BR ring end gap

[JBiff76]

We are installing a set of piston rings in the 1936 BR. What is the tolerance of the ring end gap? I checked the archives and couldn't find any info. Thanks, Jon B.
 

[P Browning]

Here is an excerpt from the JD-H Restoration Guide, 2nd Ed. which should give you some idea. Be advised that excessive ring gap is an excellent sign of wear, and that the wear can be a combination of wear to rings and to the bore itself. That's where micrometer readings come into play -- to determine what shape the bore is in. (PatB)
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PISTON RING END GAP – End gap is important because the ring needs room to expand. Without sufficient expansion clearance, scoring of the cylinder wall or a broken ring may result! For proper break-ins, end gap should be a minimum of 0.003 inch per inch of bore. Engines that will work hard and hot immediately after overhaul need 0.004 inch minimum of end gap per inch of bore. Service manuals call out a range from 0.015 to 0.025 end gap for both compression and oil rings. As a matter of interest, the range of 0.012 to 0.016 is called out in DIR214 for the 1939 “H”. Equally interesting is the fact that this clearance is not referenced in the subsequent IPLs, (DIR248 & DIR276) as the wording was changed to state, “New piston rings are provided with proper end gap. No fitting in cylinder is required.” Per FSB 95-S (3-15-39), ring gap for all rings is not to be less than 0.015. This measurement is made by first inserting a cleaned piston into the cylinder to about the center of the stroke. Then, place the ring against the head of the piston to square the ring, and measure the gap with feeler gauges. Common sense dictates that regardless of the source of your new rings, John Deere or an aftermarket supplier, you should always check the ring end gap just to be sure. If the ring end gap measures less than the prescribed 0.015 inch, place the ring in a cushioned vise and use a fine file to open this gap. Be careful: don’t take away too much, and work “square” to maintain square ends. ++++
Restoration Site, JD H

 

[F-I-T]

Do a web search for Hastings, and you'll find their tech info that advises a minimum of .0035" of end gap per inch of bore. That's what they have printed on their box flap for ring sets for over 70 years (I know because I have some Hasting boxes that are at least that old!). That has always worked well for me.

Use a new ring and a premium shim stock gauge to check top, middle, bottom of the bore to determine just what the out-of-round and condition is. It will more often than not surprise you. I bought a decent dial bore gauge some time ago (not all that expensive) and it has been very revealing as to the poor condition of many, many blocks. These engines are getting older every day, and most people don't measure the bores for fear that truth will point them to a re-bore and new pistons. Also check the more often than not neglected wrist pin bushings. Nothing will make an old engine noisier than slappy wrist pins.
Hastings tech tips for ring gap.

 

[Kris A NC]

Looking at your information and the information above from Hastings I am a bit confused. I understand you place the new piston ring at the bottom most part of the cylinder wall to make sure it is with in minimum and maximum spec. When you move the ring up to measure the middle and top of the bore how do you determine if there is too much wear there? I was looking at the hastings site trying to determine how they did their math, but I couldn't. Thanks for you help on this. I too am about to rering my '36B.

Kris A NC
 

[F-I-T]

Start by using the same ring. If the end gap grows, then the bore is increasing. Basically at a factor of 3:1 (Pi*D = circumference, and Pi is just a little over 3). So, if you move from the bottom of the bore and the gap is .020", and you move up to the mid-bore and it grows to .026", the middle is larger than the bottom by about .002". If the top is .032", then top to bottom, the taper .004". If you have a spec for .010" total out of round, taper, and wear, you have already used up .004" of it, so if you are worn an additional .007", it's out. I made those numbers up, but you get the idea.

Now, if you have a bore gauge handy, you can even "calibrate" that ring so that you know what its end gap is at a given bore size, and make a rough computation of the bore measurement, but all bets are off when you are dealing with a significantly out-of-round condition as the ring tries to be a circle, and it may not truly fill the bore cross section.

Putting rings into a bore that is true from top to bottom is what you desire, and it will be reflected in a consistent ring end gap from top to bottom, but it won't tell you if the whole bore is over-sized and requiring re-boring. Three things drive the bore out of spec: diameter, out-of-round, and taper (top to bottom parallelism). If the bore is tapered excessively AND it is out-of-round, AND it is worn just a bit, these can all stack up and combine to form an out of spec bore, even when all three are at the top limit of being in service limits.

Whether to bore or not to bore is always the subject of great debate because people will argue factors such as how hard you intend to run the engine, are parts available and the like, but if you want to put one together and have it ready to pull like it once did, be aware of how important attention to cylinders bores can be in overall engine performance and longevity.

Hope this helps.

Frank