piston to cylinder bore clearence

[Facime]

If I understand correctly this number is specific to the piston not the motor? Meaning, a ross piston will have a different spec than another forged piston or a stock one for that matter, is that correct?

If it is engine specific what is the spec as I am only able to find "oil clearence". If its piston specific can anyone tell me what the spec is on stock from the dealer toyota pistons?


I apologize ahead of time for the motornoob question. This is my first ever bottom end. (yes I have a local mentor that is going to be helping with the assembly)

 

[Adjuster]

Pistons are generally made out of aluminum alloy.
The forged ones are "pressed" or forged into a slug/blank, then machined into the final part.

The forging process aligns the grain of the metal, and actually forces the molecules into a more concise pattern. (And one that is denser than the material started out before it was forged.)

Because of this, when forged items heat up, they expand more than similar metals that are not forged.

Also different metals expand at different rates, iron v/s aluminum for example.

So, your forged pistons are going to expand more when heated than a cast piston will, so you have to leave more space/clearance when you machine the bores.

If you don't, the piston could actually expand and stick into the bore, causing engine falure.

Same goes for the rings. You should always gap the rings wider on the top ring than the second ring. This allows any trapped gas to escape via the top gap v/s causing overpressure, and possible ring failure.

The extream pressure in the combustion chamber actually forces your rings out into the piston bore during use. The small gap in the ring is a necesarry loss of pressure/blowby on pistons of this design. (The total seal rings are different, and use a multi layer approach to seal up better, but are not reccomended for boosted engines, or anyone on this motor that I've ever seen. No personal experiance here on this type of ring however.)

I gapped mine at 17 and 19k top to bottem. Slightly wide, but I never wanted my rings to bind, and cause engine damage. (Besids, you stagger the rings, so one gap is on one side, and the other is opposite. This keeps gas losses to a minimum.

OK, for ROSS stock type forged pistons, they reccomend 6k clearance. (It may have changed, check your instructions that come with the pistons.)

Coatins may change your piston specs, so you shoul allways plan for that if possible.

 

[Facime]

Once again great info. I was able to contact the manufacturer of my piston and made sure of the spec. I passed that info on to my machine shop and they are supposed to have my items ready here real soon.

As for ring gap, the book shows using a file and "squeezing" the ring against that and filing outside to in. Ive seen some other people mention a ring gapping tool (looks like a minature table saw) and even others say to only remove material from one end so you can have a square reference on the other. Any recommendation here for a one time builder?

 

[Adjuster]

Buy a ring file. (No shit, that's the name.. and they cut both ways..)

Now, get a good vise... turn it sideways.. and clamp your ring file into the vise so it's sticking out at you.

Now, using a piston crown (Upside down) to push the ring down even to the lands on the piston. (Use the second land is fine, but make sure the ring is even in the bore by sighting around the land... before you measure the gap...)

Ok, measure your gap.

Very carefully, take out the ring. (Your going to be doing this about 40 times, measure, file, measure, file and measure, file. And then you might have it right... LOL)

Grip the ring in both hands, place the end gaps flush against the file faces and push the gap on both sides into the ring file cutting faces. (You have to tilt the ring slightly to keep it from dragging on the file...)

Now pull it towards you, move it back in and pull again. I like to pull when I'm fileing, and not so much when I'm pushing the ring back up the file, but it cuts BOTH WAYS.

Now, you have cut some material, wipe off the ring with a clean cloth, oil with your fingers, and put it back in with the piston, measure and then file till you get the gap you want.

Simple? Nope, you need to do this for every bore, and there are different sized gaps for the top and second ring.. So keep them organized, and your good to go. (I put them onto the pistons for each bore as I sized them up... )

I think a wider SECOND ring gap is important to keep the top ring from seeing too much pressure.

.19 top and .17 lower is what mine are set at, and slightly wider than that was fine too. (It's not like you can put material back on if you file too much eh? LOL) But don't worry, just make sure the top has a wider gap than the bottem and your fine. Also, as the engine is running the rings are pressed into the bore by the pressure, so the small ring gap "hole" leaks a minimal amount of blow by in reality.

I've edited this, as I screwed up, and was saying the upper ring should be wider than the lower.. It is the SECOND RING that needs to be wider than the first.

Here is what ROSS suggests.
ROSS RACING RINGS for the top and second grooves are oversized
and must be file fitted to obtain proper end gap.
The proper amount of end gap required is directly dependent upon
the amount of heat to which the top ring in most cases should be the
bore size in inches x .004. (To convert mm to inches multiply the
bore size in mm x .03937). To determine the proper end gap in the
second ring on normally aspirated engines multiply the bore size in
inches x .005. It is recommended that the second rings have more
end gap than the top rings to allow any blowby gasses passing the
top rings to quickly escape to the crankcase. If the second ring gaps
are set smaller than the top ring gaps, the engine may show less
leak down under static conditions! Should the top ring land be .180
or narrower add .002 total to the above computed end gaps.
EXAMPLE: 3.625 (92mm) bore with. 150 width top land: 3.625 x .004
= .015 plus .002 for the narrow top land = .017 total end gap.
For engines that are blown, turbocharged, or run nitrous systems of
more than 30 horsepower per cylinder add .004 total to the end gaps
shown above. EXAMPLE: 3.625 x .004 = .015 plus .004 for
turbocharger - .019 total end gap.

 

[King of Cleveland]

If you leave the second ring in an old piston you can do exactly the above but there's no sighting involved; you put the new ring to be measured in the bore up at the top, then push it down with the upside down old piston until the ring sticking out of the second land sits right on the deck.

Makes this a little bit less tedious. not much. Still takes some time to get right.

 

[IJ.]

.017"

I run a little extra after closing an end gap last year and trashing a cylinder.

 

[Adjuster]

I'm with you on that IJ.

Wider on HP engines is not a problem... Mine are minimum gapped .17 upper and .19 lower respectively, but I have a few where they are more like .21... Blow by I know, but they will never expand and stick at that size I hope.

Just make sure you don't put the lower ring in narrower (less gap) than the top, and your good to go. (If you do that, pressure will not escape out via the exhaust and intake strokes, and the pressure spikes can cause your rings to flutter in the lands, further breaking the seal, causing more spikes, and wearing your rings out in short order.) It can also crack your rings, and that ends your day too.

Edited this. I've had some interesting things going on lately, and got this mixed up.

Wider on the second ring. You want the excess pressure to escape out as blowby and be burned off via the PCV system. If it does not escape, it will cause ring flutter. (That part I did not screw up, but I did get the ring gaps crossed...)

Sorry.
PS, Thanks Doward for catching my error.