’t move the piston. Then run the piston down through bdc until you can barely see light again. Read that measurement. Should be around 155-165 roughly. Take your number off the wheel, divide it by two. Then take 180 and minus the number you got after dividing. For example: 160 degrees of duration divided by two is 80. 180 degrees of total duration minus 80 = 100. Exhaust height is 100

 

[huskihl]

I’m not sure where you’re going wrong but I got 101 on a saw I thought I ground on too much. Just measure it in duration. No need to find tdc. Shine a light through the spark plug. Run the piston down until light starts to shine through the exhaust port. From here zero your wheel. Double check you didn’t move the piston. Then run the piston down through bdc until you can barely see light again. Read that measurement. Should be around 155-165 roughly. Take your number off the wheel, divid it by two. Then take 180 and minus the number you got after dividing. For example: 160 degrees of duration divided by two is 80. 180 degrees of total duration minus 80 = 100. Exhaust height is 100

Doing it that way might involve the factory bevel, which is usually pretty excessive. The intake is a straighter view for finding "0" or tdc

 

[Yotaismygame]

Doing it that way might involve the factory bevel, which is usually pretty excessive. The intake is a straighter view for finding "0" or tdc

If the ports open it’s open doesn’t matter if it’s the bevel or fully open. Open is open. I usually see the light, then it run it down just a touch more. Doing it that should get you in the ball park. Consistency in your own garage is what’s important. Numbers between people not so much.

 

[Yotaismygame]

I just can’t imagine oldguys port is at 90. If it is I can’t imagine it’s a very strong saw or has the 160 comp he’s stated. But something closer to 95-98 I could see. If I’m getting 101 and ground on the exhaust quite a bit a stock jugs not going to be 100 or less. I personally don’t know what stock 372s time at but I would imagine around 105.

 

[huskihl]

If the ports open it’s open doesn’t matter if it’s the bevel or fully open. Open is open. I usually see the light, then it run it down just a touch more. Doing it that should get you in the ball park. Consistency in your own garage is what’s important. Numbers between people not so much.

Very true if you're changing your own stuff and writing it down looking for improvements.
My point though, is that if you're using a set of numbers from someone else, and the bevel is .060", your exhaust reading is off roughly 4° from how the other guy reads it. And if you use a ring to get the transfer duration, that's an exact number. So you'd be raising the transfers an extra 4°. This is all if the other guy used a straight line of sight or a ring to find the exhaust, of course.

 

[Yotaismygame]

If the ports open then it’s open which equates to its height. If you’re simply eye balling it you would be getting false numbers. Anyway, I’d like to hear oldguy’s for sure exhaust height.

 

[old guy]

I just can’t imagine oldguys port is at 90. If it is I can’t imagine it’s a very strong saw or has the 160 comp he’s stated. But something closer to 95-98 I could see. If I’m getting 101 and ground on the exhaust quite a bit a stock jugs not going to be 100 or less. I personally don’t know what stock 372s time at but I would imagine around 105.

I ran this saw against the stock saw yesterday & it is definatly the stronger saw.
Today I may tear down the other saw to see how that one measures.

 

[old guy]

Ok, I did a bit more reading on the correct way to do this ° stuff, this time I put a ring in the cyl. & backed it up to the port, turns out my ground port actually opens at 95°, and the new cyl. at 103-4°, education is hard sometimes!

 

[Yotaismygame]

That makes a lot of sense to me. I do find 95 degrees to be an interesting height though. I've got another jug that I raised the exhaust enough that compression was lousy. It made for a lazy saw. I don't know its exact height but if the jug on my jred is at 101 im thinking this other jug is around 95-98. Even with this jred at best im getting 150psi. It could be the ring, I had used this ring originally in another cylinder for a tank of fuel and then swapped on another jug. Might not be seated very well yet or wont ever. But aside from that I have the Hyway mega pop up in it which is .050 of pop up and still only getting 150psi.

 

[Leafy]

That makes a lot of sense to me. I do find 95 degrees to be an interesting height though. I've got another jug that I raised the exhaust enough that compression was lousy. It made for a lazy saw. I don't know its exact height but if the jug on my jred is at 101 im thinking this other jug is around 95-98. Even with this jred at best im getting 150psi. It could be the ring, I had used this ring originally in another cylinder for a tank of fuel and then swapped on another jug. Might not be seated very well yet or wont ever. But aside from that I have the Hyway mega pop up in it which is .050 of pop up and still only getting 150psi.

That lazy jug probably just needs the blow down fixed and a bigger carb to be a cookie cutter. Probably still a dog off of idle but scream with a power band from 12k to 15k.