Piston ring clearance

[Arrowglassrestorer]

I am rebuilding a 3.0. I had 110psi compression in each cylinder and needed to find out why the low compression. When I tore down the engine I found that the side clearance from ring to groove was .006 on all the top rings on all the pistons. In looking at the shop manual that engine calls for .0012-.0027. I have rebuilt a lot of car engines and that clearance seems a little tight. Is the book right and could this be why my compression is low. BTW I am checking the head and block as well.

 

[alldodge]

I am rebuilding a 3.0. I had 110psi compression in each cylinder and needed to find out why the low compression. When I tore down the engine I found that the side clearance from ring to groove was .006 on all the top rings on all the pistons. In looking at the shop manual that engine calls for .0012-.0027. I have rebuilt a lot of car engines and that clearance seems a little tight. Is the book right and could this be why my compression is low. BTW I am checking the head and block as well.

The ,0012-.0027 is the ring grove clearance, meaning there is a groove all the way around the piston and the ring sits in side the grove. The ring Gap clearance is the ring inside the bore of the cylinder and the air gap between the two ends of the ring should be around 0.010-0.020 for a production run engine. A rebuilt one will be tighter if done properly, so your 0.006 is good.

Did you do a leak down test? If not your weak compression could be valves, valve seats, head gasket or sticking lifters

 

[Arrowglassrestorer]

Since I already have the head off I am sending the head to be check and valves redone. This block has never been bored and I believe that a good honing will do the job. The rings that were in the piston were .075, pretty standard. So I guess the grooves wore quite a bit. Do you think that this ware will effect compression very much if I rering with a .075 ring

 

[Arrowglassrestorer]

Correction: rings are .078 thick

 

[Bt Doctur]

 

[Bt Doctur]

screen grab cut off from the top 1st set of numbers 4 cyl, 2nd row 140, 3rd row IL6
measure the bore and measure the piston, install a used ring in the bore and measure the end gap. Most older
motors need a bore job not a rehone.
Dont even think of putting .010 over rings on a standard piston

 

[alldodge]

I also agree don't do a hone and go. The wear to cylinder walls are close to true at the top but get wider as they go down. See if you can check to bore with a bore gauge. The ring grooves wear on the outer edge more then the inner. Another 400 or 500 will rebuild the hole motor with new pistons. What year is the motor?

 

[Bondo]

Most older motors need a bore job not a rehone.

Ayuh,... Ditto here,... Original condition, still with cross-hatch, Maybe,...

Cylinders don't wear evenly,... A re-bore is like a brand new motor,...
At 110 psi, yer's probably needs it,...

What did the head look like,..??
Did ya do a leak-down test to see which way the compression was bein' lost,..?

 

[Arrowglassrestorer]

It came out of a 1977 boat but I can verify that it was used very little over the years. It has been sitting the last 20. While I still may end up getting the block machined 400 or 500 more will not get it done in my area.

 

[wrench 3]

Was there much of a ridge at the top of the cylinders at the top of where the rings traveled. Place a piston ring in the cylinder. Push a piston into the cylinder (upside down) to square up the ring. Pull the piston back out and measure the ring end gap. Repeat at at-least five different positions in the bore and check for variations.

 

[NHGuy]

I think he is talking about the ring groove clearance. Not ring gap. If it's .006 it's over twice the .0027 limit. Piston grooves might be no good, How do they look?
I'd venture a guess that ring manufacturers stop making thicker rings at some point since there would be extra contact area with the cylinder walls.

I rechecked your ring thickness and groove clearance numbers. The .078 rings will not take up enough of the extra groove clearance to get your desired .groove height.
Here's the math .075 plus .006 = .0081 groove height. With the .078 rings there will still be .003 groove clearance and that's greater than the .0012 to .0027 allowed.
An .079 ring thickness would get your groove clearance, do those exist? That would put you at .002 which is smack in the middle between the allowed limits of .0012 and .0027.

 

[NHGuy]

Get your end gap just right too. There are gap calculators on all the piston sites. Calculate marine or towing ring gaps.
I'd recommend using go/no-go gap gauges. They are great for ring gaps. The center of the feeler is the printed thickness. There is a thousandth extra thickness at the inner edge of the feeler, and a thousandth less at the tip. They make it easier to zero in on a thickness.

I updated post 11. If you originally read this before 10:34 PM forum time, look at that post again.

 

[wrench 3]

Was there much of a ridge at the top of the cylinders at the top of where the rings traveled. Place a piston ring in the cylinder. Push a piston into the cylinder (upside down) to square up the ring. Pull the piston back out and measure the ring end gap. Repeat at at-least five different positions in the bore and check for variations.

Just trying to save him paying for a re-bore if it's not needed.

 

[Arrowglassrestorer]

Thanks I just came frim the machine shop and they said it only needed honing and a set of standard rings. My only concern is the side clearance for the ring in the piston groove. I will need to check further on that.

 

[alldodge]

Good luck, hope it works out

 

[wrench 3]

You used to be able to get shims to put in the ring groove along side the piston ring to take up the extra space. You had to square up the groove first though. It might be worth asking at your machine shop but if he's not an old timer he may not know what your talking about.

 

[alldodge]

IMO pistons using oversize rings or spacers to take slop out of rings is like reusing a head gasket

 

[Bondo]

Ayuh,... If the pistons are shot, they're shot,...

That's why SoP is to over-bore .030", 'n replace the pistons, 'n rings with New,...

 

[Rick Stephens]

Pistons and rings are ~$150. Bore and replace. All of these extra steps just mean likelihood of rebuilding the motor right a second time go way up.

RIck

 

[Oshkosh1]

Thanks I just came frim the machine shop and they said it only needed honing and a set of standard rings. My only concern is the side clearance for the ring in the piston groove. I will need to check further on that.

If they're a reputable shop...listen to them. While "we" have good intentions..."we" have not personally seen, touched or measured anything on it.

In my other "world" I deal with a couple of shops who do my custom valve work on heads I rebuild. One of them is a true "old school" shop who's owner has been in business for 37 years. He wouldn't tell a customer to simply hone and go if indeed it was in need of a re-bore. I've simply used a dingle ball/starting fluid, new pistons/rings and refreshed a couple of old, low compression engines to almost new numbers.

Myself? I'd probably punch new holes and be done with it, however if the OP wants to save some $$$ and there aren't any glaring imperfections then go for it.

You can lap the valves in yourself and check your work with a sharpie as dye on the seat to save some money as well. New valve guide seals, and unless you've overheated it...clean up the face use a new viton coated head gasket, torque to spec and go.