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desco, GeoffLLLL, HayMike, kevin, KevRasen, Stuart Kirk, The Bonneville Shop
Total Likes: 10
Original Post (Thread Starter)
#891684 09/24/2022 10:04 PM
by Al Eckstadt
Al Eckstadt
Preparing to mount the cylinders. Checking the ring gap - it is well over 0.014" quoted in "General Data". This is a '72 T120RV engine with barrels bored 40 thousandths over. Pistons and rings purchased from MAP - EMGO pistons. The rings are Hastings with an Emgo label (+40), and a MAP label R11050/H040.
Setting the rings in the bore and squaring them with the piston, there is a gap of 0.024" plus or minus 1 thousandth. I have read that the ring gap should be 0.014" , but have also seen 0.017" as a maximimum gap - either way these are quite a bit over.
Is this a problem or will it be OK?
Liked Replies
#891686 Sep 24th a 10:42 PM
by Stein Roger
Stein Roger
Whenever this question pops up I post this link. Read this and make up your own mind...

2 members like this
#891863 Sep 27th a 09:54 AM
by triton thrasher
triton thrasher
I don’t have equipment to examine what is happening in the space between two piston rings in a running engine, nor time left on this Earth to thoroughly analyse the results of different hones and running-in procedures.

What do I do then? I read about others’ research and experience and I make a judgment about their credibility.

Using coarse honing, big second ring gaps, “dry assembly,” cheap running-in oil and charging up the road with a handful of throttle the instant the newly built engine fired, has produced better compression, lower oil consumption and better running than just following the old workshop manual.

Which parts of the whole procedure really helped? I’ve no way of knowing.
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#891687 Sep 24th a 10:43 PM
by triton thrasher
triton thrasher
0.014” maximum gap is best described as wrong.

Late 1960s Service Bulletins specified up to 0.020”, on new bikes.
[Linked Image from]
A top ring gap of 0.024” or more doesn’t cause my 650 any problems and I can’t see why yours would be any different.

There’s a modern(ish) recommendation that the gap on the second ring should be 25-30% bigger than the top ring gap.

What is the clearance between the piston skirt and the bore?
1 member likes this
#891725 Sep 25th a 02:22 PM
by Tridentman
+1 with the reference that Stein Roger gives.
Just to add to the authenticity----I worked at the AE R&D Cemnter at the time these tests were carried out.
In fact I shared an office with the engineer in charge of the tests.
The reference quoted by Stein Roger is accurate except in one respect.
The results were so amazing to us all that the test was continued with a ring gap of 1/8"--- and no discernible increase in blow by was measured.
So to me the lesson is--- make sure you have at least the minimum ring gap-- but dont be concerned if the gap is greater than the maximum quoted in the manual.
1 member likes this
#891724 Sep 25th a 01:56 PM
by reverb
...many use 0.015 or so in the first ring then more in the second then the same as the first with the oil ring.
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#891785 Sep 26th a 04:29 AM
by NickL
Combustion pressure gets behind the ring and pushes out.
That's why it's important to fit rings the correct way up as most are
shaped on their ID to form a pocket.
1 member likes this
#891860 Sep 27th a 07:10 AM
by Stein Roger
Stein Roger
Originally Posted by GeoffLLLL
My contention is while it seems to have been proven that larger ring gaps are fine is there a real need to use larger gaps just because it can be done?
Absolutely not, but it has become accepted and general practice to use a slightly larger 2nd ring gap. Don't take my word for it, but it's what the motor industry does since maybe the 80s.
Scroll down a bit until you find Barry Rabotnick's post.

1 member likes this
#891918 Sep 28th a 04:53 AM
by linker48x
Originally Posted by desco

Lots of really interesting info here, from knowledgeable engine builders about ring gap, big gaps on second rings, mostly at the theoretical level, including the post on the Speedwake engine builders site from Barry Rabotnick, a guy who obviously knows his stuff. One of the simpler, more plain spoken and clear statements about this second ring gap question showed up much earlier in this thread in the Franz and Grubb blog cited above, in their Hastings ring section:

“We recommend that the second ring gap be larger than the top to allow for combustion pressure to equalize between rings, and to help prevent ring flutter. We have been gapping rings like this for years and it seems to work well with the Triumph twins.”

Right in line with Rabotnick, at the cutting edge as it were. You might think these Franz and Grubb guys were really knowledgeable or something LOL
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