Redneck cylinder milling?

[eyeinstine]

What kind of lathe do you have? How about a pic? Later, Roger.

Hi Roger, et al,

the legs on the lathe have w.p. davis casted into them, but the casting in ID tag on the lathe head itself has Frevert Machine company..
Here's some pics, finally!

the 2nd pic has the dolmar 100 piston in the lathe. The 3rd pic is the dolmar in pieces prior to the piston work.

I chucked the piston in the lathe with a scribed mark on the piston at .020. I made three small cuts and different angles. think of a 3 angle valve job on a automotive head. I then took the piston out, and smoothed over the tips of the angles, and then i smoothed and polished the newly domed area of the piston with some heavy leather strapping. Came out real nice.. I had a great close up pic of the piston, but dang it, i accidently deleted it before saving to the computer.

If Ed or anybody is still following this thread; how does my piston work sound?? I wish i had that dang picture, it would have shown exactly what im talking about and what i did!

Thanks
Ron

 

[njforestfire]

Eye - let us know how it works out. Did you turn the piston crown down to look like that drawing - the one in the earlier post?

 

[canguy21]

Does the 100 have a separate jug? If memory serves me right and it is questionable, that saw has the horizontally split cc like most of today's consumer saws.

 

[ehp]

Hello, my Dad has that same lathe at his house, it works fine.
Now on your 3 angle piston, this is just my thinking ok so it maybe right but then again it could be wrong.

what I try and do when cutting a piston is first look at the chamber in the jug, is it flat on the outside, is it angled up and at what degree is it angled up.
you want the top of your piston to be the same as the top of the jug before it goes into the dome.So when you measure the squish band it should be the same thickness at the outside edge and at the point just before you enter the dome of the jug.
Now when you cut the piston you need to lower your jug because you have increased your cc area in the head so you have ,lowered your compression, you need to get your compression back up or you have lost power not increased it, I think you were going to pull your base gaskets out correct , if it is .020 or a little thicker you are fine, plus if you donot lower your jug with you cutting the piston you have increased the tranfser and ext. porting numbers.
What I can see you do a very good job and take your time and do it right, this is good and I am sure she will run fine once you get her back together

 

[eyeinstine]

Eye - let us know how it works out. Did you turn the piston crown down to look like that drawing - the one in the earlier post?

John,

actually, i forgot about that draing that was posted, so in a way, no i didnt really follow it.. I think it is talking about the same thing in his post right after this one, so i'll describe more about it on thta reply..

Thanks,
Ron

 

[eyeinstine]

Does the 100 have a separate jug? If memory serves me right and it is questionable, that saw has the horizontally split cc like most of today's consumer saws.

Yup, seperate jug.. But no, the case is split vertically just like most other quality, or pro grade saws.. It really is a well built little saw..

Ron

 

[eyeinstine]

Hello, my Dad has that same lathe at his house, it works fine.
Now on your 3 angle piston, this is just my thinking ok so it maybe right but then again it could be wrong.

what I try and do when cutting a piston is first look at the chamber in the jug, is it flat on the outside, is it angled up and at what degree is it angled up.
you want the top of your piston to be the same as the top of the jug before it goes into the dome.So when you measure the squish band it should be the same thickness at the outside edge and at the point just before you enter the dome of the jug.
Now when you cut the piston you need to lower your jug because you have increased your cc area in the head so you have ,lowered your compression, you need to get your compression back up or you have lost power not increased it, I think you were going to pull your base gaskets out correct , if it is .020 or a little thicker you are fine, plus if you donot lower your jug with you cutting the piston you have increased the tranfser and ext. porting numbers.
What I can see you do a very good job and take your time and do it right, this is good and I am sure she will run fine once you get her back together

Ed, if your Dad has the frevert lathe, than that i sthe only other one i know of, or could locate! There is NOTHING about them i could find on the web! Great machine though..

Thanks for sharing a more indepth description of how you work the piston.. This was my first attempt, and looking back, after reading this, i think i may have been able to do a tad better job. I never though of looking at the port angle, and matching that when cutting the piston edge. Makes sense, so it is a straight smooth shot, right up into the dome of the cylinder. Also, i didnt measure and scribe for the horizontal amount of material to be removed from the piston. I should have paid more attention to that drawing that was posted, as that gave a clue for the importance of that measurement. But i'm pretty sure i am very close anyways. I did remove the base gasket, and it mic'd out at .021. So i removed .020 from the piston; assuming the gasket was going to squish a tad to .020 or .019 when it was tightened. I used the loctite flange stuff instead of the base gasket. So hopefully, i did what i set out to do, and bump the compression up while maintaing close to the original port timings..

Now if i could just find a ???? ignition module for it! I really cant justify spending the 65$-70$ for a new one.. I already have about 30$ into this thing (not counting the 14$ for the loctite flange gasket stuff!); and this is just a fun experiment saw.. So if it craps out, i can scrap it and not feel so bad.. Sooooo, anyone out there with a dolmar 100/102 coil thats good, i'm in the market! Or a whole parts saw! I cant believe there hasnt been another one on ebay in months.. i check every day... oh well..

Thanks again, and have fun!
Ron

 

[jimbo1490]

To put in perspective what might happen to total power output if you lower the peak power RPM, consider how torque, HP and RPM are related mathematically.

(Slight math alert; skip to last paragraph if allergic )

One horsepower is set at 33000ft-lbs of work per minute. Since these foot pounds describe work done in a straight line in one plane, it is necessary to use 2pi as a multiplier to quantify the work being done by a rotating shaft, since 2pi(r) describes the distance around a shaft with a radius r of 1 unit. The radius is always set at 1 to effectively remove the shaft radius measurement from the calculations. The torque notation in lb-ft on the shaft sets a 'virtual' radius anyway, which is again, 1 unit. The minute part of RPM accounts for the time notation in the HP definition.The HP definition uses the 'foot' as the linear measurement, so the radius unit is likewise 1 foot, rather than 1 meter, fathom, cubit, mile or whatever. So on a rotating shaft, 1 HP = Torque X RPM X 2pi[around 6.28]/33, 000, or simplifying, HP= Torque X RPM/5252, since 33,000/6.28 = 5252.

If you play around with this equation by plugging in various torque and RPM numbers, you will soon figure out that it's much easier to make horsepower by increasing RPM's. For instance, if torque stays flat and RPM doubles, HP doubles. Even if torque falls a bit when RPM doubles, HP will rise a bunch. Even with a $hit load of torque, without high enough RPM's you have little HP. If RPM falls to zero for instance, the HP is zero no matter how high the torque.

A very popular small gas turbine engine has only ~50lb-ft of torque, but makes it a 50,000 rpm, which equates to nearly 500HP. A turbocharger on a car can easily dissipate 30 HP or more. On a diesel truck the turbo dissipates more like 100HP. It's easy when you're spinning at 100,000+ RPM.

If your proposed modification increases torque slightly while lowering peak RPM, HP will likely fall slightly, or stay nearly equal. An engine like this will be said to have a broader power curve, which is an advantage in some applications, especially when the engine is expected to accelerate under a load. Chainsaws don't generally have to do this trick when they are operated properly; we operate them a full RPM always and select a bar and sprocket appropriate for the expected load.

Jimbo