MS461 AM pistons...Popup vs flat top

[James Sawyer]

I've been working on this old saw of my dad's, and I can save the OEM cylinder, but it needs a piston. I don't have a mandrel so I'm looking at a base gasket delete. I'm trying to decide on what piston? The OEM timing numbers aren't bad with gasket. The exhaust is at 100 and blow down is 19... the intake with gasket delete not quite what I'd like.

So... I did some piston comparison between the OEM, Meteor and Hyway Popup. Bottom line there are some differences. I know I could have bought an OEM piston for what I paid for these AM pistons, but I wanted to learn something about these popups.

Hyway piston is heavy...105.5 grams, you'll have to decide for yourself if you think they are close to the OEM measurement as far as piston skirt lengths etc. My thoughts the Hyway could help a little with the intake time if I did a base gasket delete. If I went with the Hyway a course I would want to check the timing number before deciding. The longer Hyway skirt would give me about degree on my intake numbers after thinking about it. The Meteor is direct replacement with the OEM. Here are my measurements (PT = piston top). I included a few photos as to show how I did my measurements, they were done on a mapped machinist granite so their accurate!

I'm assuming I'd increase the exhaust and intake timing about 2.25 degrees with a base gasket delete. The squish is like .043 if I remember right.

OEM Meteor Hyway
Wt. 97.0grams 98.0 grams 105.5 grams
intake to PT 1.560" 1.556" 1.690"
Ex to PT 1.723" 1.722 " 1.728" (popup is 022" or the number would be 1.750)
PT to bottom pin 1.357" 1.361" 1.3775

The casting on both the Meteor and Hyway both need some work, hard to say which one is worse. I think the window on the Hyway are better thou. The skirt on the intake isn't flat on either AM. The Hyway had casting flaws on the intake.

I was always curious about these popup pistons but never used one and I really couldn't find much on them. This MS461 with its wrist pin height I think as a tendency to rock in the cylinder... with the extra wt. of the popup it would make it worse.

Has anyone had any experience with these popups and done a base gasket delete. Should I delete the base gasket with popup? I just thought I'd ask for some input.

 

[singinwoodwackr]

I’ve used a couple of the pu Hyways in the past but may not repeat after reading several threads on OPE of late. The pu may not really help performance after all.

 

[thompsoncustom]

Whats your current intake timing? I'm not a fan of pop up's if you want more compression get machine work done the pop up just doesn't add enough to matter.

you don't want to add more weight to the rotating mass (piston) if you don't have to the lighter the piston the better. I think saw salvage (The Dukes) coated pistons tend to come in the lightest tho all of the pistons in the pictures above look like you could grind some weight off of easily.

 

[James Sawyer]

Whats your current intake timing? I'm not a fan of pop up's if you want more compression get machine work done the pop up just doesn't add enough to matter.

you don't want to add more weight to the rotating mass (piston) if you don't have to the lighter the piston the better. I think saw salvage (The Dukes) coated pistons tend to come in the lightest tho all of the pistons in the pictures above look like you could grind some weight off of easily.

ya... the wt.. not good. I read where some thought they weren't even sure if popup add any extra height, maybe it was just factory piston machined to a dome. That why I measured it from the pin I wanted to know. The add compression I read was only about 10psi.

My intake at 111abdc so about 69btdac

 

[cookies]

The am pop up design adds about 10psi that i've seen and imo it messes with the transfers ability of directional airflow. I scored a pop up piston trying to set it by ear in a 390, had to set it by tach...am pistons always need work to clean up casting and often require reshaping the windows. I'd get a oem piston and deck/set the cylinder port numbers up for it that way next time you can just slide a new oem piston in it and not need to deal with am inconsistency.

 

[thompsoncustom]

ya... the wt.. not good. I read where some thought they weren't even sure if popup add any extra height, maybe it was just factory piston machined to a dome. That why I measured it from the pin I wanted to know. The add compression I read was only about 10psi.

My intake at 111abdc so about 69btdac

Well if your squish is .043 and your intake opens at 69 degrees I say delete the base gasket and run the shortest shirt as you need more intake duration anyways.

 

[James Sawyer]

The am pop up design adds about 10psi that i've seen and imo it messes with the transfers ability of directional airflow. I scored a pop up piston trying to set it by ear in a 390, had to set it by tach...am pistons always need work to clean up casting and often require reshaping the windows. I'd get a oem piston and deck/set the cylinder port numbers up for it that way next time you can just slide a new oem piston in it and not need to deal with am inconsistency.

Love to deck the cylinder but I'd have built a mandrel. The only good ports I know these days in my area are Matson or I heard there's a guy in Florence or Coos bay Oregon. I'm sure there might be others but I don't know anyone that would just deck my cylinder. I comfortable with porting but just don't like machining my cases.

 

[thompsoncustom]

Love to deck the cylinder but I'd have built a mandrel. The only good ports I know these days in my area are Matson or I heard there's a guy in Florence or Coos bay Oregon. I'm sure there might be others but I don't know anyone that would just deck my cylinder. I comfortable with porting but just don't like machining my cases.

most porters would like the whole saw but if they have a builder case than some will do the machine work on just the cylinder for ya without sending the whole saw.

 

[James Sawyer]

probably go with the meteor piston with base gasket delete see what the squish comes out at. think about the cases.