shortening and lightening a piston

mitchg95 Tue Sep 04, 2012 5:56 pm

i have the means todo this and want to give this a second shot, i saw second because i tried shortening a .049 piston today in my machining class at a local community college and accidently snapp the con-rod in half DAMMIT!

is there a better way to keep the con rod out of the way like using a piece of tooth floss tied to the open end of the connecting rod? and how should i go about lightening it?
thanks

Jason_WI Tue Sep 04, 2012 6:15 pm

To bore out a piston you will need to grind a HHS blank tool to make a small boring tool.

I was thinking about using a magnetic base and a piece of elastic band with enough tension to keep it out of the way.

EXModelEngines Tue Sep 04, 2012 6:20 pm

Yes, a custom boring bar will be necessary. I usually use a piece of music wire to hold the con-rod while machining.

Matt

mitchg95 Tue Sep 04, 2012 6:21 pm

EXModelEngines wrote:Yes, a custom boring bar will be necessary. I usually use a piece of music wire to hold the con-rod while machining.

Matt

ill try that thanks Smile

lousyflyer Tue Sep 04, 2012 6:29 pm

Here's why I'm not a machinist.

My first thought would be to stick the piston down to a table with double faced tape and then fill the skirt with hot melt glue or even candle wax.
Then you could chuck it up and cut away - until you stick a tool in and popped out the plug.

I'm a wood guy by trade. Does it show? Shocked

EXModelEngines Tue Sep 04, 2012 6:42 pm

Also, I have had better results shortening the piston first. A lot of time if you lighten first the material is so thin it will sort of buckle when trying to shorten it later.

Matt

lousyflyer Tue Sep 04, 2012 6:50 pm

EXModelEngines wrote:Also, I have had better results shortening the piston first. A lot of time if you lighten first the material is so thin it will sort of buckle when trying to shorten it later.

Matt

Just curious -
Are you thinning your piston skirts by dimensions or are you using the TLAR method? (That Looks About Right)

dckrsn Tue Sep 04, 2012 6:52 pm

Here's a link Matt.
Lots of info on shortening and lightening pistons.
Bob
http://www.rcuniverse.com/forum/m_8088423/anchors_8255167/mpage_1/key_/anchor/tm.htm#8255167
shortening and lightening a piston Poor_m10

shortening and lightening a piston Poor_m10

DICKEYBIRD'S Poor Man's Lathe.

mitchg95 Tue Sep 04, 2012 6:58 pm

lousyflyer wrote:
EXModelEngines wrote:Also, I have had better results shortening the piston first. A lot of time if you lighten first the material is so thin it will sort of buckle when trying to shorten it later.

Matt

Just curious -
Are you thinning your piston skirts by dimensions or are you using the TLAR method? (That Looks About Right)

Tlar, i dont have any dimensions

EXModelEngines Tue Sep 04, 2012 7:17 pm

lousyflyer wrote:
EXModelEngines wrote:Also, I have had better results shortening the piston first. A lot of time if you lighten first the material is so thin it will sort of buckle when trying to shorten it later.

Matt

Just curious -
Are you thinning your piston skirts by dimensions or are you using the TLAR method? (That Looks About Right)

Yes and no. When we build custom engines for customers who want a lightened piston I usually go to about a .015" wall thickness which is around what the OEM Killer Bee had. Even that is a bit thin for structural integrity and will have a short life as compared to a regular piston. I have tried to go as thin as I could and it ended up about .005"-.007" which, needless to say, will run great but longevity of the piston is rather short lol!

Matt

Jason_WI Tue Sep 04, 2012 7:18 pm

Dimentions from my shortned and lightned piston from Bernie:

Piston length: .3565"
Piston wall thickness: .015"
Depth of lightning: .150"

This is for use in a surestart cylinder with twin slit exhaust.

mitchg95 Tue Sep 04, 2012 7:20 pm

Jason_WI wrote:Dimentions from my shortned and lightned piston from Bernie:

Piston length: .3565"
Piston wall thickness: .015"
Depth of lightning: .150"

This is for use in a surestart cylinder with twin slit exhaust.

will that work in my cylinder? has no # is modified and has 2 boost and bypass ports?

Jason_WI Tue Sep 04, 2012 7:26 pm

does this cylinder have SPI with a standard piston? If it does then no it will be too short and have too much SPI.

Cox International Tue Sep 04, 2012 9:30 pm

Jason_WI wrote:does this cylinder have SPI with a standard piston? If it does then no it will be too short and have too much SPI.

Correct, do not further shorten a piston that already has SPI in its current cylinder setup.

If a setup does not have SPI, there are 2 ways to induce such.

#1) Shorten the piston (we shorten by aprx 1,500µ)
#2) Lower the exhaust port cutouts by the same amount as #1

Either way is fine, depending if one prefers torque over mass/inertia or vice-versa.

mitchg95 Wed Sep 05, 2012 7:03 pm

Jason_WI wrote:does this cylinder have SPI with a standard piston? If it does then no it will be too short and have too much SPI.

it did not have spi, but now it does

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