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DIY cylinder decking/squish Need machinist help!

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so i work with some very talented machinist's and talked to them about doing some work for me. They are very willing to help me out and we have every machine I would ever need. bridgeport, lathe, whole wizard, milling machine's. 

 

I want to deck my cylinder and re-chamber my head on my 2001 cr250.

 

In eric gorrs book he talks about indexing the spark plug hole on these engines as well as turning the base down 2thou 

 

How should i explain what i want to the machinist? i assume he would know how to deck the base 2thou but what about the head? 

 

thanks guys!

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Wow only 0.002"?

Are you doing this to adjust the squish or the port timing?

Are you sure its not 0.02"

And indexing the plug.

I've noticed on some bikes they are quite far up in the hole

Does Eric reccomend making them flush or are you just trying to get the ground strap aimed a ceartain direction like at the boost port?

Tell them something like hey I need to cut this spark plug boss perpendicular to the plug hole untill theres only 1 thread left?

Or tell him hey I want to machine this boss perpendicular to the the plug hole a few thoulsands at a time untill the ground strap faces this way."

You could also figure out the spark plugs rotation per 0.001" and tell him to take x amount.

Don't expect it to index perfectly all the plugs are probably different and you have to crush the washer as well.

And I'm sure they can figure out how to take what ever you want off the base.

Have fun

Two strokes are great fun to modify!

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Wow only 0.002"?

Are you doing this to adjust the squish or the port timing?

Are you sure its not 0.02"

And indexing the plug.

I've noticed on some bikes they are quite far up in the hole

Does Eric reccomend making them flush or are you just trying to get the ground strap aimed a ceartain direction like at the boost port?

Tell them something like hey I need to cut this spark plug boss perpendicular to the plug hole untill theres only 1 thread left?

Or tell him hey I want to machine this boss perpendicular to the the plug hole a few thoulsands at a time untill the ground strap faces this way."

You could also figure out the spark plugs rotation per 0.001" and tell him to take x amount.

Don't expect it to index perfectly all the plugs are probably different and you have to crush the washer as well.

And I'm sure they can figure out how to take what ever you want off the base.

Have fun

Two strokes are great fun to modify!

 

well eric says to cut .5mm which is 0.019685in so i guess i was a little off my fault. 

 

As far as indexing the spark plug he says .040 in off of the lug 

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well.

Tell the machinists that!

"Whack .04" here

And whack 0.02" here and make Damn sure its indicated in well" a piston/rod/crank won't like it much if you try to run it crooked

Edited by englertracing

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It's pretty difficult to cut the base of the cylinder in a mill what is often used is a special mandrel that goes in the bore of the cylinder that allows the base to be turned in a lathe. The issue is the base needs to be inline with the bore not with the head surface.

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Well a good machinist like a mold maker could do it any way.

rotary table on a mill.

On a cnc machine

Or in the lathe.

It was originally machined in the first place.

I know the fixture your talking about its like a inside out colet doohickey

Or reverse pot chuck.

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It's pretty difficult to cut the base of the cylinder in a mill what is often used is a special mandrel that goes in the bore of the cylinder that allows the base to be turned in a lathe. The issue is the base needs to be inline with the bore not with the head surface.

Well what's my best bet just have the head shaved? I was going to bring an old spark plug to see if we could make a chuck out of it

Can you elaborate on decking with a mill?

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How the cylinder was originally manufactured is irrelevant.  What matters is how you work with it at this point. 

 

Without having looked into Gorr's recommendations, I would imagine that the purpose of turning the base is to square it with the bore.  A lathe seems to me to be the easiest choice for this.  Chuck it by the inside of the top of the bore, then check the free end for concentricity by mounting a dial gauge on the lathe ways so it reads from as far down the cylinder as possible.  Once the bore axis is centered, trim as little as possible off the base to get a full cut on the whole surface.  You're not trying to lower or shorten the cylinder, just square it up.  The top deck of the cylinder should also be minimally clean cut, but now that the bore and base are in  the correct relationship, you can do this on a mill with the jug sitting on its base.

 

If you really wanted to go all out, you want to cut the crankcase cylinder deck to be square with the bore, too, but setting that up and determining what "square" really is can be challenging.

 

Cutting the head and squish can be done on a lathe if you have a way of chucking it up.  Most plug holes are centered well enough to be used for this with an arbor, but not all. 

 

Plug indexing as I know it is an effort to have the ground electrode facing in a certain direction.  Doesn't do a lot, really.  What often does is completely relocating the spark plug to another part of the head.  This can sometimes produce startling results.  Chevrolet produced a version of their high performance head for the old small block that differed from the previous version only in that the spark plugs were angled about 10 degrees toward the exhaust vale from their original location, and this change alone was worth 8 hp on engines producing 350-400.

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How the cylinder was originally manufactured is irrelevant. What matters is how you work with it at this point.

Without having looked into Gorr's recommendations, I would imagine that the purpose of turning the base is to square it with the bore. A lathe seems to me to be the easiest choice for this. Chuck it by the inside of the top of the bore, then check the free end for concentricity by mounting a dial gauge on the lathe ways so it reads from as far down the cylinder as possible. Once the bore axis is centered, trim as little as possible off the base to get a full cut on the whole surface. You're not trying to lower or shorten the cylinder, just square it up. The top deck of the cylinder should also be minimally clean cut, but now that the bore and base are in the correct relationship, you can do this on a mill with the jug sitting on its base.

If you really wanted to go all out, you want to cut the crankcase cylinder deck to be square with the bore, too, but setting that up and determining what "square" really is can be challenging.

Cutting the head and squish can be done on a lathe if you have a way of chucking it up. Most plug holes are centered well enough to be used for this with an arbor, but not all.

Plug indexing as I know it is an effort to have the ground electrode facing in a certain direction. Doesn't do a lot, really. What often does is completely relocating the spark plug to another part of the head. This can sometimes produce startling results. Chevrolet produced a version of their high performance head for the old small block that differed from the previous version only in that the spark plugs were angled about 10 degrees toward the exhaust vale from their original location, and this change alone was worth 8 hp on engines producing 350-400.

The purpose of turning down the base of the cylinder is to increase compression get a closer squish and advance the port timing

I think I'm just going to mill the head in a lath

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The purpose of turning down the base of the cylinder is to increase compression get a closer squish and advance the port timing

 

 

You're going to accomplish that with a .002" cut?  Please.

 

Lowering the ports retards the opening of the port and shortens the duration of the opening.

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You're going to accomplish that with a .002" cut? Please.

Lowering the ports retards the opening of the port and shortens the duration of the opening.

Further down he corrects to 0.02" which is a couple degrees less duration.

And he was talking about taking 0.04" off the plug boss because the hole sparkplug sits way recessed in the head.

I really think you should do what Eric says there's a reason he says to do it.

The base can be cut 2 good ways.

You split a piece of barstock larger than the bore drill and tap it for a pipe plug on the lathe. from then face a few inches back so when your looking at the round "o" end there's a + cross. Install he pipe plug a untill it just barley swells the prongs. Then turn this down to exactly the bore size. Loosen the pipe plug slide the cylinder on tighten the pipe plug and whamo o your running true as tits to the bore and not tearing your bore up with the chuck.

You could also take this assembly out and put it in a rotary table on the mill.

This requires extra indicating but theres less of a chance of the part going ballistic across the room.

It's not hard if your a good machinist (in not but I have a lot of old timer friends who show me tricks)

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We use various thickness washers on plugs for indexing on HP  auto engines, all plug mfg differ on thread cut/ gnd strp location as well as torque setting of crush washer.

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Give this guy a call he will set you straight.   www.tdc2strokeperformance.com     He fixes a lot of that other guys stuff.

 

Where is your piston at BDC, relative to transfer opening?

Where is your piston at relative to deck height?

Where is the squish at now?

Whacking .002 without calculating msv?

Have you calculated compression ratio now?

What will be after all these changes?

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Give this guy a call he will set you straight. www.tdc2strokeperformance.com He fixes a lot of that other guys stuff.

Where is your piston at BDC, relative to transfer opening?

Where is your piston at relative to deck height?

Where is the squish at now?

Whacking .002 without calculating msv?

Have you calculated compression ratio now?

What will be after all these changes?

I was just going off of what I read in Eric gorrs book

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I was just going off of what I read in Eric gorrs book

The "ONLY" reason I stepped in was I went down that same road. It was littered with dollar bills though!!! :cry:

 

Brent at TDC led me in the right direction by contacting me privately and we've had many hours of conversation in the last few years. He is an anal mad scientist and is never content and is constantly making changes and trying things on the dyno. And has a pretty good group of racers around the country giving him great feedback. I'm not a big fan of "off the shelf kits", things need to be setup from the dimensions in your particular motor.

Edited by JBFL

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The "ONLY" reason I stepped in was I went down that same road. It was littered with dollar bills though!!! :cry:

Brent at TDC led me in the right direction by contacting me privately and we've had many hours of conversation in the last few years. He is an anal mad scientist and is never content and is constantly making changes and trying things on the dyno. And has a pretty good group of racers around the country giving him great feedback. I'm not a big fan of "off the shelf kits", things need to be setup from the dimensions in your particular motor.

Thanks! I will contact him can I use you as a reference?

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Thanks! I will contact him can I use you as a reference?

Sure, just tell him "Jake's Dad"

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cutting some off the bottom of the cylinder lowers the ex port

ex port is measured in degrees -degree wheel on crank dial indicator on piston at TDC ,shine a light in the ex port and watch the dial indicator 185/190 probably the light will start shining into the bore

transfer ports are timed all so probably about 122/126 ish

once the cyl is cut and dropped a bit -checking to see If skirt of piston still covers bottom of ex port is done

next comes squish gap-the area between piston top edge and squish band on the head guesing say maybe 58 thou so put modeling clay in and check, chamber volume-

so much to learn kinda depends on what and whare you want your motor to run good on what fuel at what rpm

I,m a cr500 guy at heart and I,m beting theirs all kinds of 250 guys got it all figured out just do a search in the Honda 2stroke section

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indexing the plug is a waste of time IMO

 

Try a protruded tip plug if you want the 'trode in farther rather than cutting the surface.  We used to try em on the dyno (sleds) and if they made more power it usually indicated they could use a little more timing, +2deg was typical. Never found power indexing.

 

Before you deck the cyl base you might see if you have clearance to build it without a base gasket to see how it runs. I used loctite 515 or prematex anaerobic on the surfaces with no problems.

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So long time to update, but I machined the radius for a dome piston off so the squish is now flat for a flat top piston.

I'm going to install and measure squish then cut the gasket surface, what and how do I cut the chamber? I read in bells book to cut 2 times the material you take off the gasket surface for std compression, if I run AV100LL I can get away with more compression so I am thinking of only taking 1.5 times the material out of the chamber ??

ImageUploadedByThumper Talk1384360309.949580.jpg

ImageUploadedByThumper Talk1384360332.246032.jpg

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