Building a 372XP Big Bore With 272XP Piston

[Rudolf73]

Intake timing ending up a little longer than I would have liked it.........but we'll see if it's a runner. I've got a few more pics.....but my camera is in the shop and I ain't unlocking the door. :msp_wink:

Did you have to do much to the exhaust roof? Some of the 372 BB kits have a funny shaped one.

 

[Mastermind]

Did you have to do much to the exhaust roof? Some of the 372 BB kits have a funny shaped one.

I raised it about .100 anyway. :msp_sneaky:

 

[4492011]

Its tomorrow where is the vid

We get to run it tomorrow?

ok its tomorrow where is the vid?

 

[Mastermind]

ok its tomorrow where is the vid?

We had a problem.........It seized up after about a minute of run time. I used a Meteor piston and a no name cylinder kit. I wonder if the piston being a "B" made it too tight. The jug will clean up.......I think we'll try the kit's piston and see if it runs.

If you're gonna make an omelet ya gotta break a few eggs. :hmm3grin2orange:

 

[blsnelling]

We had a problem.........It seized up after about a minute of run time. I used a Meteor piston and a no name cylinder kit. I wonder if the piston being a "B" made it too tight. The jug will clean up.......I think we'll try the kit's piston and see if it runs.

If you're gonna make an omelet ya gotta break a few eggs. :hmm3grin2orange:

Ahh man! I hate it when that happens:mad2:

 

[Rudolf73]

We had a problem.........It seized up after about a minute of run time. I used a Meteor piston and a no name cylinder kit. I wonder if the piston being a "B" made it too tight. The jug will clean up.......I think we'll try the kit's piston and see if it runs.

If you're gonna make an omelet ya gotta break a few eggs. :hmm3grin2orange:

Hmmm that's no fun, but a little bit of extra tightness can do that to ya.


But at least you didn't have zero squish issues right...

 

[tbone75]

opcorn:

 

[Mastermind]

Ahh man! I hate it when that happens:mad2:

Jon pulled it apart.......we cleaned up the piston and the jug......it was pretty minor scuffing. A new ring in it and we'll try again tomorrow. I really want to see how this does with the 272 piston. It's a little looser now.

Hmmm that's no fun, but a little bit of extra tightness can do that to ya.


But at least you didn't have zero squish issues right...

It's all good...these kits make it cheap enough the play with some stuff I probably wouldn't be able to otherwise.......it's fun at least.

opcorn:

I hope you brought enough for us all. :cool2:

 

[davebull]

Here's a few pics......I'll be adding to this one.

Opened up the transfer inlets a bit since the 272XP piston is a windowed design.





Here we are test fitting to the case. The match is worse than you can see here.....



Here it is after being case matched.......



Like I said......I'll post more as I set the squish and port it.

Hey Randy,

Got any info you want to hand over on the popup 272 piston? How much did you take off the top and how far in from the skirt did you come?
Any info would be great, I'm doing they same build with an xt carb and BB kit
Cheers from down under

 

[Mastermind]

Hey Randy,

Got any info you want to hand over on the popup 272 piston? How much did you take off the top and how far in from the skirt did you come?
Any info would be great, I'm doing they same build with an xt carb and BB kit
Cheers from down under

No popup here.......

 

[thomas1]

No popup here.......

Have you tried the blue pills?

 

[Mastermind]

Have you tried the blue pills?

Nope......that's "fuel doping".

:bang:

 

[davebull]

No popup here.......

Mmmm.....

 

[Mastermind]

Mmmm.....

I cut the squish band instead.

 

[greg storms]

squish band & relationship to compression ratio

You'll see us talk about cutting the squish band in these engines. That decreases the volume of the combustion chamber......then we turn the base of the jug to set the squish clearance which raises compression. At times lowering the jug also allows us more control over the port heights.

thanks for the explanation. I have a few saws to rebuild & would like to do it right!
Once, a 1978 evinrude 100hp x 4 cyl haqd a piston melt & when I rebuilt it, the replacement piston came with the top ring about 4-6 mm lower than the original....as it turned out, the mfg designed the original with the top ring too high, therefore too close to the combustion chamber/intake& exhaust ports. Learning is fun!

 

[Mastermind]

thanks for the explanation. I have a few saws to rebuild & would like to do it right!

You are welcome. I don't mind sharing at all.

As a matter of fact I ran this saw some yesterday at the East TN GTG.....and will share my thoughts.

I don't like the way it runs at all. I'll not be using windowed pistons in these engines anymore. I just needed to try it to see how well it works.......not as well as a non-windowed piston as far as I can see. It has too little crankcase pressure in my opinion......Now I need a different top end.

 

[greg storms]

windowed piston vs non type

You are welcome. I don't mind sharing at all.

As a matter of fact I ran this saw some yesterday at the East TN GTG.....and will share my thoughts.

I don't like the way it runs at all. I'll not be using windowed pistons in these engines anymore. I just needed to try it to see how well it works.......not as well as a non-windowed piston as far as I can see. It has too little crankcase pressure in my opinion......Now I need a different top end.

exactly what's the difference? I went to eKartingNews.com & they discuss strengthening the piston by glass beading the high stress area of piston lightly to increase resistance to melting. Also, they seem to advocate making windows in thinner areas on non windowed pistons. If we can get the saws to last like 2 stroke bike engines, they may be better, eh?

 

[Mastermind]

exactly what's the difference? I went to eKartingNews.com & they discuss strengthening the piston by glass beading the high stress area of piston lightly to increase resistance to melting. Also, they seem to advocate making windows in thinner areas on non windowed pistons. If we can get the saws to last like 2 stroke bike engines, they may be better, eh?

Getting them to run like a two-stroke bike is what we long for......they already seem to last just fine. :msp_wink:

In the 372XP the designers used a non-windowed, full skirted piston. I think that since the transfers are fed from the bottom of the jug they used this type piston the help more fully evacuate the crankcase on every stroke. Using a open type piston seems hinder the performance on the engine.....at least it does with it ported the way I port one. There is probably something to be gained using this piston that I'm missing......but I like the way they run with a full skirt better at this time.

Yesterday there was a Dolmar 7900 with a new slab sided type piston at the GTG and a Solo 681 with a 272 windowed piston. I built both saws for two different guys. The 7900 was holding over 1000 rpm more in then cut. That helped me to decide on what piston design was working better in that saw as well.....

 

[davebull]

I cut the squish band instead.

Ah yes I was being a little narrow minded, cheers
I noticed on my project the 272 piston doesn't really have enough meat between the ring land and the top to made a pop worth while...
I think I'll do the same

 

[MCW]

Ah yes I was being a little narrow minded, cheers
I noticed on my project the 272 piston doesn't really have enough meat between the ring land and the top to made a pop worth while...
I think I'll do the same

How do you intend to cut the squish band and how much do you need to take out?

Pretty funny after mentioning you'd ordered 5 x 272 pistons in the other thread for pop ups You'll have a few spares now