Learn Me: Squish, Domes, Flattops and CCV

Tim Walsh

I had my second event yesterday with the kart, and everything went well as can be with VERY old tires. While there, Keith Vail, who has been a huge help getting into this whole karting thing, and I ran into something a little disconcerting. Evidently the squish area on the engine is more than 0.062"! The PO did a ring/piston job just before he sold it with a stock '99 Honda piston, the same as what he bought it with. I don't have any info on the engine, just that it's a modded cr125 with a vortex PI module and always been run on 5oz of Motul to ever gallon of Sunoco 110. The head is not oringed, and it looks to be a stock cr125 base gasket.

So it looks like I've got some choices to make. In order of increasing cost I'm see them like this:
1. Run it as is.
2. Correct the squish with a thinner gasket.
3. Replace the ring/piston with a 2005 piston
4. Replace the ring/piston with a Wisco RS piston
5. Send engine to builder.

Number 5 really isn't in the budget for the winter of 2011/2012. Thoughts? Ideas? Rants?

Tom, Should I just drop the whole motor in the garbage, and pick up a TM instead? Cool

EDIT: I've been through most of the honda threads here on EKN.

Benn Herr · Joined: 18 Jul 2001 Posts: 1580

You can take the head you have and machine off some of the mating surface. Cut it so the combustion chamber protrudes into the cylinder the amount you need to get the squish you want.
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Brian Garfield · Posted: Mon Oct 24, 2011 11:25 am Post subject:

I'm guessing that it was cut for a domed piston and you're running a flat top? If so, and you can confirm that, just change to a domed, preferably RS Piston.

Brian

Tim Walsh · Posted: Mon Oct 24, 2011 11:56 am Post subject:

Alan Sheidler · Joined: 09 Aug 2001 Posts: 471

So many variables....

You checked squish with the old standby... soft solder wire?

I'd start with the basics first, like checking to see whether or not the piston that is in the motor with that big squish is indeed a Honda or some other maker's part. Does not make sense to me that the squish measurement would exceed the Honda stock number without a spacer of some sort under the cylinder.

I can conceive of the possibility that a non-stock crank or piston rod "could" have a shorter stroke or length than original, but I can't for the life of me guess as to why someone would want to de-stroke a motor. A longer rod has lower forces trying to wreck the engine in the transition between just before and just after TDC. Using a shorter one defies logic.

The actual piston pin to dome edge measurements for the various pistons available should be able to be found somewhere. I'm thinking that the culprit is the piston itself. Maybe the correct bore, but the wrong pin location for your motor.

Running it that way won't hurt anything other than performance. The lower compression should be fine with premium pump gas, no need for that 110. Before I'd machine anything, I'd check the true identity of all of the parts, including the year of the cylinder, if possible. A Honda domed piston might fill the bill, might not.

Keep us informed....

Larry Andrews · Posted: Mon Oct 24, 2011 9:58 pm Post subject:

The RS piston is about 0.040" (1mm) taller than any CR piston and requires a 1mm aluminum spacer under the barrel - is one there? What thickness is the base gasket? That's probably the first place to start looking and it won't cost anything.

Also, it might be good to talk to TDCgaskets.com to see if they still sell an assortment of different thickness base gaskets. Used to be about $30 for a 1/4" thick stack.

You won't likely need to send the motor to a builder, just keep posting your questions and someone will try to help. As long as you don't get too ridiculous, the motor will run and you can try different stuff. It's fun once you get used to it.

Do me a favor and grab a Bojangles ham biscuit for me - dang I miss them thangs.

Tim Walsh · Posted: Tue Oct 25, 2011 5:44 am Post subject:

I'm reasonably sure that what came out of the motor, and what went into the motor this year were both stock CR pistons. The old piston came in a Honda box with part number 13110-kz4-506. The old piston doesn't look to bad either. A hint of pitting/detonation, and the machining on the side is worn off, but not really scuffed.

We looked at the base gasket when we tried to measure the squish, and it seems to be a stock OEM base gasket without any shims, but it's entirely possible I missed them. I have some thicker lead solder, I'll try and get an accurate squish measurement tonight. I think it's entirely possible that the motor was setup for an RS piston and since has been replaced with a CR.

Is the o ring modification relatively standard on mod motors? This motor has a stock head gasket on it. Also what is the accepted method for adjusting piston to cylinder clearance if I switch to the Wiseco piston?

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2002 Birel CR32V/ StockMoto w/PI

Benn Herr · Joined: 18 Jul 2001 Posts: 1580

Some points to consider on the direction you take with your engine.

Stock Honda pistons are much cheaper than RS pistons.
Genuine RS pistons are getting harder to find.
The required clearance for a forged piston is much greater than a stock one.
The warm up time for a forged piston can be longer (and more critical) than for a cast one.
Many builders prefer the stock cast piston to the forged Wiseco. They seem to be more flexible than the forged ones.
The cast piston can handle any power the engine may make. The extra life a forged piston offers would only be noticeable after several seasons of Autocrossing.
The cast pistons are usually lighter.
The head can be cut to match any piston you might put in there.

Side note: Get your head or cylinder cut for o-rings - it'll save you a ton of money on head gaskets!
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Benn Herr
Come see our Superkart Build Off @
http://www.lostenduros.com/?page_id=1534
beherr3@cox.net
http://www.southwestkarters.com/
http://www.lostenduros.com/
Kart Design
Chandler, Az.

Tom Reynolds · Posted: Tue Oct 25, 2011 7:19 am Post subject:

I agree for the most part with your options except I would not run it the way it is. I ran my mod moto built for a RS piston once with the squish too low due to an 05 piston and not spacing it right. The squish IIRC was .045. It felt flat especially at the top end. When I went back and reset it to .030 I could not believe the difference, its huge. With your gap, the change should be pretty remarkable. Vendors sell different spacers and advice I was given is never put a metal spacer to metal so make sure you get the paper ones as well.

Keith Vail · Posted: Tue Oct 25, 2011 8:38 am Post subject:

I can weigh in only a little here as I was the one checking the squish. I used 0.062" Rosin core solder and when I rolled the engine over it didn't even touch the solder. So we are not entirely sure where the squish is actually set at but I am sure if things got tightened up inside the motor it would make a lot more power. My Mod Moto has the squish set at 0.032". There is a huge difference in how hard my motor pulls compared to how soft feeling Tim's motor is but I also have an RS piston in my engine.
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Scott Clontz · Posted: Tue Oct 25, 2011 1:04 pm Post subject:

Alan Sheidler · Joined: 09 Aug 2001 Posts: 471

With compression that low, I wondered how there could be any evidence at all of detonation with 110 octane fuel. Programmable ignition answers that....

Lots of options for how to proceed with the motor, and it helps to have a friendly machinist!

Very Happy

Tim Walsh · Posted: Tue Oct 25, 2011 1:50 pm Post subject:

I'm currently thinking that the motor was built for an RS piston, and when they got scarce, put in a CR piston as well.

Just as a note: I have no reason to think that Scott is anything but a standup person, and have known him for a couple years now.I don't believe he had been into this engine before this year and had no reason to suspect that anything should have been in it except a CR piston.
_________________
2002 Birel CR32V/ StockMoto w/PI

Tom Reynolds · Posted: Tue Oct 25, 2011 1:57 pm Post subject:

You could try an 05 CR125 piston, its domed, not sure if its enough if you don't have a spacer already. Note the come in different thicknesses:

http://www.fastech-racing.com/honda-cr125-2005-piston-kit-rs125-alternative.html

and

http://www.fastech-racing.com/98-99-cr125-base-gasket-various-thickness.html

Otherwise you need an RS125 piston from here:

http://www.rscycles.com/images/honda_rs125_pistons/honda_rs125_pistons.htm

Jim McMahon · Posted: Tue Oct 25, 2011 3:11 pm Post subject:

Insufficient squish can cause detonation. Yes, thats right. Too MUCH squish can cause detonation. You want the squish as low as you can get it and it should be set independently of your compression. The reasons behind this are probably best explained by somebody else. Often people say less squish leads to detonation, but that's only if you reduce the squish by half-measures that lead to a change in compression in addition to squish, for example removing base gaskets. Whenever you can, correct the squish with the cylinder head and not base gaskets.

If the deck height is the same for RS and CR piston setups then I'd just get a stock CR99 cylinder head and run it with a CR piston for now. Keep it simple.
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