372 bb kit first porting attempt

[sprintmotor45]

Well I did my first attempt at porting a 372 for myself. I have a couple of questions about the compression amount, I am thinking I changed port timing too much. Well here goes with the story: First thing I did was used a new 372 75cc big bore kit from Baileys for my project. I first cut .025" from the base of the cylinder in the lathe, Used a base gasket that was .021 thick, cut the squish area of the piston top down .005" to produce a little popup and not end up with too tight of squish for a work saw. I will check my squish amount tomorrow. I removed the area on the bottom of the cylinder next to the transfers until the area was flush with the transfers. matched the transfer bottoms to the base gasket, widend the intake and exhaust ports.080" raised the exhaust port roof.060" lowered the intake port .040" , I did not touch the transfers in the cylinder. After puuting together with oil on the rings I had 150 pounds of compression. After 5 to 10 mins. of running I have 140. I was hoping for 160. Will the compression stay at 140? or will it climb after a few tanks? I have not modded my muffler yet but after these mods it feels equal in power to my fresh 372 with a muffler mod. Did I get too aggresive with my ports? I know I should of checked with a degree wheel but my printer is junk and had no way to print the wheel.

 

[rmihalek]

I think you'll get a better idea about the compression after 5 tanks, maybe 10. I think there are stories here about saws still getting higher compression up to 20 tanks of fuel, so don't worry right now.

Definitely measure the squish using the solder-on-the-piston trick to see where you're at before you run it too much. If it's 0.020" or so, then run it a bunch, maybe even get some video on a log and watch the video with a stop watch to see how fast its cutting.

Measure the port timing duration in degrees of crankshaft rotation so we can get a better idea where it's at.

 

[mdavlee]

Did you measure the squish before you cut anything? You only widened .080" or that is what you left on each side of the piston skirt? Raising the exhaust will hurt compression some. It will gain some after a little bit of time on it.

 

[parrisw]

The way I read what you wrote was that you didn't check anything before grinding?? Say it aint so. You need to check your numbers before doing anything.

 

[mdavlee]

That's what I gathered from what he posted. I think it will probably be pretty tight with only a .005 popup. and .025 off the base.

 

[nate cutter]

my compression also went down on the BB from 168 to 150 after 10 tanks or so I'm trying to get a good picture of the rings they have a groove down the center of them does yours?

 

[mtngun]

raised the exhaust port roof.060"

I would not have raised the exhaust.

 

[mtngun]

the rings they have a groove down the center of them does yours?

All the BB rings have a tiny groove in the center.

 

[AUSSIE1]

It doesn't sound like you have done anything drastic at this stage from what I can see.
You should do a complete check of everything before any cutting.
At this stage your best bet is to check your No's with a degree wheel.
An 064 piston turned into a popup will gain you some compression.