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maddog
12-04-2004, 10:36 AM
Hey all,

I have a need to make the combustion chamber
larger (lower comp) on 2-cycle motors.

I want to increase the head volume without
affecting the shape of the chamber. The
chamber is a dome shape and I need to make
a cutter (for aluminum) that conforms to
the origional shape of the dome.

Any sugestions out there for making a cutter
like this?

TIA... MD

toolmakerjim
12-04-2004, 10:38 AM
if all you want to do is lower the compression ratio add a couple of head gaskets...jim

Swarf&Sparks
12-04-2004, 10:55 AM
Or a piece of annealed copper the appropriate thickness.
Lin

maddog
12-04-2004, 11:06 AM
Thicker gasket is not an option. These
heads have whats called a squish band.
(Perhaps I should have mentioned that)

The squish clearance is critical (.025-
.035") to engine performance.

So, I need to make a cutter that conforms
to the inner dome shape (x).

GDamn it!


.../--\
--/ x \--



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[This message has been edited by maddog (edited 12-04-2004).]

[This message has been edited by maddog (edited 12-04-2004).]

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I hope you can get the idea,,, this POS
MB won't take my crude graphics without
f--king them all up.

[This message has been edited by maddog (edited 12-04-2004).]

ibewgypsie
12-04-2004, 11:27 AM
On my old flh panhead I had all them years.

I put four extra cylinder base gaskets under it to lower the 11.5:1 factory compression. It sounded like popcorn was poppin in them jugs.

If your block is a one piece, no help.

David

maddog
12-04-2004, 12:08 PM
Raising the cylinder is of no use. It would
change the port timing for one thing. Not to
mention screwing up the squish clearance.
(same as thicker head gaskets, no dice!)

What I need is to make a cutter that has
the same shape as the combustion chamber
dome. Has anyone made custom cutters like
this?

Thanks...

plastikosmd
12-04-2004, 12:13 PM
I mounted head on a faceplate and made up a radius turning attachment to re-cut a chamber. may work for your application

scott

shawnspeed
12-04-2004, 12:25 PM
I have re-cut 2-cycle heads on the lathe,using an old spark plug hole as a mandrel. grind a a HSS tool to the contour you need and start with light cuts. Make a bondo squeese of the head before you start, so you have a reference to re-cut your squish band. (wax the chamber, cut a section of thin wood,plastic or metal, close to the shape of the chamber,apply bondo to this edge and squeese into the waxed chambr,allow bondo to kick, Instant template.)You can also adjust compression in a 2-cycle by altering the port timing. (raising the exaust port a milileter or two, this will also enhance performance http://bbs.homeshopmachinist.net//smile.gif But If it was me I would add a few Copper gaskets first, to make sure you get the desired results,as the squish,in a low compression 2-stroke are not as critical. Are you trying to increase the RPM of your engine? Reducing compression should help. Trying to get it from detinating? retard the ignition timing,it's qucker than milling the head.

JRouche
12-04-2004, 12:35 PM
Whats the radius of the chamber. And how deep from the flat surface of the head to the top/inside of the dome?

Don't know how much material you want to remove but consider sanding it off.

You could make a dome out of wood (oak) or hard rubber the same shape as yer head then mount the dome to a shaft that fits your drill press or mill.

Clamp the head down, chamber up. Use some powdered abrasive in the chamber and spin the dome shape into the chamber. Would be very slow but doable.

Use oil to make a slurry from the abrasive.

If you could locate a grinding stone as large as the chamber you could form it to the correct shape then use the drill press/mill to turn it?

I have some large grinding cups for concrete that would work, check that avenue.
JRouche

sdeering
12-04-2004, 02:11 PM
Maddog, do I sense a turbocharger. If so I am planning on putting one on a 2 cyl 2stroke Polaris 700 and we should talk.
As far as a cutter that big it would most likely chatter real bad. I would make a template of how you want the radius of the pocket and squish and do it with a normal tool. Or cnc mill the chamber.
Stephen

ibewgypsie
12-04-2004, 02:40 PM
Yeah, my cnc mill got ran about five minutes this week. I got the huge indexer on it now. (for my harley wheel) My back is throbbing so it still ain't gonna see no action.

Take the digitizing probe, scan into machine. Raise table cut old pattern out. Same pattern just deeper.

There you are.

What time should I expect you? Are you bringing the beer? Actually the probe is apart, but I do have one.

David http://bbs.homeshopmachinist.net//biggrin.gif

Carl
12-04-2004, 03:28 PM
I have a video tape showing a machinist milling a convex dome using a flycutter on a vertcal milling machine. The head of the mill was set at an angle, and the work was on a rotary table. Maybe this method could be used for a concave area such as your project.

maddog
12-04-2004, 04:02 PM
<font face="Verdana, Arial" size="2">Originally posted by sdeering:
Maddog, do I sense a turbocharger. If so I am planning on putting one on a 2 cyl 2stroke Polaris 700 and we should talk.
As far as a cutter that big it would most likely chatter real bad. I would make a template of how you want the radius of the pocket and squish and do it with a normal tool. Or cnc mill the chamber.
Stephen </font>

I don't have the luxury of CNC. I'm stuck
with the old fashon manual lathe/bp.

I modified the squish shape on the lathe.
Bacically, I took a 1" round bar, turned
the end down and cut 14mm sparkplug threads
on it. Then I thread it into the head and
chuck it up in the lathe.

I wish I could draw a picture...

I'm going to need a cutter that has the
same shape/contour as the combustion
chamber so I can make it bigger without
changing the basic shape of it.

And yes, turbochargers are the ultimate goal
(I work on vintage Kawasaki 750 two-strokes)

maddog
12-04-2004, 04:09 PM
<font face="Verdana, Arial" size="2">Originally posted by shawnspeed:
I have re-cut 2-cycle heads on the lathe,using an old spark plug hole as a mandrel. grind a a HSS tool to the contour you need and start with light cuts. Make a bondo squeese of the head before you start, so you have a reference to re-cut your squish band. (wax the chamber, cut a section of thin wood,plastic or metal, close to the shape of the chamber,apply bondo to this edge and squeese into the waxed chambr,allow bondo to kick, Instant template.)You can also adjust compression in a 2-cycle by altering the port timing. (raising the exaust port a milileter or two, this will also enhance performance http://bbs.homeshopmachinist.net//smile.gif But If it was me I would add a few Copper gaskets first, to make sure you get the desired results,as the squish,in a low compression 2-stroke are not as critical. Are you trying to increase the RPM of your engine? Reducing compression should help. Trying to get it from detinating? retard the ignition timing,it's qucker than milling the head.</font>


Shawn, I'm trying to get rid of a high rpm
pre-ignition rattle. The EX port has been
raised considerably, I played with the
timing with limited success.

I think the problem is exagurated by the
huge expansion chambers we built. It runs
like a scalded dog but that rattle has me
bugged. (adding some race fuel eliminates
it altogether, but thats not a long term
option)

BillH
12-04-2004, 04:41 PM
Could you put a few washers underneath the spark plug to raise it a bit? probably wont lower your compression anywhere near enough to what you want though.

gunsmith
12-04-2004, 04:56 PM
A friend of mine races his snowmobiles and I do a lot of the engine work for him. Last year we lowered the compression by using a radius turning tool and mounting the head in the lathe chuck. On one smaller head I loosend off the top slide and rotated it within the dome by hand. I used a 750 rpm speed setting and wd40 for lube. I used a long bar fastend in the turret opposit to the cutter to control the rotation. Nice finish and no problems in setting it up.

aboard_epsilon
12-04-2004, 05:16 PM
Carl has the method right.
you can use a small boring head as well.
for concave and convex.
i see my friend Alwyn doing this almost every other day.
he make hemipheres for lense grinding.
put a tube up against a sphere and you will then know the secret.
or you can mount the head in a lathe and use a ball turning attatchment.

all the best.......mark



[This message has been edited by aboard_epsilon (edited 12-04-2004).]

sdeering
12-04-2004, 05:30 PM
Maddog you could build a tool the same shape as your current dome. Feed from the side only, not changing the squish zone or the dome depth. Just cut from the side area of the dome.
I take your reply as you are turboing the 750. What boost pressure are you running, what have you done to prevent boost loss through the exhaust side when both in and ex ports are open. Have you done any ignition mods or fuel enrichment with boost, or maybe H2O injection.Did you install a separate oil system for the turbo. I will try to post a drawing.
Stephen
rough drawing
http://img.photobucket.com/albums/v510/sdeering/MVC-001F.jpg
I realise your dome most likly more complex but you should get the picture.

[This message has been edited by sdeering (edited 12-04-2004).]

darryl
12-04-2004, 05:51 PM
I have no experience with the characteristics of head design and hop-up, but it seems to me that a formed cutter would work, and chatter would be a problem. The shape of the cutter could be broken into two areas, one on one profile, say the left side of what would essentially be a wide speed bit, and the rest of the same profile on the right side. This is a very poor explanation, let's see if I can make a picture of this in 'paint'.
http://img.photobucket.com/albums/v136/heinrich/profilecutter.jpg

maddog
12-04-2004, 07:12 PM
<font face="Verdana, Arial" size="2">Originally posted by Carl:
I have a video tape showing a machinist milling a convex dome using a flycutter on a vertcal milling machine. The head of the mill was set at an angle, and the work was on a rotary table. Maybe this method could be used for a concave area such as your project.</font>


Yeah, I'm torn between the lathe and the
mill. I'm leaning toward the lathe because
thats how I surfaced the heads to start
with.

I like the rotary table/mill idea too

maddog
12-04-2004, 07:28 PM
<font face="Verdana, Arial" size="2">Originally posted by sdeering:
Maddog you could build a tool the same shape as your current dome. Feed from the side only, not changing the squish zone or the dome depth. Just cut from the side area of the dome.
I take your reply as you are turboing the 750. What boost pressure are you running, what have you done to prevent boost loss through the exhaust side when both in and ex ports are open. Have you done any ignition mods or fuel enrichment with boost, or maybe H2O injection.Did you install a separate oil system for the turbo. I will try to post a drawing.
Stephen
rough drawing
http://img.photobucket.com/albums/v510/sdeering/MVC-001F.jpg
I realise your dome most likly more complex but you should get the picture.

[This message has been edited by sdeering (edited 12-04-2004).]</font>

I like your idea. Its a step beyond what I
had in mind. I have no experience making
custom cutters but I think your idea would
work nicely.

I think the cardboard/bondo idea will give
me the perfect pattern. What type of material
(HSS/Carbide) should I use to
make the cutter? And who sells blank
cutter material that I can braze onto
the tool holder?

I'm going to be doing alot of these so I
want something that's easy to setup and
reproduce.

Thanks everyone, you guys are great.

P.S. That picture is EXACTLY what I'm after!


[This message has been edited by maddog (edited 12-04-2004).]

sdeering
12-05-2004, 12:09 AM
Since you are machining aluminum I don't think you have to be too picky on what type of cutter. I purchased some 1"x about 5" HSS at a local surplus store Princess Auto cheep, use them for parallels. It would take a little grinding but you should be able to get the right contour. I think you would have to run the slowest speed possible because of chatter. How many CCs do you have to remove?
Another idea might be to make a pattern of the chamber and somehow make a system on your lathe to trace it. I cant think of the right terminology for it but it would allow you to use a smaller cutter to prevent chatter.
Stephen

Old Time
12-05-2004, 01:32 AM
From having worked on chainsaw engines that had brutal compression, remember these things are pull started. The solution I used was to put the piston in the lathe and remove metal from it,IIRC .020" made a noticicable improvement and the performance actually improved from the change in the port timing.This was on a 88cc engine with no starting compression release.

Stanko
12-05-2004, 03:14 AM
Hi Maddog, heres a link to a gizmo that will allow you to copy a profile from a bit of tin cut to your desired chamber shape using a standard cutter. Its adapted from a taper turning attachment. Its an interesting read anyhow.

http://easyweb.easynet.co.uk/~chrish/profile.htm

Stanko
12-05-2004, 03:16 AM
Hi Maddog, heres a link to a gizmo that will allow you to copy a profile from a bit of tin cut to your desired chamber shape using a standard cutter. Its adapted from a taper turning attachment. Its an interesting read anyhow.

http://easyweb.easynet.co.uk/~chrish/profile.htm

shawnspeed
12-05-2004, 09:17 AM
Maddog, Sdeerings sketch is exactly the way I have cut chambers, works well. Just another couple of ideas,Spark plug heat range, maybe dropping to a colder plug(b9 to a 8 or 7 ) could cure the high speed death rattle.(dieseling on a hot spark plug electrode) Another cure ,depending on what year H-2 you have is the igniton boxes that came with them. They were made for the gas of the day ,that had a better detonation threshhold than even premium has today, so you could upgrade to an aftermarket ,tunable ignition and increase performance along with engine longevity.(no rattle) A customer I have drag races a polaris 900 up fitted with aftermarket 1200 cylinders and custom pipes,lectron carbs, and a 3-stage NoS system,and now he has added a MSD ignition, because the stock igniton would not keep up.Just changing the ignition and being able to open the spark plug gap back up netted him a all time best in the 9.50's at a little over 150 in the quarter... not bad for a sled ! Good luck, keep the sunny side up.... Shawn

Forrest Addy
12-05-2004, 09:25 AM
I suggest dishing the top of the piston inside the squish band.