Piston in cylinder - How tight should it be?

[Dadofour]

I suppose I should have more faith, but here we are again.

I installed piston 1 into cylinder 1 (twice) and the piston is tight in the cylinder. Visually the cylinder has room, but the rings must be exerting a lot of pressure against the cylinder wall.

Is this normal? I can move the piston in the cylinder, but it take two hands and makes a sucking noise as it moves.

I have installed the number two and three pistons in their cylinders and they are the same, leading me to believe all is well.





One final question: Do I have the piston oriented in the cylinder correctly?

[2.7RACER]

Than,
I suspect your rings may need a little trimming.
Carefully remove the top compression ring from number one piston and insert the ring into the number one cylinder. Square up the ring about one inch from the top of the cylinder, then check the end gap of the ring.
Should be 0.003" (three thousandths) minimum, up to 0.006" OK.
If the rings check out OK, the fit of the ring in the piston may not allow the ring to fully collapse around the piston when the piston is inserted into the cylinder. Easily checked by observation.
All three rings need to be checked in the cylinder as described above.
The piston dome (high side) should be down facing the exhaust, or said differently the dish up facing the intake and spark-plug.
From your picture it looks upside down to me.

[Dadofour]

Doug,

I will pull the number one piston tomorrow and measure the ring gap in the cylinder.

If the rings need trimming, why would that be the case, and how would I trim them?

How easily should the piston move in the cylinder?

Thanks for the detailed response.

Than

[edevinney]

I can't tell you how easily the piston should move in the bore, but setting piston ring gap is a pretty standard procedure to get the fit exactly right for the actual bore size of the individual cylinder in question. Too much gap and you give up cylinder pressure (and hp) through the gap. Not enough gap and the rings can break when hot.

If the gap is too small you can file the ends of the ring. There are relatively inexpensive tools made to file rings so that the ends are square, looks like our hosts don't carry one but many hotrod and tool sources do.

It's not all that tough to do and you'll be happy it's done right.

[2.7RACER]

Than,
Once you have checked the ring gaps and have them right, the piston will move in the cylinder without the need of two hands.
The sucking sound is from the cylinder sitting on the newsprint on your bench, thus trapping the air in the cylinder.
Place the cylinder on a couple of spacers (small pieces of wood), so the cylinder is up off the bench. A small gap under the cylinder to release the air.
This could very well be the reason you need two hands in the first place.
Try this first.
I would also wipe the cylinder walls with a oily rag just to lubricate the walls. Wipe the piston too. Just a light amount of oil, not dripping.
Please let us know what you find.

[Dadofour]

Doug,

I am coming up with a .018" gap. Obviously not too small, but it raises questions about to big.





Thanks

Than

[jpnovak]

Did you measure that with a feeler gauge or the calipers?

[sammyg2]

Use the piston crown to push the ring down into the bore, that will keep it square and give you a fighting chance of measuring it correctly. Then use a good set of feeler guages to measure the gap.
I'm really good with tools and I doubt I could measure the gap accurately with calipers.

[Dadofour]

Heading out to get feeler guages....

[911pcars]

Are these new or used cylinders? If used, slide the ring down toward the bottom of the cylinder where there's less wear, then remeasure.

Factory specs for a 90mm factory Mahle piston:
1st comp. ring: .0039" - .0078" with a wear limit of .0314".

At .018", I'd say you're okay, but use a feeler gauge to verify the gap, not a vernier caliper. There are specs for the other rings too as well as side clearance on each.

Some sources:
http://home.jps.net/~snowbum/engineinternals.htm
http://www.aa1car.com/library/ring_end_gap.htm
http://www.hcpresearch.com/ring_gap.html

Getting back to the ease with which the pistons slide up and down. Realize the three rings provide friction against the cylinder walls. Did you verify correct piston-to-cylinder clearance too? If all measurements are okay, put it together. Don't assume anything, even new parts are sometimes whacko (e.g. wrong part in new box).

Sherwood

[ChrisBennet]

Piston orientation: I'm going to have to disagree with 2.7Racer. That looks like how I remember a 3.2 piston should be oriented.
I don't have a picture but I did come across a picture of a 3.0 motor I did and that had the tall part of the pistons near the intake as well.
-Chris

[Eagledriver]

I'll second Chris on the piston orientation. The dome should be at the top.

-Andy

[Dadofour]

Thanks guys.

These are used Pistons and cylinders. The seller of the engine got them to replace the original P&C's. They are OEM stock Nikasil 3.2's.

The machinist who did the rod bushings checked them and honed them indicating they were good.

I got a feeler guage (building an engine is a great excuse to buy tools!)

The measure meant at the bottom of the cylinder is .015"
The measure meant at the top of the cylinder is .017"

The piston moves through the cylinder well now that I have removed the cylinder from the table (not too bright).

I have checked Wayne's book three more times and verified piston orientation as correct in my pictures so I have restored all pistons to this orientation.

With a little practice installing pistons into the cylinders was not as hard as I thought.

I am going to call my machinist to ask about the gap.

Than

[Dadofour]

Spoke to my machinist, and he indicated factory spec for new P&C's is:

Top Compression Ring = .2 - .3mm or .008" - .016" wear limit = .8mm or .03"

2nd Compression Ring = .2 - .3mm or .008" - .016" wear limit = .8mm or .03"

Top Compression Ring = .3 - .6mm or .012" - .023"

Can anyone confirm these numbers? If these are right, then I should be pretty good. I'm off to measure the 2nd compression rings as well.

[Eagledriver]

Those numbers sound right from what I remember. They are published in Waynes book and even the dreaded Haynes manual. I prefer to use the old rings if they are in spec. Looks like yours are fine.

-Andy

[911pcars]

"Spoke to my machinist, and he indicated factory spec for new P&C's is:

Top Compression Ring = .2 - .3mm or .008" - .016" wear limit = .8mm or .03mm"

Andy,
Check my math. Wear limit is .8mm which is 0.03149". At .017", your end gap is well within spec (for the top ring).

Sherwood

[2.7RACER]

I stand corrected regarding my view on the piston orientation.
Good to hear once the cylinder was off the bench surface the piston moved smoothly thru the cylinder.

[Wayne 962]

Sounds about right - the pistons should be stiff within the cylinders with the rings installed...

-Wayne

[Dadofour]

Thanks everyone! Wayne I think you said it best. The pistons are stiff in the cylinders. After moving the ring in the cylinder by itself I realized how much pressure the rings exert against the cylinder walls and realized the pistons were fine.

Gapping the rings just confirmed it.