Piston--Cylinders--Connecting Rod Fitment

[JTR249]

When you assemble a piston with rings and it is so tight you have to force the gap closed in order to get the piston into the cylinder, is that normal?
Also, after it is in the cylinder, can you use WD40 to slightly free the piston to get it started?
Basically, should the piston move freely if the piston and rings are brand new?
Is the tightness a sign the bottom ring is upside down and installed incorrectly?
-JR

[1939wl]

Hello JR,
The guidelines for setting piston ring end gap do vary depending on the type of ring and the application, but for a standard, normally aspirated rebuild with a fresh bore and hone on the cylinder, most piston ring manufacturers recommend ,004” per inch of bore as a minimum. With a standard bore of 3.4375 X .004” = .01375” (set at .014”). Each application will utilize a different gap but the .004” rule of thumb for a standard rebuild is common. This is measured 1” down in the bore. I make a tool that squares the ring in the bore, but a piston upside down can also be used to square the ring in the bore to measure your gap. Too tight will cause the engine to fail at temperature. I wet hone with torque plates, that I have made for all of the H-D and Indian cylinders and set my ring gaps with the plates torqued in place as well.
Do not use WD-40 to lubricate a piston / ring assembly. There are many assembly lubes that are fantastic for initial fire up, and will prevent the rings from scuffing the walls, or the pistons. Break away torque on a fresh engine can feel quite tight, but the cylinders, ring grooves, rings and the entire reciprocating assembly should be lubed with a heavy lubricant (i.e. assembly lube) when assembled.
Checking to make sure that the chamfers on the rings are correctly oriented, rings have enough top, and back (depth) clearance in the piston itself and gaps are set correctly takes patience to do correctly. Ring material (stainless, moly coated, etc.) will determine the final hone on the bore, so that needs to be established first. There are several styles of bottom oil ring. The most important thing with oil rings is to make sure that the “butt” of the expander is not overlapping. Some manufacturers use two different colors so you can see that they do not overlap. Manufacturers always have recommendations for their products. Here are a couple from Hastings and Wiseco:

Everything You Need To Know About Ring Gap! | Wiseco

Checking Compression Ring Gaps – Hastings Piston Rings

Hope this helps.

[otis71]

There is also a tolerance to follow for the ring in relation to the ring groove in the piston. Most manuals will give this to you.

[JTR249]

Yes, you suggestion on end gap is correct for an Indian Vertical, but when you mentioned the cylinder hone, I think that might be the issue if the assembly lube does not work.
I didn't know there was a certain kind of lube for that and I did not have the cylinder honed. It was in very good condition and does not appear as if it was ever used.
I don't see an overlap on the bottom ring. These are Hastings fitted at Jim's Preussner's shop and so I didn't question anything he did. He checked to make sure the piston was grooved before placing the rings in position. The problem is my lack of shop knowledge when I try to learn assembly procedures.
When I fit the piston in the cylinder before I sent it to the shop, it was very snug and a clean fit. I think the situation might just need the lube as you suggest.
I will post how it goes.
Thanks to both of you.
-JR

[pisten-bully]

...nice post 1939wl!

[T. Cotten]

...nice post 1939wl!

Absolutely, Harry!

(Meanwhile, my torque plate collection and AN Sunnen rig are for sale.. .. .)

...Cotten
PS: My Chief top plate is magnesium for convenience.

[1939wl]

Yes, you suggestion on end gap is correct for an Indian Vertical, but when you mentioned the cylinder hone, I think that might be the issue if the assembly lube does not work.
I didn't know there was a certain kind of lube for that and I did not have the cylinder honed. It was in very good condition and does not appear as if it was ever used.
I don't see an overlap on the bottom ring. These are Hastings fitted at Jim's Preussner's shop and so I didn't question anything he did. He checked to make sure the piston was grooved before placing the rings in position. The problem is my lack of shop knowledge when I try to learn assembly procedures.
When I fit the piston in the cylinder before I sent it to the shop, it was very snug and a clean fit. I think the situation might just need the lube as you suggest.
I will post how it goes.
Thanks to both of you.
-JR

Hello JR,
I am really fussy about fitting pistons and rings. Regardless of the engine, the bore needs to be perfectly round, ......perfectly round. That is why I use torque plates top and bottom of the cylinder when I bore, hone, and fit rings. If you can get ahold of a bore gauge and even without setting it, check the bores in your cylinders at the top, perpendicular to the pin, and parallel to the pin. Then move down about 1.5" and do it again in both places, and move down progressively to finally check the bore at the bottom, at both positions. There should be no movement of the needle on the bore gauge from top to bottom. Here is where my advice gets dicey. I would not check this without the cylinders being torqued at the base and at the head. I have been doing this for 52 years and have proven to many people through the years how important this is to ensure a round bore.... did I already mention that, how important a round bore is? If you do not have access to a bore gauge, a telescoping gauge and a micrometer can give you the numbers. The bore gauge is easy because you read it directly, and any out of round, or taper is obvious.
The bore size after finish hone is determined by:
1) The actual size of the piston (not a "nominal" value i.e. .020 oversize printed on the piston) The piston is measured usually perpendicular to the pin, towards the bottom of the skirt. There may be .003" difference in some pistons at the top towards the deck and bottom of the skirt, so the "size" has to be accurately measured and checked against actual bore size.
2) Material that the piston is made from (Hypereutectic alloy, aluminum casting, aluminum forging, "Dow Metal" which is a magnesium alloy) as all of these have different expansion rates.
3) Application, i.e. street, turbocharged, supercharged, racing.
Most piston suppliers will provide the dimensions for ring gap, piston to bore, pin fit, balance etc. I did mention the ring to groove fit in my first post. Starting with a new piston, whether NOS factory or brand-new manufacture even with the other parameters correct. Of course, using new pistons (whether NOS factory, or aftermarket) you shouldn't have to worry about this, but I always check it. New manufactures will almost always have the grooves correctly cut, for size and depth for their recommended rings. Years ago, engines were re-ringed at 12,000 miles, so valves were reseated, and rings were replaced at approximately those intervals. We find it hard to believe these days, but an "overhaul" was a common service station job. Carbon would build up behind the rings in the ring grooves, and a tool was used to fit in the ring groove with little clearance top, bottom and back wall of the groove. This tool should remove the carbon, but nothing else. A mechanic could just keep cutting beyond the back wall carbon build up, and remove aluminum, thus making the groove deeper. When the rings "flutter" on deceleration, this causes excessive blowby, even when all other parameters are correct.
Assembly lubes are made by many manufactures. Years ago we used STP, Motor Honey, 60 weight engine oil etc. There are many lubricants to be used for specific applications, cam journal and bearing lubricants as well. These are formulated for EP (extreme pressure) as well as for that initial startup, to remain in place and not wick away. A quick search will yield results for assembly lube or check with the shop that put the rings on your pistons, to see what they recommend. A heavier motor oil is fine if your oil pump is primed, and you know that lubrication will be delivered immediately upon start up. The assembly lubes stay in place, so initial startup is seamless.
Hope that helps.

[JTR249]

Yes, that helps alot. I will let you know what happens.
Thank you.
-JR