A rider ...

[pd]

Ha . That would be a sight .

Pete .

[carryall]

Oh c'mon, we've all done it... distributor vacuum advances and supply lines, EGR valves, fuel petcocks, (that one just don't sound right) etc...as long as you don't do it for money, it's O.K.

[pd]

BTDT , But , NEVER a Simplex crank case . LOL

I'm sure that I've bounced a choice word or two off it , but ...

Anyway , I'm working on cleaning stuff and I looked into the exhaust port from the front of the cylinder . Something seemed a bit odd . I slipped the piston in and brought it's dome exhaust edge to the bottom of the ports . The ports are not level . One is a couple MM below the other . That goes for the transfer ports as well , due to the fact that transfer and exhaust ports are drilled in the same stroke , from the exhaust side .

As for what I thought might be a crack , I'm leaning real hard toward it being a casting 'snake' . I found no corresponding crack or snake on the inside of the case . I found the other snakes I'd mentioned before .

Pete .

[carryall]

  I wonder if those uneven ports would be negatively affecting the scavenging of air flow then?  Do you think "that" could be a/the contributing factor to your seizing possibly??? 

[pd]

In my thinking , the uneven ports have to create a different flow pattern . I'm not sure if that's good or bad in this case , but , I know it most likely was not the original design . That different flow pattern might contribute to excessive heat , which would help warp things like pistons and exhaust manifolds .

Every motor and every mechanical drawing I've ever seen has 'like' ports even .

Port position is the key to port timing , just like intake and exhaust valves in a 4 stroke . Get those wrong and the motor will suffer performance , if it runs at all .

I think I have some 'clean up' work ahead . Fortunately , I have 'reworked' 2 stroke porting in a couple motors and had excellent results . So , the work is not totally strange . I don't intend on changing things very much . Just enough to hopefully gain OEM intent with better/smoother flow .

Pete .

[pd]

Those slots may help aid lubrication, but I think they where put in to keep the piston from seizing when it got hot.  Wayne's replacement, as well as the ones that used to be available from Bob Kerr, had no slots.

Rick

In researching port timing and piston skirts I came across :

"Fitting pistons into the cylinder properly is very important. Because metal expands when heated, space must be provided for lubricants between the pistons and the cylinder walls. Pistons must have features built into them to control expansion. Without these features, pistons would fit loosely in the cylinders when cold, and then bind in the cylinders, as they are warmed up. This is the problem with aluminum because it expands so much. The pistons (fig. 3-25) may be designed with the following features to control expansion:

It is obvious that the crown of the piston gets hotter than the rest of the piston. To prevent it from expanding to a larger size than the rest of the piston, it is machined to a diameter that is approximately 0.03 to 0.04 of an inch smaller than the skirt area.
One way to control expansion in the skirt area is to cut a slot up the side of the skirt. As a split-skirt piston warms up, the split merely closes, thereby keeping the skirt from expanding outward and binding the piston in the cylinder.
Another variation of the split-skirt piston is the T-slot piston. The T-slot piston is similar to the split-skirt piston with the addition of a horizontal slot that retards heat transfer from the piston head to the piston skirt.
Some aluminum pistons have steel braces cast into them to control expansion.
The skirt, or bottom part, of the piston runs much cooler than the top; therefore, it does not require as much clearance as the head."

From : http://www.free-ed.net/free-ed/Courses/15%20Transportation/HeavyVehicles/HeavySystemsShow.asp?iNum=0201

Rick nailed it .

It seems the OEM pistons may have been ahead of their time . Unless skirt slotting was common practice in those days . Anyway , now we have conformation , of sorts , and a better understanding .

Pete .

[carryall]

That's a interesting "fix", and although it would help with the uneven heat expansion/seizing issue, and promote better lubrication, it just seems it would weaken the skirt too much, and promote the possibility of those cut areas cracking/breaking off?   What's the top RPM a good Servi motor will produce?

[pd]

The piston pictured is my OEM piston , replaced by a Wayne piston that has seized way too many times ( for unknown reasons , at this point ) .

I'm tempted to slot the Wayne piston , but I don't know if it is designed for that . I'll likely simply hole the back skirt just to help cooling . Lubrication doesn't seem to be a problem as every time I've torn the motor down the piston lower ( below the rings ) has been wet .

RPMs :

For those interested, I read a Simplex advertisement relating to the Simplex Automatic which lists its design improvements and specifications.  It states that the 125cc engine has a 6:1 compression ration and developes 4 hp at 4000 rpms.

Pete .

[Ricks]

Pete,  I have run Wayne's pistons in 3 motors and know many more motors running them.  The piston is not the problem, something else is.  Bob Kerr also made pistons, and his didn't have any relief cuts in the skirts either.  I would have to think the metallurgy in pistons is far better now than was available in the 1930's, when this was designed.  I would bet the thermal expansion of a modern piston is much less than the olden days. 

Now that the motor is apart, see if you can measure the clearance between the crank and the case port.  If that is too large, the case may need to be sleeved.

[pd]

I've been thinking how I can measure that difference , with the tools I have . I think I might be able to get a decent measure , but obviously won't know until I try .

The motor has only 'spit' back through the carb a couple of times , early on and that was barely noticeable , IIRC .

But , I'll give it a shot , if only to know for sure .

Pete .

[pd]

Check my math , please .

With a machinists dial caliper I measured the depth of the carb throat to the crankshaft ( 1 ). Then removed the crankshaft and measured from the same spot on the carb mount to the deepest part of the rotary bore for the crankshaft ( 2 ) .

After multiple tries and coming up with the same numbers :
1 - 1.960"
2 - 2.947"

And the crank measures .984"

I ended with a .003" clearance .

I think that .003" is right at the service limit .

I'll do a couple chores and remeasure , again .
Like my master craftsman father told me : "Measure twice or thrice and cut but once ."

Pete .

[Ricks]

The math checks out. 

Probably not the way I would have done it.  I would have used a telescoping bore gauge to measure the bore, and a micrometer to measure the crankshaft.

I would think .003 would be well within tolerance.

[pd]

Like I said "with the tools I have" ...

Anyway , I went back out and measured the same way again and I just didn't feel good about the slight differences I was reading . I put the caliper away and found my feeler gauges . Put the crank in place ; lined the crank port with the intake port ; inserted a .003" into the crank port and turned the crank . I felt a little tension . Then on to .004" , more tension . .005" , the tension felt closer to right . .006" , way too much tension .
So , it seems , with that method , I have .005" vacuum clearance . That's probably a couple hairs out of spec .
I'm not really sure that method would be NASA approved , but ...

Pete .

[Mike Sal]

Holy Cow....you guys have turned this original thread into a novel....if you could bust it up into all the different topics covered in these 40 pages & put them into the categories we already have on this site, new guys would have a heck of a leg up on finding information. 

Off the soapbox....PD sorry to hear you've had so many issues.....I can somewhat relate....my bike has sure been a challenge....I've still not messed with it since we got home from Portland.....they say it will stop raining someday so I can tinker with it.....we'll see.....
Mike Sal

[pd]

Mike ,
It didn't happen over night . Like fine bourbon , it's aged .
Hot and humid between rains , here . Wanna sweat ? Just walk a few steps . They say it'll cool ... Likely the day before we head South .
We just put up with it . Less stress that way .

I hope you can get to yours some day .

Pete .