The instructions say .003 to .004 per inch of cylinder diameter. Calculates out to .011” to .015”. What end gap should I use? Anyone used this combination?
Thanks,
Jsaum

For stock piston rings in all the 4 cylinder engines it calls for .014 to .020. What do the rings measure out at out of the box?

They’re tight .003.

That's too tight. I'd shop around for a inexpensive ring grinder tool. Using a file by hand is hard to do and you might break a ring.

I bought a ring grinder to do it right.

Check the internet, I was surprised to read the KB (Keith Black) pistons call for increased ring clearance . I don't remember the fraction But I gapped the top ring at ..024. The KB top ring is closer to the top and gets hotter ergo more gap.

I’ve looked and found several different call outs. The Haynes manual is calling out .0138 to .0216 for both the top and lower compression rings for a 2.0. This is a 1911 engine build with 96mm AA pistons and cylinders using Grant rings. If I go to the top call out in the Grant instructions at .016 I should be safe. I was hoping someone might have used the same combination and could give me the gap they used to confirm I have it right.

As far as I know, they are all cast iron rings and should have similar specs, BUT make sure you get it right or the rings can break. Have you called the factory? Grant

I have the Grant factory instructions which call for a range of .003 to .004 per inch of diameter of the piston. That’s between .011 to .015. I’ll give AA performance a call Monday and see if they can give a recommendation.

Here’s a picture of three different pistons, from left to right, KB, AA performance and the stock dished 1.8. I see how the KB top ring is closer to the piston top so the .024” makes sense for KB pistons. The AA ring distance is the same as stock so the question of which gap should I use for the 96mm pistons for a 1911 build is the 1.7 to 1.8 or 2.0 from the Haynes manual.

Ok, after a ton of google searches and watching a bunch of YouTube videos I think I have zeroed in on the numbers. .017” top ring and .015 second ring.

Personally, I would favor over .015 than less than .015, but less than .020

Keep us posted with your results

Thanks, 930cabman for the advise.
I did call AA performance this morning and they confirmed they recommend.0045” for the top ring and .0004” for the second ring times the diameter of the piston. So .017” and .015” ring end gap. If I was to go over this slightly how much, .0005 or .001? I was hoping some of our members with engine building experience might share what they use but also understand some reluctance to do so. This is my first air cooled engine build, I’m using this as a learning experience and am hoping for the best but planning for some road bumps while trying to do my do diligence.

Jsaum

Give Mark Henry some time and he should chime in.

He's one of the top T4 engine builders and must be busy with the spring rush from customers who want to drive this summer.

OK, it would be great if he will. I follow a lot of his posts and know he is highly respected here.

Thanks,
Jsaum

PM him.

Not as critical as you would think, too small could be very bad but a little to large... not so much.

Interesting ring gap article. ( much about staggering, but also about gaps)

------------------------------------------------------------------------

Ring Gaps vs Knowledge Gaps
During school summer holidays there occurs a period known as the "silly season" when breaking
news is slack and newspapers, radio and TV issue forth all sorts of drivel to justify their existence.
In our industry I sometimes think that we use ring gaps to fulfil a similar purpose – and we do it all
year round. There are more old wives tales about ring gaps than any other facet of our job, so lets
spend a while and bin a few of them.
Frequently I hear in court that when an engine is dismantled it is discovered that the ring gaps were
not staggered when they were installed. Frequently I read workshop manuals that go into great
detail on the necessity to stagger ring gaps. Frequently these manuals specify ring gap limits – which
is yet another myth we can bin before we are finished.
In days gone by, some compression rings used to have a ‘threepenny bit’ expander behind them.
These braced against the base of the ring groove and forced the ring onto the cylinder bore but
these are very much the technology of yesteryear and, as such, an exception to what I am about to
say.
Piston rings are free agents and can rotate or not rotate as they see fit. They are not in touch with
the base of the groove and neither are they trapped between the upper and lower faces of the
groove. The rings are entirely free to rotate – except where a stop peg is fitted – so what's the point
in staggering the gaps on installation?
We used to work on a minimum of 0.020" back clearance on radius or, to put it another way, the
inside diameter of the ring when installed in the cylinder must be at least 0.040" bigger than the
groove root diameter. Minimum side clearance was generally held to be 0.0015" and if you could fit
a 0.006" feeler gauge in the groove along with the new ring then the groove was "goosed" so the
piston was replaced.
The piston ring was manufactured with a tangential load – the force with which the ring presses
against the cylinder wall – but apart from that it is completely uncontrolled. There is no way, under
these circumstances, that you could prevent the ring from turning so, to repeat the question, why
stagger the ring gaps on installation?
Staggering ring gaps when installing pistons is every bit as daft as having four tyres fitted and placing
all the valves at "twelve o'clock". One trip to Tesco and they end up all over the place.
Equally daft are those who expect ring gaps to stay staggered when the engine is in operation.
When there is clearly nothing to prevent the ring from rotating, why should the gaps stay staggered?
More than one county court judge has fallen for the hocus pocus that because the gaps were in line
when the engine was dismantled, they must have been in line when installed. Absolute poppycock!
Yet the inclusion of such rubbish in workshop manuals does admittedly give it an air of authority.
When you think about it, you don't need me to tell you – but I will anyhow – that rings do rotate in
operation. Every now and again the gaps do line up – and once lined up there is a tendency for them
to stay lined up at least until the vehicle hits the next pothole in the road when one or other will
rotate and break the line. Staggering ring gaps when installing rings is a myth that we can bin
forthwith.
Even worse is the preoccupation with the size of the ring gap. Yes, there is a minimum but this
varies considerably depending on the material used. Normally 0.003"/0.004" per inch of bore size is
given but where, for example, low expansion SG (spheroidal graphite) iron is used, it can be
considerably less.
It’s no problem if the gap is too small because it can, in the final analysis, be filed bigger but what do
you do if it's too big? You cannot stick a bit extra in there!
Well, the answer to that was that you melted them down and started afresh – until AE research
asked the question, "What is too big?" and set out to quantify that. The results were interesting –
very interesting – and what you are about to read was kept quiet because it bestowed an enormous
commercial advantage on AE. This is probably the first time the information has been published
although the research was undertaken in the late 1970's – almost 25 years ago.
A Ford Kent engine was stripped and fitted with compression rings which had end gaps of 0.015"
when fitted in the bores. The engine was wired up with the usual telemetry to measure blow-by and
oil consumption and then run in one of the test cells. After making due note of the blow-by and oil
consumption, the engine was stripped and fitted with new compression rings with gaps of 0.025"
and the test cycle repeated.
These rings were subsequently replaced by ones having end gaps of 0.035" and the test cycle
repeated again. It had been planned to stop at 0.035" gaps ----------> but the results were so
interesting that it was agreed to proceed to 0.045” and then not to 0.0055” but to 0.0625” – 1/16"!
Whoever heard of rings with 1/16” gaps – a ridiculous figure – but the interesting thing was that the
increase in blow-by and oil consumption at 0.0625” was only marginally above the figures obtained
with 0.015” gaps.
Practical tests established that the gap was not the villain of the peace. To all practical purposes the
size of the gap didn’t matter. It is important to stress at this point that we were dealing with
compression rings that were brand new when fitted to the test engine.
The gap was specially manufactured for the tests. So how come all oil burners and heavy breathers
have ring gaps you can back a bus through? Well, the tangential load that the ring exerts onto the
cylinder wall is a direct function of its radial thickness.
As the periphery wears in contact with the bore, the radial thickness obviously decreases, as does
the tangential load. Peripheral wear means a smaller ring o/d and this manifests itself as an
increase in the ring gap.
So, whilst all oil burners and heavy breathers have big ring gaps, it's not the gap but the reduced
tangential load that is detrimental to the performance of the engine. The ring gap is a complete red
herring.
Imagine four top compression rings all with 1/16” gaps. The total gap for all four would be 1/4”.
Now imagine the seal provided in an 80 mm diameter bore. Pi x Diameter = Circumference, so we
have 3.14” x 3.15” = 9.891”. Multiply that by four cylinders and we have 39.564” – over a yard of
contact seal between piston rings and bore. Now visualise the many litres of blow-by and consider
whether all the gas is squeezing through 1/4” of total gaps or passing through 39½” of reduced
pressure contact seal!
But even this ignores one important facet of the argument because there is not just the one
compression ring on a piston – there are usually at least two and that is because rings work as a
team to form a labyrinth seal.
For gaps to be the villain of the peace, the gas would have to find the gap in the top compression
ring and pass through. It would then have to circulate to find the gap in the second ring and pass
through that and so on. Now this may be possible if the power stroke lasts for 10 minutes but it
doesn't, does it?
At 3,000 rpm the power stroke duration is a mere 1/100second. Quite simply, the power stroke
does not last long enough for the combustion gas to find its way around the maze – or labyrinth seal,
so the villain of the peace has to be the reduced tangential load of the ring on the bore caused by
peripheral wear or reduced radial thickness of the ring.
This was our hypothesis based on the results obtained in the engine test cell but it took a very clever
American to prove it. This genius invented telemetry that measured gas pressure between the
piston rings in a working engine.
Use of his brainchild revealed that some gas did get through the top ring gap sufficient to generate a
hell of a pressure between the top and second rings – so it clearly was not finding the gap in the
second ring. The labyrinth was working well. Caterpillar and IHC must have thought it was working
too well because they increased some second ring gaps to 0.050” and 0.070” thought to be
beneficial.
Come to think of it, the exception proves the rule – as usual. Two stroke engines would not need
stop pegs to prevent the end gap from crossing a port if the ring didn't rotate. The people who
allege that ring gaps were not staggered when installed just because they are in line when the
engine is dismantled don't need stop pegs.
They could simply position the end gap away from the port and expect it to stay there – and we all
know that this would get them into more hot water than a Yorkshireman’s tea bag! So why do
they always blurt out their rubbish on the importance of staggering gaps?
The very presence of a stop peg also proves my point about the size of ring gaps. Where a peg is
fitted, the end gap has got to be 1/8” to accommodate the peg. There would be one hell of a
draught through that if the 0.015”/0.018” boys were correct.
This knowledge was commercial dynamite because, instead of the '0.015”/0.018” spec., it meant
that new rings with gaps over 0.018” could be used without any detriment to the engine's
performance. The gap was only detrimental when it was the result of peripheral wear. Customer
acceptance was the only problem.
To re-educate the customer would let the cat out of the bag, thereby losing the distinct commercial
advantage. It was decided, therefore, to accept rings with gaps of up to 0.030” in a nominal bore but
even then there were arguments. 0.030" in a nominal bore is 0.045” in a bore worn by 0.005” – and
its normally engines with this sort of wear that get new rings.
The gaps on the new rings were bigger than the gaps on the ones being replaced which led to
several heated conversations – and how the hell do you pacify the customer and keep the cat in the
bag?
It made diagnostics a lot harder too because when all rings set off in the 0.015”/0.018” area gaps of
.040” meant something but now, when you don't know what they set off at, what they measure is
meaningless.
Well, now you know. All rings are free agents to rotate as they like, making staggering of gaps on
installation a joke and ring gaps are not a problem provided that the gap is not the manifestation of
reduced ring radial thickness caused by peripheral wear.
In the light of the logic expressed in this article we are offering an expensive prize to anyone who
gives us a good reason for staggering ring gaps on installation – a free burial at sea.
Author:
M H Booth F.I.Diag.E

@Olympic: thanks for sharing. The engineering behind much of this stuff is barely understood, but the numbers don't lie.

Thanks again

+1 with the knowledge gaps

.017" is good, if you're over a hair I wouldn't worry about it.
You don't really have to stagger the gaps, as the rings spin in the grooves, but old habits die hard so I still do.
On the oil scraper do have the gaps 180 from each other and the wavy ring 90 degrees from the gaps. These rings do have bigger gaps right out of the box, don't worry about it. With grant rings care must be taken on the wavy ring that you don't overlap the ends, you must see both painted colours on the ends once installed.

After grinding lightly deburr with a jewelers file, but only enough to remove any burrs.

I don't know if running .015" on the 2nd ring helps much, but I like to keep gaps as small as possible. Nickies you can run much smaller gaps.

Thanks Mark for the input and info. I appreciate the help.
Jsaum