Question on Piston Ring Gap

Markus · Post by **Markus** » Fri May 10, 2019 10:42 am

Hello together,
I use Aluminium Pistons in my 26 Roadster with .040 oversize and the Grand Rings for Aluminium Pistons .040. The standard Gap for the rings is 0.004-inch per inch of diameter of the cylinder. That means 0.004 x 3.790 = 0,015.
My question is now, what is the maximum accept gap for this combination? is 0,024 to much???

Thanks from Germany
Markus

Post by **SurfCityGene** » Sat May 11, 2019 3:00 am

I think that's too much. I was advised by the guys at Grant to use the next over size rings and then you can get the proper gap.

Kerry · Post by **Kerry** » Sat May 11, 2019 3:16 am

0.012" is what I set a fresh rebore rebuild on a T but according to the likes of Hastings web site, max can be .022" to .026" for around a 4" bore, give or take 1/2"
If your new rings are giving you that much gap and are the right size for the pistons then what are your pistons giving you in the bore?

Aarongriffey · Post by **Aarongriffey** » Sat May 11, 2019 11:45 am

I think .012” is a little tight for the top ring, okay for the oil ring though.
I’d not worry about a little extra ring gap in the top ring, put it together.

Kerry · Post by **Kerry** » Sat May 11, 2019 6:21 pm

Aaron,
to some it may seem a tight spec but for 45 years of engine rebuilding never any issues, even by the book which is a min of .003" per inch. Even at .012" is still just over the min on a 3.750" T bore.
By the way the spec book I've used all those years, (Digest of Automotive Engine Re-Conditioning) is a max of .005" per inch, unless ring makers specs say different.

Markus · Post by **Markus** » Mon May 13, 2019 6:29 am

Thanks for the answers.
I want to explain, why i asked the question:
I got a complete rebuild motor. I put the motor in a teststand to run the motor to see that the motor is running fine. After about 10 minutes run (motor is running fine), i removed the sparkplugs. The sparkplug from the first cylinder was complete black with a oil film. The others are fine. So i removed the head and i found this:

: 2019-05-13 11_10_24-IMG_1632.jpg (32.95 KiB) Viewed 4493 times

The Pistons are Aluminium with 0,040 oversize.
Than i start to investigate what is the problem. I removed the first Piston to see where are the gaps located from the Pistonrings. That was fine.
Then i checked the installation direction of the piston (Wrist Pin goes toward the cam. The slot in the Piston goes away from the Cam). That was fine.
Than i started to measure. Here are the results:
Top Pistonring 0,026 Gap
Middle Pistonring 0,022 Gap
Oil ring 0,020 Gap
Gap from the piston rings in the piston groove 0,002

Piston:

: Piston diamention.jpg (43.35 KiB) Viewed 4493 times

Diameter from the Piston bore 3,791.
When i put the feeler gauge in (Piston in the piston bore without Piston rings) there is a Gap from 0,008.

I will now remove the second piston to measure it.
what is your opinion?

Markus

Post by **Charlie B in N.J.** » Mon May 13, 2019 7:00 am

Is there an outside oil line over feeding #1 ?

Markus · Post by **Markus** » Mon May 13, 2019 7:19 am

Yes, there is an outside oil line.

Markus · Post by **Markus** » Tue May 14, 2019 6:36 am

Hi together,

Does anyone have an idea for this topic????

Post by **DanTreace** » Tue May 14, 2019 7:47 am

Might check position of the rings on that piston. Had one rebuild that fouled #1, valves were ok, but pulled the piston and found one ring up-side down and the taper was pushing up the oil.

Used a hone and scuffed the bore, replaced the ring correct side up, put piston back, and no more issues.

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