New piston rings

[StevenPH]

I just reinstalled the first piston with new rings, and am finding the piston assembly to be VERY tight in the cylinder. With one piston installed, it takes about 40 Nm to move the crankshaft, and when it does move its very jerky.

Is this abnormal?

I measured each ring gap in the cylinder and they were accurate, and oiled everything for installation. I had a significant amount of blowby before, and Ive never done this before, so could it be I'm just not used to how tight these pistons are supposed to be?

Thanks!

[air-cooled or diesel]

they are tight esp w/new rings, and with only 1 installed it may make it (turning) feel funny

[Quantum-man]

Are you sure that it's the piston rings binding and not the big end bearing? Try slackening the cap, if not already checked for...

[82vdub]

40Nm is somewhere close to 30 lb/ft, which seems very high.

What is the measurement without any rods in the engine (main bearings and front and rear seal loads)? On a new engine, this should be pretty light.

Did you plastigage the rod and main bearings?

Did you lube the bearings before checking resistance in rotation?

Did you install the cap on the rod the correct way, and the rod into the cylinder bore the correct way (and piston on the rod the correct way)? There may be a chamfer on one side of the rod where it works one way with the crank journal.

Was the piston/ring assembly lubed prior to install?

[StevenPH]

I think i may have found the issue -

i found each piston was going in way too tight, and when removed there was ton of crud (i assume leftover carbon buildup) all over the rings and piston. So much that it caused some of the rings to stick already. I think that when deglazing the cylinders i didn't realize how much was there, and didn't spend enough time on the upper ridge, and the rings were scraping off that buildup during installation. Then all of the crud kept the piston from moving smoothly when down in the cylinder.

I removed all of the pistons again, and honed the cylinders until all visible buildup was gone. Cleaned pistons and rings of crud and the first i installed went in much more easily and seems to move much more smoothly. I had to leave it for the day but i'm hoping this will solve my issue.

There is still a slight ridge on the upper cylinder wall, but the diameters from top to bottom are all well within spec. The ring gaps are spot on.



To your questions 82vdub -

the measurement without rods is very light - i can hand turn it with the crank pulley. Prior to my initial rod and piston removal, the crankshaft turned easily with a ratchet on the crankshaft bolt.

i haven't plastigaged any bearings - i am not planning on opening the main bearings unless i have to. Is it critical to plastigage when installing new rod bearings? The old bearings and shaft show no signs of wear and i had no other driving symptoms other than blowby and very low compression.

Everything was lubed for installation with motor oil, bearings, rings, pistons, cylinder walls, crankshaft.

I never removed the rods from the pistons, and made sure the chamfers on the the cap and bearings matched and the cap was positioned correctly.


Thanks for your help!

[82vdub]

I've personally plastigaged every motor I've built (not that many) because it checks: the machinists work, the bearing manufacturers work, proper labeling of the bearing for it's size, and proper boxing of the parts. I've found all sorts of mis-boxed and bad parts in factory boxes over the years. It happens.

Some may not know, but the rod and rod cap are a machined pair. You can't just mix and match them as they will more than likely end in quick bearing failure unless the journal is resized for the new assembly. It's the same with the main bearings and where those caps are installed as well.

Make sure you get all that honing material out of the motor before it gets into every oriface, crevice, crack etc.

[Quantum-man]

Be sure to remove the carbon that was behind the rings.

The carbon will have formed to take up the wear in the old rings and they will be pushed too tightly against the the bore, possible premature wear, or breakage.

Vice grips and a piece of old ring good for decarbonisation IMO

[StevenPH]

Great thanks. I picked up some plastigage and will remove the carbon from behind the rings.

Any suggestions on using a solvent of some kind to help dissolve the carbon to help with removal?

[JRM]

I did a backwoods budget rebuild on mine back in 2004 with just rings, berrings and seals. Ran amazing! I just soaked the pistions for a week in paint thinner, my engine spun over very easy with all the new parts installed- I spun it over several times then re-torqued everything. This is a video taken just before I sold it with 50K on the rebuild
http://www.youtube.com/watch?v=yo-YUR1_DaU

[StevenPH]

Great idea - i'm soaking the pistons for at least a few days until some new rings arrive (I broke two oil rings already - those suckers are brittle!)

[Quantum-man]

Never could wait for a soak; so a piece of ring for the grooves, kept at an acute angle to avoid scratching, and finish off with some oven cleaner foam.
Wipe off after an hour or so.

I would consider leaving band of carbon above the rings on the pistons, if not scored, for extra compression.

Set ring gaps at 180 degree spacings, contrary to 120 deg dogma for maximum compression. Search for my rationale if you wish.

[air-cooled or diesel]

before installing pistons make sure you sand a cross-hatch pattern into cyls, i like to use 330 wet or dry w/kero.

[StevenPH]

Would you do this in addition to the honing? I used the 3 stone hone from harbor freight. Honing left scoring on the bore walls - its mostly cross hatched - I moved the hone up and down fast enough to achieve some level of cross hatch.

Thanks

[82vdub]

The cross hatch from the honing should be just fine.

[air-cooled or diesel]

thats if you go in both directions!!