Fitting Piston Rings

It's OK guys, I'll take care of it. Since all you hot shot engine builders won't take the time to show us how you properly fit piston rings, I'll cover for you this time. The procedure to follow was documented during final assembly of my 400 Cleveland motor. Let's review the application:

Block: 351M/400, 4.030" Bore
Piston: Probe FPS 351C, PN P2379F-030, 5/64", 5/64", 3/16" Ring Grooves
Ring Set: Federal Mogul Sealed Power, PN E-251K30, 5/64", 5/64", 3/16"

So What's on the Workbench?
Here's a shot showing critical items I used to fit my piston rings. Included here is a piston and rod assembly for squaring the rings, the piston ring set, a fine grade file, a sharpening or "whetstone", and a feeler gauge. Not pictured, but required, is a piston ring expander. It's also good to have standard engine building supplies like lacquer thinner, a number of clean shop towels, and a squirt bottle of motor oil.


About the Rings
Federal Mogul Sealed Power Piston Ring Sets are a standard in engine building for production and performance builders. They are proven for street performance motors like my 400 Cleveland. I do not have experience with anything other than Sealed Power type ring sets, so I will not go into a comparison with other ring types. With that said, here's a breakdown of what you should know about a Sealed Power Piston Ring Set.


How Much Do They Cost?

The Official Product Description at Federal Mogul

Here's the basics, one ring set is made up of 5 rings. From the top of the piston down, you have a single Compression Ring (aka Top Ring), a single Wiper Ring (aka Second Ring), and a three-piece Oil Ring Set made up of two Rail Rings and an Expander Ring. Compression Rings and Wiper Rings have a "dot" stamped into them to indicate which side faces up when installed since their cross-sections are asymmetrical by design. Rail Rings and the Expander Ring are symmetrical which means there is no proper up or down "side".

Compression Ring
A Sealed Power Compression Ring is made of iron and can be distingusihed from a Wiper Ring easily if you know what to look for. Compression rings have a moly and chrome face that shines while Wiper Rings have a face with a flat black finish. While not easily detectable to the naked eye, Compression Rings and Wiper Rings are tapered on the face.

Wiper Ring
As mentioned, a Sealed Power Wiper Ring while made of iron, does not have the moly and chrome treatment to the ring face. Wiper Rings are tapered on the face like a Compression Ring, but they also have an inner bevel along the inner diameter.

Compression Rings vs. Wiper Rings... in pictures


Compression Rings on the left and Wiper Rings on the right. So what's the visual difference?


A clear shot showing the Compression Ring moly and chrome faces.


And a clear shot showing the Wiper Ring matte faces.


Note the nearly rectangular cross-section of this Compression Ring.


And note the bevel along the inner diameter of this Wiper Ring.


This technical drawing also shows the cross-sections of Compression and Wiper Rings, exaggerated for effect.


The stamped "dot" on both Compression and Wiper Rings indicate which side faces up during installation.

Oil Ring Set
Federal-Mogul - Sealed Power SS-50 Oil Rings

Fitting the Rings
It seems you should be able to take your rings straight from the box and wrap them onto your pistons. While this may work, you don't want to take any chances when you're this deep into an engine build. It's always possible that the wrong rings were placed in the right box... and with 40 total ring related components, it CAN happen.

Test fitting and filing rings for an optimum fit will prevent anguish and ensure the best possible combustion chamber seal. So, when you are wondering what difference filing off 1/1000 of an inch of ring material makes, keep in mind you are also conducting one last quality control procedure for the Federal Mogul Corporation to confirm a 4.010" bore Compression Ring didn't make it into your 4.030" bore package.

Before I get started. Here's are the basic formulas I am going to use to determine proper ring gap for the Compression Ring and the Wiper Ring. The Rail Rings in the Oil Ring Set should be checked for proper size, but since they can run gaps as large as .030" and do not serve a sealing function, no filing is needed. The Expander Ring should butt together and obviously does not require filing either.

Compression Ring Gap Formula -.0045" x Bore (4.030") = .018"
Wiper Ring Gap Formula - .0030" x Bore (4.030") = .012"

Let's get started...


I began by arranging pairs of Compression and Wiper Rings in a box corresponding with each cylinder.


Then I wiped down all the cylinder walls with motor oil.


With that done, I fit a Compression Ring in cylinder one.


And squared it with a piston.


Here, I've indicated the "out-of-the-box" Compression Ring gap in cylinder one, a little tight. Too tight of a gap will close during heat expansion and cause rapid wear.


A feeler gauge indicated a gap of .013"


Good news was these were the right rings, just needed to shave off about .005" to increase the gap to .018". Five or six forward rubs across this fine grade file is all I needed. I only worked one side of the ring gap.


Here, I'm verifying a true mating surface by closing the gap and checking for light.


Next, I smoothed the filed surfaces with a "whetstone".


I also used the whetstone to gently break the edges of the ring.


Here, I wiped off any filings with a shop towel and thinner.


This is a close-up of the re-fitted Compression Ring in cylinder one.


With the feeler gauge I verified a proper gap of .018". Of course its necessary to "sneak" up on the proper gap to avoid going too large, so be prepared to file, fit, remove, file, and re-fit.


I removed the Compression Ring for cylinder one and placed it back in the box in corresponding order.


I repeated the procedure 7 more times for each cylinder's Compression Ring. In some cases, the Compression Rings required no filing whatsoever.


With the Compression Rings done, I started back at cylinder one with the Wiper Rings.


Same procedure, just shooting for a smaller optimum gap.


The Wiper Rings run a smaller gap as I indicated earlier. Most of these required no filing since they hit .012" right out of the box. If they don't, simply file and fit like the Compression Rings.


Now it was on to the Oil Ring Set. There was no need to arrange these by cylinder since the gap for the Rail Rings are large.


Still, I test fit the Rail Rings and the Expander Rings to confirm I had the right ones from the manufacturer.


There's the large gap. In this case, .025", which is fine.


With all the rings gapped and confirmed, I proceeded to fit them on the piston assemblies. First, I wrapped the bottom Rail Ring in the bottom groove for the Oil Set.


Then I added the Expander Ring and butt the black and red tabs together.


After that, I wrapped the top Rail Ring into the Oil Set groove.


Note the offset of the gaps for each Rail Ring as they relate to the black and red tabs of the Expander Ring


Obviously, you don't want the open ends of each ring in the Oil Set to be vertically inline.


Using a ring expander tool, I fit the corresponding Wiper Ring to the piston assembly for cylinder one.


And finally I fit the corresponding Compression Ring to the piston assembly for cylinder one.


The final product.


When done, I placed the piston assemblies in a box also corresponding with each cylinder. Repeat 7 more times.

Im assuming this could be followed for a 289? Since Im using the same piston and ring combo you are. I have NEVER put together a motor, and I am going to pick my fresh cut/clean block up next week.

Thanks for the write up.

Yes, you may get some different opinions on the optimum gap but everything I used here is standard for a NA street motor. Also, not required but some prefer to use one of these rather than the hand file... Powerhouse: Piston Ring Filer with Carbide Wheel

Originally Posted by Mikelonis

Yes, you may get some different opinions on the optimum gap but everything I used here is standard for a NA street motor. Also, not required but some prefer to use one of these rather than the hand file... Powerhouse: Piston Ring Filer with Carbide Wheel

yup always more then one way to skin a cat.

great article. I used a ring filer and it took me a better part of a day to do the job. Like mentioned, you will file, fit, file, fit, file,............... on the same ring. The piston/ring pkg you get will have a formula for the gap measurement that you should apply to your application. If you go a thousands over that's ok. I'm not sure how much is too far.

This is a GREAT writeup, including good and comprehensive pictures.
It isn't a hard job indeed, but when doing it for the first time without having it seen "live" once, this is a good start! (for me, in the couple next months are definately going to use it, as i'm rebuilding an engine also)

Another Q:

I have an engine with brand new pistons in it (now disassembled) and i bought flattops for it (same brand, Keith Black hyper, same weight).
How do i remove the "old" pistons, and put the new ones om?
They are heat crimped..
i know sort of how it works, but not exactly enought to go to the barn, and do the job right away...

Anybody got some tips and hints? (preferred including pics..?)

thanks!!

They are heat crimped.. I never heard of that. Please explain.

maybe i tell it wrong, sorry.
The rod's are heated, and then the piston pin is put through the rod, and cooled down. Due the expansion during heating, the hole in the rod is slightly larger, letting the pin go right in, and holding it in place when cooled down. It's the way that stock pistons are connected to the rods i believe.

Now, i wonder: do i just push the pins out, or do i need to heat them agan, having everything enlarge due the heat? I think they'll expand both, as you heat the rod and the pin as well... Maybe not, due being different materials... i don't know...

Hope its a bit clear what i mean now

Thanks.

Originally Posted by dutchlincoln79

maybe i tell it wrong, sorry.
The rod's are heated, and then the piston pin is put through the rod, and cooled down. Due the expansion during heating, the hole in the rod is slightly larger, letting the pin go right in, and holding it in place when cooled down. It's the way that stock pistons are connected to the rods i believe.

Now, i wonder: do i just push the pins out, or do i need to heat them agan, having everything enlarge due the heat? I think they'll expand both, as you heat the rod and the pin as well... Maybe not, due being different materials... i don't know...

Hope its a bit clear what i mean now

Thanks.

We normally call that a press fit pins. in the machine shop we just press off he old pistons and heat the small end to install again. Now if you want to save the old pistons there are special fixtures we use to remove the old piston with the risk of damage.
Nice job on the article Jon, in the case of our race engines, we also install a torque plate on the block when we file fit the rings. It is amazing the difference here also.
Quick question Jon, the picture of the piston & rod titled "The final product" looks like the rod cap is incorrect for that rod. If you look at the parting line of the rod, the radius of the rod and cap does not match. The first picture at the top is the same, but reversed. Maybe the cap is on the rod back wards.

Thanks Tim. Yeah, the cap was on backwards, I noticed that too when I finally went to install them. I corrected the last picture, thanks for watching out.