All I know about Piston Ring end Gaps

Posted by CSR on March 22, 2007 at 09:23:54 from (192.249.47.8):

I have previous posts on the subject. I had a good conversation with Dawn @ Hastings and thought I'd pass along what I've learned.
------------------------------------------

Info from a follow-up phone call with Dawn.
1) My top and second rings are both regular iron, but the top one is triple chrome plated (dull silver color) and the 2nd one is bare.
2) The top ring is described as having as a "barrel face", where ~.030 of the .092 will be in contact with the bore.
3) The 2nd ring is described as a "reverse twist torsional taper face" where the bottom of the ring (OD) is larger than the top (OD) of the ring. The sharp lower edge helps scrape the oil down. Eventually the entire end face will be in contact due to wear.
4) The end gaps allow the ring to be hotter than bore which makes sense because the smaller cross-section of the ring means they will thermally respond quicker than the sleeve/bore. Per my calculations, .004 inch of gap/inch of bore on my 3.125 bore means I want .0125 min gap (alpha of 6.3 e-06). The .0125 min gap will close if the ring gets 200F than the bore. In a pulling situation where quick heat up is more likely, being bigger on gap may prevent a binding situation and be to my advantage. Dawn said some pullers run top gaps nearer .030 particularly if they run hot and have the ring very close to the top of the piston.
5) The 2nd ring gap can be and sometimes is designed to have a bigger gap than the top ring gap because when the pressure gets below the first ring, if trapped by the second ring, it can get under the top ring and cause it to lift. It is better to let the pressure exit towards the oil ring and be vented thru the crankcase. The pressure already made it past the important top ring so where it goes after the fact is of secondary importance.
6) The ductile iron rings she suggests have more steel in them and do not creep as much as the regular iron. They are not available for my bore size but are desirable when you can get them. They retain more of their spring after exposure to temperature so provide a better seal to the bore. With the regular iron, the ring can take the size of the bore, so exert less force against the bore.
7) The part numbers she lists are for regular iron rings but are longer so could be hand filed if I wanted to. She said I could buy them from her with a credit card. They are $5-6 each.

-----Original Message-----
From: Dawn Hill @ hastings
Sent: Thursday, March 22, 2007 9:42 AM
To: Richardson, Carl S.
Subject: RE: Farmall A Piston Rings

Carl
Bore of 3.1250, ring end gap .010min/.020max factory specs the 3.126 bore will open the end gap by .003

Exp; if the rings leave the factory at max .020 the end gap will be ..023 when installed in the 3.1260 bore

Oil ring rail end gaps .010min/.050max factory specs

Im not sure the end gaps you are stating below should be of a big concern in the use of the engine. Likely the heat is extream and the thermal expansion of the top ring would be greater than in the normal use of the tractor. The larger end gap may be helpful in not allowing the end gap to butt together in an extream heat situation.
But, if you choose to have an end gap less than the below stated, you can use oversize top and 2nd rings and file the end gap to what you feel to be an advantage.
Top ring #34378 3.1350 bore
2nd ring #34377 3.1350 bore

My feel of a greater worry is the Top ring being regular iron chrome plated, likely not the preferred choice but possibly the only choice.
In the Performance engines, it is always nice to be able to use the Ductile Iron top ring, The ductile Iron rings are less apt to loose strength and tension under the higher heat and stress.

Not sure if this is helping you in making any decissions on your next step, but feel free to call or email if you have other questions.

Regards
Dawn Hill
Ring Engineering

-----Original Message-----
Sent: Thursday, March 22, 2007 8:20 AM
To: Dawn Hill
Subject: Farmall A Piston Rings

>
> 1940 Farmall (also known as International Harvester and Case IH)
> farm tractor
> Original engine was 113 cu in and the factory overbore kit went from
> 3" bore to 3.125" bore, raising the displacement to 123 cu-in.
> I am installing the overbore kit.
>
> C123 engine 123 cu in, nominal 3.125 diameter bore
>
> TISCO overhaul kit contained new sleeves, pistons, rings etc.
>
> Ring boxes identified as RP-539383 SGL and HBR 060919
> Piston has two compression rings and the typical 2 piece and spacer
> style automotive style oil ring.
> Top ring cross-section measures .092 thick and .142 wide. It is a
> rectangular cross-section with no indication of a top-side. Ring is
> silver.
> Second ring is the same cross-section but has a chamfer (~.025
> measured parallel to the width direction) on the inside, lower edge. Ring
> is black.
>
> The top and second piston grooves measure .093 wide and .165 deep
> The piston diameter is 3.098-3.095 (a little difficult to measure
> with my vernier)
>
> The gaps I presently have are:
> Top .018-.019
> 2nd .020-.022
> Oil .028-.032
>
> New sleeves measure 3.126 diameter (bore)
>
> The compression ratio was 6.5:1 with flat top pistons. I am using
> the 9000' high altitude pistons which are aluminum instead of cast iron
> and have a large dome to increase the compression. The max rpm is ~2000,
> but some pulls we actually stall the engine....
>
> I am using this engine for tractor pulling and want to
> optimize where I can.
>
> What is your impression of my present gaps and would you suggest an
> oversize ring ?
>
> thanks,

Replies:

• Re: All I know about Piston Ring end Gaps JayWalt 15:35:55 03/22/07 (0)

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