Box Stock

Clone engine clearance

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bricklayer653

Member
I'm confused why everyone says that you need at least 3 thousands piston to cylinder wall clearance (which I agree and think a little more than that is better for hp)but everyone says you need 4 thousands in compression ring gap ...aluminum expands at 2.73 and steel expands less at 1.63 so if your aluminum piston doesn't expand to much and bite fast to cylinder at 3 thousands gap why would a steel ring need to have such a large gap ...the steel bore would be growing at approximately the same rate as the steel ring so wouldn't the gap stay close to the same ...just wondering is all
 
The thin compression ring sees more heat, being nearly directly in the fire. The piston has a large mass compared to the area exposed.
Also, the ring is forced against the cylinder wall, both by ring tension, and combustion pressure.
Metals expand from heat equally in all directions. The thin section will expand a very small amount. The circumference of the ring lends itself with more material to expand further. If the gap is enough, it expands harmlessly into the gap. If not, the ends bang together, and with nowhere to go, it forces ring into cylinder wall.

The steel bore only sees localized high heat, constantly being dissipated into the rest of cylinder, and aluminum block.

Generally, the rule of thumb is .004 inch of gap per inch of cylinder bore diameter.

If you measure your piston above the ring, it is much smaller than at the skirt, where you measure clearance.
 
Firstly, I've found over the years about 1/2 of what you read on Bob's is good info, and the other half of it is garbage. Whether it's intentional misinformation, I'm not sure. As for setting the top ring end gap to .004", IMO that one is garbage. I think you want at least .004" per inch of bore.

As for why the the ring and the cylinder, while made of similar materials, don't expand the same: It's a surface area to volume thing resulting in a different rate of expansion.
Let me explain.
Surface area is to the 2nd power or square, and volume is to the 3rd power or cube. As an object gets very small, it's surface area and its volume become closer to a 1:1 ratio. As an object gets larger, this changes rapidly. See link below, also see table.

https://en.wikipedia.org/wiki/Surface-area-to-volume_ratio

surface area to volume.jpg

The above table is for a cube, and our shapes are different but the same principle applies.

The point here is because the ring is much smaller than the cylinder bore, the ratio of surface area to its volume is smaller. This results it the ring heating more rapidly than the cylinder, and therefore expands at a much faster rate.

It is the rate of change in temperature and the resulting expansion that is the reason they don't achieve their final dimensions at the same time. Cold seizure in a two stroke is
a perfect example of this. Or think of two parts of aluminum, equal in volume (for this example weight). One is .062" thick sheet metal, the other is a small billet cube. The size and surface areas of the two parts are completely different but the volume (weight) is the same. You expose both shapes to the same torch for the same amount of time, which heats faster?

We could also discuss the opportunity each part has to cool, and the eventual equilibrium of an engine running in steady state in terms of heat loading of the different parts and heat flow out of them. The ring relies on its physical contact with the cylinder and piston to transfer heat. The cast iron cylinder relies on its contact with the aluminum block and the air-flow over the block to transfer its heat. But that's more than I care to type.

I probably did a crummy job of explain this....but I hope you can understand what I'm talking about.
 
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Not disagreeing with anything said ...but as long as cylinder didn't lock up from close clearance on ring then even if it butted ends of ring it would be ok ...I'm asking cuz some say ring will wear faster because of not enough clearance but once it wears to clearance it needs wouldn't it stop wearing quickly
 
bricklayer653 said:
Not disagreeing with anything said ...but as long as cylinder didn't lock up from close clearance on ring then even if it butted ends of ring it would be ok ...I'm asking cuz some say ring will wear faster because of not enough clearance but once it wears to clearance it needs wouldn't it stop wearing quickly
Click to expand...

Some say the earth is flat. Doesn't make it so.
 
Everyone has different ideas on all this and most of them are fine if it works for ya. I've tested .0005 to .001 comp ring gap with zero problems on these engines. Piston to cylinder clearance I've seen anywhere from .0018 to .0035 dyno well. I prefer just over .002 on our best engines. Alot of variables and not a big difference in peak hp to run more clearance so unless you have the equipment to machine properly and understand these engines don't take a chance on tighter clearance for minimal gain.
Www.racingcarb.com
 
bricklayer653 said:
Not disagreeing with anything said ...but as long as cylinder didn't lock up from close clearance on ring then even if it butted ends of ring it would be ok ...I'm asking cuz some say ring will wear faster because of not enough clearance but once it wears to clearance it needs wouldn't it stop wearing quickly
Click to expand...

The chrome faced top ring required by most rules wears very little. That leaves the bore to wear, or score.
Your mileage may vary.
 
I agree with Racingcarb we ran these with minimal clearance I mean tight, we never experienced a problem or failure because of it. I would inspect the ends of the rings, I would look for the file marks after hours of running and never saw evidence of contact or polishing. As far as gain we did not see consistent gain we could attribute to ring end gap. I did pay particular attention to ring grooves, side clearance and would lap rings.
 
This might apply or not, but after gapping each ring, I used to take the piston and rod upside down and push each ring through the bore. Just to be sure.
I also used to lean a little heavier on the bottom of the bore with the machinist's hone, in order to make the bottom of the bore about a thousandth larger than the top. Squeezes the rings tighter at TDC.
So, in this case, with .003 piston to wall clearance, I would have the cylinder tapering from .003 clearance at the top to .004 at the bottom.
FWIW
 
Listen to Bob here folks, this is an old trick that is still in use today. The slightly larger bore at the bottom, or forced "taper" also makes for less friction at the bottom of the stroke and allows more oil to get on the cylinder wall and piston skirt for cooling and lubrication.
 
The taper may have been a theory that you felt worked, my entire goal was to make the cylinder as straight, true and round, perfectly perpendicular to the crankshaft all the whie maintaining the tolerance for clearance as accurate as possible with the desired wall finish. I would never stray from that trying to make my cylinder taper, this is now causing the rings to move in and out against the cylinder lands.
 
Kart43- I see a very valid point in what you're saying also especially from a durability standpoint on an engine that isn't torn down every 8-10 races....
 
bricklayer653 said:
Not disagreeing with anything said ...but as long as cylinder didn't lock up from close clearance on ring then even if it butted ends of ring it would be ok ...I'm asking cuz some say ring will wear faster because of not enough clearance but once it wears to clearance it needs wouldn't it stop wearing quickly
Click to expand...
You have gotten quality information from 6 differnt people, all in close agreement.
Only thing left to do is try it.
 
I am old school like Bob. I do let a .001 taper from bottom to top. I never felt that .001 larger at the bottom was enough to cause a problem
 
If the top of the bore just happens to be hotter than the bottom, a tapered bore just might be close to straight at temp. Or does the top actually end up larger than the bottom at temp? Is it common on air cooled engines to have the bore choked at the top? Is .001 taper even enough? How close to the top or bottom do I need to be to set my ring gap? Just Wondering
 
My opinion, the cylinder should be straight or cylindrical. The piston is tapered so the top expands with heat. The clone stock pistons are tapered far to much for their size.
 
Your ring gap needs to be set at the top , no more then one inch down.
The cylinder should expand some , testing to find out exactly would be quite complicated .
You don't want more taper then .001 .
JMHO.
 
Racingcarb.com said:
Everyone has different ideas on all this and most of them are fine if it works for ya. I've tested .0005 to .001 comp ring gap with zero problems on these engines. Piston to cylinder clearance I've seen anywhere from .0018 to .0035 dyno well. I prefer just over .002 on our best engines. Alot of variables and not a big difference in peak hp to run more clearance so unless you have the equipment to machine properly and understand these engines don't take a chance on tighter clearance for minimal gain.
Www.racingcarb.com
Click to expand...

A very interesting thread this one......Am I allowed to ask what type of bore gauge are you using in order to measure accurately to a 4th decimal place???........for honing I am just using a conventional T-bar shaped bore gauge, it was not an expensive one for sure!.....its with the wheels on one side which I rock about to get upper/mid/lower bore readings, its with a DTI on the end, it can indicate to 0.0005" increments but no lower........have always felt that a far more accurate device must be out there.......as a final check I always put the new piston in upside down and feeler gauge the skirt top and bottom of bore as a final sanity check.
 
For what it’s worth I set my top ring at .005 and my second and oils at .010 gap. Total ....not per inch of bore and have not had an issue. Piston to bore clearance is .002 for me. Sealed up tight and makes good power. I guess everyone has their way this is just what works for me.
 
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