View Full Version : Milling the head of a 2 stroke engine
12-13-2003, 12:42 PM
I want to mill the head of a two stroke engine. The head has o ring seals. Once I mill the head I then have to mill the o ring grove. It is a two cylinder engine with one head.
How do I mill the o ring grove and match the radius on the four corners?
LES A W HARRIS
12-13-2003, 01:55 PM
1. A VOLSTRO HEAD $$$$$$$.
2. X-Y ROTARY TABLE.
3. N/C MILL.
12-13-2003, 02:05 PM
I don't know what a volstro head is?
This is a Seadoo PWC, 650cc with dual carb.
But I have a rotary table and bridgport, How do I locate the thing and get it perfect? or do you some how follow the original line free hand?
I take it the O-ring groove is a square with rounded corners, or something like that. Oh, boy.
Maybe some of the more experienced folks will have a clever way of doing this, but here's what I think I'd do:
Mount the head on a plate (the surface you're going to machine "up") so you can move it around and not worry about having to re-level it.
Machine the head flat, or whatever you're doing to it. I assume you'll be leaving quite a bit of the O-ring groove intact.
Now move the fixture plate (and the head) to your rotary table and start picking up the radii of the groove by eye using a pointed wiggler; shift the plate around and rotate the table until the wiggler follows a radius. Then switch over to an appropriate ball end mill and mill that radius. Move to the next.
Do the straight sections of the groove the same way, picking up the alignment with the wiggler and traversing the table until you follow the groove.
Happily, the groove doesn't have to be absolutely perfect, just good enough. Correct depth is probably more critical than perfectly following the groove, although of course you want it pretty close.
This could all be pretty tedious.
12-13-2003, 07:40 PM
It would be easier to do on a CNC mill. Possibly impossible on a manual.
Might be cheaper to look into a aftermarket racing head.
12-13-2003, 07:54 PM
To mill o ring grooves I use a homemade 90 deg. adjustable fly cutter, grind a tool bit to the groove size and go. The fly cutter uses a straight shaft and a 90 deg. adjustable shaft.
LES A W HARRIS
12-13-2003, 08:30 PM
1. A Voltro attaches to the quill of the bridgeport mill. It is powered by the mill, it has a slide that you can offset by upto six inches, and the whole thing can rotate about the spindle 360 deg. So you can machine any radius up to the slide capacity, by useing the attachment and the table x & y axis you can mill some complicated tracks or "o" ring grooves. As I indicated in my first reply, $$$$$. n/c did away with making parts this way. I think they are still available in the industrial supply catalogs,J&L etc.
2. x-y rotary table or attach an x-y table to the rotary table that you have.
2a. SGW's method will get you there.
2b. x-y rotary method.
X=machine X axis. x=rotary x axis.
Y=machine Y axis. y=rotary y axis.
I do not know your rectangle or radius on the corners. So for this example: two one inch diameter bores, spaced 1.500" apart, the "o" ring groove is 1.000" away from the bores, and say a .250" radii at the corners. So we have a rectangle of 3.500" x 2.000" with .250" radii at the corners. Align the machine spindle on the centerline of the rotary table. Align rotary to zero degrees with the "x" axis parallel too machine "X" axis. Mount the part with the bores parallel to ?x? also. Move ?x-y? to center of first bore, zero ?x-y?. Move ?x? to pickup the second bore, do not move ?y? if it is off location, rotate rotary a few minutes & re-zero. The centerline between the bores is .750" away in ?x?. Go there. X-Y & x-y should all be zero, also the rotary degrees setting.
The centerline of the first radius is at ?x? (-)1.500" ?y? (+).750". Set the radius, move one or the other, not both, ?X? machine axis .250". Bring the cutter into the part & go to depth, Lock ?z?axis. Rotate counterclockwise 90 degree?s. The rotary ?x? is now in line with the machine ?Y? axis. Move the rotary ?x?axis to ?x? (+) 1.500. Rotate the rotary table counter clockwise 90 degrees, Now reading 180 degrees total. Move the rotary ?y? axis to ?y? (-) .750". Rotate the rotary another 90 degrees counterclockwise, (270derees total), move the rotary ?x? axis to ?x? (-) 1.500". Rotate the rotary the final 90 degrees counterclockwise. Move the rotary ?y? axis ?y?(+) .750", cutter off the part, stop spindle, you are done.
12-13-2003, 10:01 PM
Thanks for all the great ideas, it seems it will be dificult without a cnc. I am doing a retro on a cnc so perhaps this will be my first prodject for it.
12-13-2003, 10:06 PM
I have worked on 2 strokes for many years and done quite a bit of study on them. I'll pass along a little bit of information that I think you should know. The cylinder head on a 2 cycle engine has quite a bit of engineering put into it. If you mill it, it's not like milling the head on a 4 cycle engine. Yes, it increases the compression ratio on both but on the 2 cycle there is an area right next to the outer edge of the combustion chamber where the hemispherical shape of the combustion chamber tapers out to nothing. This is called the squish band and it's engineered into the head to provide complete combustion. If you remove stock from the head this area should be put back in but if you don't know what the angle is or how deep to put it in you are going to completely alter the combustion process of your engine. Unless you know what you are doing I wouldn't even go there. That's not counting having to remachine the "O" ring groove. You would have to measure the radius and set up for this center, then put an indicator in the spindle and sweep the radius to make sure that you are on center and have correctly measured the radius. I agree with one of the other posts. Buy an aftermarket head. It will certainly save you a lot of work and possibly you will end up buying one anyway if you have and oops while you are machining the stock one. Hope this helps.
12-13-2003, 11:47 PM
How bad is the head? Does it have a blown out or eroded area? If the O-ring cleans up OK I wouldn't worry about the flat except to lightly stone it with plenty of WD-40 (world's finded stoning lube; good for nothing else). If the head will bolt down to the block flat without a gasket I wouldn't worry about it. if you can slip a feelet between the bolts, thats another matter.
Personnaly, I wouldn't face the head and re-cut the groove. If the groove was blown out I'd locally repair the erosion and locally machine the groove then I'd draw file and scrape the head to fit the block.
The block and head faces don't have to be flat. All the have to do is match contours.
12-13-2003, 11:52 PM
i had a Yamaha Banshee and put a Moto-Fast milled head on it. all they did was take a few thou off the factory head, they didn't recut anything in the combustion chamber itself. when i sold it, it was still screaming like a banshee, so to speak. i would say it depends on just how much you are planning on milling off. also, what does the o-ring seal? i doubt it contains combustion gases. is it just a seal for the water jacket? is there a corresponding groove in the cylinders, or is the cut just in the head with a flat surface on the cylinder? if the cylinders are flat, i would try to recut your groove manually. as long as there is a groove there in the general area where it shoud be, i would think the o-ring would seal. of course, maybe NASA assumed this and look what happened there. several others have mentioned you should just buy a new head. i say give this one a shot first. the worst that could happen is you have to buy a new head anyway.
12-14-2003, 12:04 AM
What I'm trying to do with this motor is increase the compression. I want to get it up around 165psi. Lots of guys have done this to get a little more performance and still be able to run pump gas. My manual gives a depth for the squish band of .035-.050. It doesn't say anything about the angle. The diagram shows the oring grove in the head, but this is incorect, the oring grove is actually in plate on the jug. I would like to think I can match the squish band, what do you think? After market heads are very hard, if not impossible to find, for this machine. I can send the head out and get it machined for $95.00. But then, what is the point of being a home shop machinist?
I also wonder if I could mill off just enough to leave the minimum squish?
That do ya think?
[This message has been edited by Bob Quale (edited 12-14-2003).]
12-14-2003, 02:31 AM
I'd leave it alone if higher performance is what you're after. Unless you have a racing budget and can cinduct your own instrumented tests, its seldom that application of this mod or that has a significant effect on power without a high price paid in reliability. I've seen a lot of perfectly good engine parts turned into shrapnel thanks to guys doing what the other guys are doing on a "monkey see monkey do basis".
I've know serious racers who leaked destructive "secrets" as a means of thinning the competition.
That said, increasing C/R is a classic means of increasing torque. Mill the head with regard to squish and head clearance and forget the O-ring. Fill the groove with RTV.
12-14-2003, 09:53 AM
There is a plate on top of the jug with the oring. I had two options,
mill down the plate and oring grove, leaving the head alone. The plate is cheep and replacable.
or mill the head and squish band, new head being expensive. I think I am going to mill the head and squish band if needed, I will try to mill untill minimum squish, and see what I get for compression. I talked to a shop that I do buisness with and does this type of thing, he said I need to take off about .020-.030 to get a good improvment. He told me about the squish band and said to open it if needed, but that was all. I have learned alot about that, from this post.
Once again I am amazed at the knowledge pool of this board! This is not a home shop machinist fourm, but a every thing you need to know forum. I'm beginning to think there is no limit!
Anyway I would like to learn more about the two stroke stuff, If there are any more ideas or cautions please let me know! I plane on doing the job next weekend, just so I can recheck to make sure I have not missed anything!
gbritnell-I had no idea what a sguish band was or was for, thanks!
12-14-2003, 12:08 PM
gbritnell and Forrest have some good advice.
I would further suggest that before you start to irreversably modify an expensive part some basic math might be in order.
You mention raising the compression to 165 pounds, this is not compression, but cylinder pressure. It is not a particularly good method of predicting potential power, but is a good method of determining health of a cylinder, piston ring sealing, bore wear etc. Milling 0.030 from a cylinder head will result in a negligable increase in cylinder pressure. If you assume a 3" bore and stroke, the most increase would be a little better than 1.6 psi.
Without knowing the combustion chamber volume and bore and stroke, it is difficult to predict the increase in true compression ratio. Just playing around with the same 3x3 engine and a couple of CC volumes, I cannot begin to approach even one point in compression ratio gain. This was assuming a cylindrical combustion chamber. A wedge shaped chamber would result in much less of an increase.
I recall from a dim and distant past of working with small block Chevys that there is a predictable HP and torque gain per point gain in compression ratio. This will be different in two cylinder, two stroke engines, but information is probably available, and any potential gain is probably as predictable.
To make a long story short, milling the cylinder head may result in a very small gain in power, but is just as likely to destroy the effeciency of the basic head design and result in a major loss of power.
12-14-2003, 02:05 PM
Here is how the whole thing happened, I have two older seadoos, They are underpowered by todays standards. I want to get a few more years out of them with my family but also want to increase the fun factor. Now I also have a stand up ski with a 200psi radical race motor, the thing is a rockett!. I do have to run high octane fuel (race fuel)do to the high compression. Now if I keep the compression at 165, I can get more tork and still run pump gas, stock is around 135psi. I talked to the speed shop about this and some other stuff, they tell me to reach 165psi I need to take off .020-.030 and then redo the squish band. Stock parts are available for this ski but not much aftermarket. This particular motor was only in production for a few years. I figure if I take .010 at a time and see where I go I should be OK. As they say in the two stroke work "it's all about compression".
12-14-2003, 02:10 PM
<font face="Verdana, Arial" size="2">Originally posted by Bob Quale:
Here is how the whole thing happened, I have two older seadoos, They are underpowered by todays standards. I want to get a few more years out of them with my family but also want to increase the fun factor. Now I also have a stand up ski with a 200psi radical race motor, the thing is a rockett!. I do have to run high octane fuel (race fuel)do to the high compression. Now if I keep the compression at 165, I can get more tork and still run pump gas, stock is around 135psi. I talked to the speed shop about this and some other stuff, they tell me to reach 165psi I need to take off .020-.030 and then redo the squish band. Stock parts are available for this ski but not much aftermarket. This particular motor was only in production for a few years. I figure if I take .010 at a time and see where I go I should be OK. As they say in the two stroke world "it's all about compression".</font>
12-14-2003, 03:02 PM
I used to do snowmobile heads all the time for the local ice drag racers. We had the fastest woman in the nation. I did it in the lathe.It made recutting the sqish band a piece of cake. To hold the head in the lathe I had a piece of 4" round with a 3/8" threaded hole tapped in the center. I would chuck that up and face it off to make sure it was true and put the head on it with a bolt and washer through the sparkplug hole. you can unmount it and put it back on the lathe with no hassle, just make sure there are no burs on the top of the head before you start.Put a last word indicator in your tool holder and tap your head in the jig until it is concentric with the spindle and adjust your compound so you have zero movement when moving it in and out. I could do 3 heads in 30 minutes once everything is set up. The boss charged 1 hour a piece. For the guy that wanted a little boost to thier every day sleds we would usally take off .015 to .020" off both the face and the squish band.
12-14-2003, 04:50 PM
I like the idea of the lathe, This head is a two cylinder so I don't think I can mount it well, I think i'm going to try mounting it on the mill through the plug holes and use a indexing table for he squish.
What do you think?
12-14-2003, 08:58 PM
We did it with 2 cylinder heads in the lathe also. If you have enough swing it's no problem. Either way you do it, use sharp tooling with WD-40 for cutting lube. You want a smooth finish. If you have a bad finish it will creat hot spots.
12-14-2003, 08:58 PM
Go to MacDizzy.com, its free until 1/1/04. He has some very interesting projects for two stroke motors. Also many formulas to calculate compression and proper squish band.
I used modeling clay to determine combustion chamber volume and compression.
12-14-2003, 11:00 PM
Just a side note-- You can also raise compression by boring the cylinder.
12-15-2003, 12:52 AM
I went to MacDizzy! Wow what a sight! Thanks