Issues after work-over on Makita 6401

[PES+]

.004˝ per inch. of bore is the standard but a little more is better than a little less when choosing.

 

[NPKenny]

Insult to Injury

My new 7900 piston and cylinder:

So compression reads at ~125psi, I have not even started the saw or even installed the carb and this score is on the virgin piston and cylinder!!! The score is only visible as I can't feel it when I run my finger over it. I can only begin to guess what the cause is.

I think if I were not resilient, I would begin purchasing firewood. However, this is not the case and I am determined to make this saw run (with the help of arboristsite that is).

 

[RED-85-Z51]

You DID oil the parts up before assembly right?

I coat the piston/ring(s) with some fresh 2 stroke oil before I slide the jug on, looks like you were pulling through dry, metal to metal contact is the key to wear. When a 2 stroke is running, the oil is all over everything, creating a thin film that prevents metal from touching metal. It is only when this layer breaks down, or there is debris intrusion that wear occurs.

Let me ask you this. If you rebuilt your car engine, would you put the rods on dry, main bearings on dry, and slide the pistons into dry cylinders, then crank it for 15-20 seconds? Of course not. Same principle applies here.

 

[NPKenny]

You DID oil the parts up before assembly right?

I coat the piston/ring(s) with some fresh 2 stroke oil before I slide the jug on, looks like you were pulling through dry, metal to metal contact is the key to wear. When a 2 stroke is running, the oil is all over everything, creating a thin film that prevents metal from touching metal. It is only when this layer breaks down, or there is debris intrusion that wear occurs.

Let me ask you this. If you rebuilt your car engine, would you put the rods on dry, main bearings on dry, and slide the pistons into dry cylinders, then crank it for 15-20 seconds? Of course not. Same principle applies here.

I am a fool and my own worst enemy.

Good thing it is my saw and not someone else's. Can I buff the score marks and try it again or is it an unsalvageable situation. I can only see the slight score, I can't feel it.

 

[Freakingstang]

Oil helps, but even without, there will be oil there before it fires. It looks like a dirt mark. I bet the p/c wasn't clean.


How deep is the gouge? It doesn't appear that deep in the pics. Rough it up with some scotch brite and WD-40, clean it down with brake parts cleaner. do the same with the cylinder.

 

[Deebo]

Balls of steel!

Kenny I applaud you of your bravery w/no 2-stroke knowledge to get in there and grind on your cylinder. Port timings are very precise and need to be for a 2-stroke to perform well. When you lube your cylinder next time you assemble it, it may even bring your compression up. My bet is that your compression guage is off(fairly common in the world of cheap compression testers) especially seeing your compression reading when you only had a .008 squish. It is possible to have hogged out the ports enough to make your compression numbers lower substantially. But you said you just touched on them I believe...

 

[NPKenny]

Kenny I applaud you of your bravery w/no 2-stroke knowledge to get in there and grind on your cylinder. Port timings are very precise and need to be for a 2-stroke to perform well. When you lube your cylinder next time you assemble it, it may even bring your compression up. My bet is that your compression guage is off(fairly common in the world of cheap compression testers) especially seeing your compression reading when you only had a .008 squish. It is possible to have hogged out the ports enough to make your compression numbers lower substantially. But you said you just touched on them I believe...

Bravery sometimes comes due to ignorance. I figured I would only be able to get so far without digging into something and seeing for myself. I have learned more than I thought existed in the process of this thread and the saw work that accompanies it.

I will touch up the piston and cylinder as described, clean and lube and get back into it. After pondering on it last night, I do agree that the cold compression should come up slightly with oil as the oil should also act as a bit of a seal as well.

As a note, I have not performed any port work on the 7900 P&C, only on the original, low compression 6400 P&C. I did smooth some casting errors that were very obvious near the bottom of the transfers.

 

[RiverRat2]

Sent ya some rep Bud,,,, I figured you could use a boost!!!!! Your saw should rock,,,,, You will be smiling!!!!!!

Stock 7900's do well with just a simple Muffler Mod,,, BTW

But whats bad is you will be planning your next saw before you know it!!!!!


LOL

 

[timberwolf]

+1 I'l rep ya too, shareing your learning experience as it was takes some balls.

There are a lot of details involved in moddifying an engine, and they are all important, sometimes for reasons that are not clearly evident from just looking at things.

It was mentioned earlier on squish that the clearance was needed for thermal expansion, I don't think this is realy 100% true. Only the portion of the piston from the rings up realy heats up and lengthens, and it's a very small amount, the rod is cooled by the charge so it should not be getting hot at all. On the other hand the jug heats up and lengthens so in reality I think squish clearances might be greater in a warm engine. And problems start long before there is physical contact betwean piston and head.

The issue of squish though is more complex, one aspect is that the rod of an engine is under tension as it comes towards TDC, in some cases there could be as much as 1000 pounds pulling up on the rod streaching it out. The other issue is in knowing the peak flame temperature in an engine can be over 2000f but yet aluminium melts at about 1200f, so then why does the heat of combustion not melt and burn away the surface of the piston? This is to do with boundry layers, there is a thin layer of fuel and air about .005 to .010 tick that does not burn during combustion and protects the piston and head from the heat of the flame front. If squish is run too tight this boundry layer is forced away allowing the combustion flame to directly contact the pistons and head surfaces heating them and literally boiling or burning away metal. I have seen this on an experimental saw I ran down to about .007, there were little craters and visible signs of errosion in the squish band. Once the boundry layer has been stripped away more heat will be directed into the piston crown heating it up over 500F this swells the piston crown and you risk seizing the saw up, but the killer is once the piston crown heads up over 500F it hits the auto ignition temp range of gasoline and starts to light up the charge in the squish area which makes for additional heating and a runaway situation.

Call this overthinking, say that you don't need to know this to build a saw and you may be right, but if you are interested in building a fast saw understanding some of these things and why to do or not do stuff helps goes a long way to wards avoiding problems. There is a fair bit of good reading out there on engine design and modification, even some 4 stroke car engine info can be drawn in to help understand 2 strokes..

From the dammage on the piston and cyl, makes me question what are the chanfers or bevels on the port edges like? Got a phot of your port work? Though rereading, I see the 7900 exhaust port was not touched, so I would guess some dirt got into it on start up.

 

[PES+]

More info

+1 I'l rep ya too, shareing your learning experience as it was takes some balls.

There are a lot of details involved in moddifying an engine, and they are all important, sometimes for reasons that are not clearly evident from just looking at things.

It was mentioned earlier on squish that the clearance was needed for thermal expansion, I don't think this is realy 100% true. Only the portion of the piston from the rings up realy heats up and lengthens, and it's a very small amount, the rod is cooled by the charge so it should not be getting hot at all. On the other hand the jug heats up and lengthens so in reality I think squish clearances might be greater in a warm engine. And problems start long before there is physical contact betwean piston and head.

The issue of squish though is more complex, one aspect is that the rod of an engine is under tension as it comes towards TDC, in some cases there could be as much as 1000 pounds pulling up on the rod streaching it out. The other issue is in knowing the peak flame temperature in an engine can be over 2000f but yet aluminium melts at about 1200f, so then why does the heat of combustion not melt and burn away the surface of the piston? This is to do with boundry layers, there is a thin layer of fuel and air about .005 to .010 tick that does not burn during combustion and protects the piston and head from the heat of the flame front. If squish is run too tight this boundry layer is forced away allowing the combustion flame to directly contact the pistons and head surfaces heating them and literally boiling or burning away metal. I have seen this on an experimental saw I ran down to about .007, there were little craters and visible signs of errosion in the squish band. Once the boundry layer has been stripped away more heat will be directed into the piston crown heating it up over 500F this swells the piston crown and you risk seizing the saw up, but the killer is once the piston crown heads up over 500F it hits the auto ignition temp range of gasoline and starts to light up the charge in the squish area which makes for additional heating and a runaway situation.

Call this overthinking, say that you don't need to know this to build a saw and you may be right, but if you are interested in building a fast saw understanding some of these things and why to do or not do stuff helps goes a long way to wards avoiding problems. There is a fair be to good reading out there.

From the dammage on the piston and cyl, makes me question what are the chanfers or bevels on the port edges like? Got a phot of your port work?

As a side note Nich is @ 6,000 feet elevation so he kind of needs more compression than lower elevations.

I can't help but think that that is partially why he is tightening his squish in the first place.

His choice to change exhaust time and intake time plus the dropping of transfers by lowering the cylinder coupled with the worn out distorted piston makes for an unknown powerband crapshoot.

I suspect his compression tester is also low reading and when coupled with his elevation can make for readings that can cause worry over nothing.

 

[NPKenny]

OH Man, Buddy...

The 7900 set-up is stinkin' fast.

Thanks to all who have helped in the course of this post. I buttoned the saw back up over the last two nights after cleanining up the piston and cylinder (Thanks Freakingstang) and finally ran it last night. Saw burbled on the first pull with the choke and fired on the second pull at 1/2 throttle. I slightly adjusted the idle and ran the saw through a few rpm ranges staying away from full throttle and then put it in some wood with the H setting very rich. I tached it at ~11,500rpm and it cuts like a wild beast. I will lean out the H setting after about a tank of fuel, working up towards the 13,500 mark.

Final squish is at .023" accomplished in the final version by using the supplied base gasket and honing the cylinder base. Only work done inside the cylinder is some touch-up work on the transfers (very rough) and polishing of the ports. I guess I did lighten the piston some as well. Oh yes, a very simple muffler mod was performed...my wife wanted to help and I gave her a design she could handle. All I have left for this saw is a can of black paint for the top plastic and the muffler.

I also adjusted orientation of the kill switch since I kept accidentally turning this saw off to start it or inadvertantly killing it while running. Much better now.

I suspect his compression tester is also low reading and when coupled with his elevation can make for readings that can cause worry over nothing.

I concur, on both accounts. Both of my compression testers are going to take a rest as they have seemed to cause undue grief to me on multiple occasions.

 

[NPKenny]

But whats bad is you will be planning your next saw before you know it!!!!!


LOL

This is truth...I think I didn't sleep last night because I was so excited about the 7900 running that all I want to do now is get into the 660 I have in my shop.