Hints and Tips - Piston Ring Breakage

From: Maggie Shapland
Sent: 13 August 1999
To: british-cars-pre-war
Subject: piston rings

Any ideas what causes piston rings to break on a regular basis (apart from a heavy foot)? or how I can stop them breaking. Cant remember how many were replaced in 1991 on the 1925 Talbot but 2 oil control rings broke on no 1 cylinder (one in lots of little pieces) in 1997 and now an unknown number on no 2 cylinder. I only replaced the broken ones in 1997 since they appeared fine on the others. This time I have ordered a set and will replace all regardless. The car is used regularly so the pistons should not have a chance to seize up in the bores, and it is different bores this time to last. It is also years ago that a rebore was done so the pistons are not too tight

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Maggie Shapland
Web page: http://www.cse.bris.ac.uk/~ccmjs/
1925 Lanchester 21, 1925 Talbot 10/23, 1929 Peugeot 190S, 1986 Moss Monaco


From: Lawrence J Alexander
Sent: 13 August 1999
To: Maggie Shapland
Subject: Re: piston rings

Maggie....

The most usual causes, in our experience, are.......

Insufficient ring gap (should be at least 2 thou per inch of bore diameter).
Badly worn ring grooves, allowing rings to rock up and down and twist.
Frequent engine overheating.
Detonation (or pinking, as I believe you call it).
Badly worn, scored or tapered bores.

Lawrie Alexander
British Sportscar Center


From: TA TERRY
Sent: 14 August 1999
To: Maggie Shapland
Subject: Re: piston rings

In a message dated 8/13/99 Maggie Shapland writes:

<< Any ideas what causes piston rings to break on a regular basis >>

Maggie, when I drove a MG TD back in the 50's, it would break its rings once a year because I had a collapsed piston....the PO had overheated it....so once a year just before I went off to University, I'd replace the rings on no. 3 piston. Didn't have enough money to buy a new piston!!

Terry


From: Maggie Shapland
Sent: 16 August 1999
To: british-cars-pre-war
Subject: Re: piston rings and brake springs

Thanks for the many replies about piston rings, not all were sent to the newsgroup so I have compiled a summary for others who I am sure will be interested. I have now worked through the history and am sure of the reason now and also will not fall into the same trap again:

New pistons and rings in 1992

Overheating problems 1995 so radiator recored- this obviously started the rot

Number 1 piston rings replaced in 1997

Number 2 rings fail in 1999.

My mistake was not to replace all rings in 1997 since visually they looked OK, I shall certainly check the gaps on my new rings when they arrive and I shall replace all. I will also order another set of rings so that that next time they fail (well I have to be a realist!) I dont have to wait for a month.

I also have got my brake springs on having surveyed various tools and in the end used a clamp and a very big screwdriver with both springs already in situ on the shoe (my shoes had holes in to put the springs through and I found that tools tended to pull the spring away from the hole since the u-shape nature of the spring end meant the spring had to be at right angles to get it in the hole which was impossible for me). Halfords brake tool (about 9) is for brake shoes which have a stud on the shoe for the spring to go round. Eastwoods sell long brake pliers for 12 I also bought a couple of vintage brake tools at an autojumble very cheaply which in theory should have worked- they were in the shape of a long oval with an extension at each side of the oval- one of which had a hook on the end, the other had a thread and a handle so one could pull the spring.

The Peugeot is coming on fine now- I cleaned all the clutch levers and springs etc while watching the Hungarian Grand Prix. The engine is now sitting in the chassis- so small I didnt need a crane especially without the flywheel on. When I get the rest of the clutch back from being relined the car will be on 4 wheels instead on 2+milkcrates. Current problem is that some idiot in the past cut the end off the crankshaft so there is no room for the pulley so I am getting someone to make an extension so that I can have both a pulley and a pin for the starting handle dog. I dont have a starting handle either so at least it can be made to fit! The gear box is in the back axle so I am leaving well alone since it seems to work. (I have a fixed axle). So who knows- it may be running before the millenium. I have no body but that is a different problem.

*****
Robert A. Johnson wrote:

Some years ago I began doing the following when doing rebuilds and I've not had a ring failure since. The basic concept is that I don't rely on the manufacturers to always provide perfection.

1. I check each individual ring side clearance on each piston groove where they will be assembled. I have encountered more than one set of pistons that had to be sent back due to excessive groove width. In one case I opted to have a custom set of thicker rings made which worked out fine.

2. I hand install each ring in the bore where it will go and carefully measure the end gap on each ring. More than half of the time it is necessary to do some filing on the ring ends to meet the manufacturers specs. When no specs are available I have used a Rule of Thumb of .003" gap per inch of bore diameter.

3. When I assemble the rings to the pistons I make sure that any rings that have a different top and bottom or a taper are installed right side up. I space the ring gaps at 120 degrees to each other making sure that none of the gaps are lined up over the gudgeon pin ends. I dip the whole piston assembly in a bucket of oil before inserting into the bore to insure free movement to seat in during the first few minutes of run-in.

But then, maybe you do all this already and the problems you have experienced are due to other causes.

Cheers.

Bob Johnson
Walnut Creek, California

webmaster of Early MG Classifieds located at http://consultantbob.com/earlymgs/

*****

Ralph Steinberg wrote:

Besides a heavey foot too much piston to bore clearence can cause rings to break {piston slap} or you can have worn small end bushings on the rods or the ring gland clearences are wrong and you are not getting the proper movement of the rings on the pistons. You really have to check the thickness of the ring to the thickness of the ring gland on old cars. Unless you bought your pistons with rings as a matched set, and even then, I have found the rings to be to thick for the piston. I also asume you checked the ring end clearence in the bore before you assembeled the rings to the pistons?

Since you sent the piston to the ring manufacturer you should have gotten the right thickness rings back. For an older engine with cast pistons about .005-.007 is a good ring gap. Ring clearence in he grove should be around .001-.003 {we use these settings on pre-war jags} Remeber to check the ring gap half-way down the bore. If all you clearences are good and the bore and pistons are not scuffed the next thought would be hot spots. If the cyl. bores get to hot the rings will start to drag and come right apart. I am not sure I really know your engine so I can not say how the water flows around your bores. My MGA twin cam uses syphon transfer to get water from the head to the block and back. So overheating a piston even though the engine never shows hot is always a concern. Also check the conditione of the piston ring glands very carefully. You are looking for nicks, or slight compression of the grove that will keep the ring from moving properly. It does not take much. Did you use a ring tool to clean the groves when you replaced the rings? You also may think about a different ring supplier. Many makers of parts for old cars really do not believe you drive them or drive them hard. To many, including parts manufacturers, 500 miles a year is a lot. We were running a 38 SS100 around 5000 to 6000 miles a year in rallies for around seven years in a row. We still race a 39 SS quite hard every year and have had a lot of experence with parts that would not hold up but were fine for trailer queens.

*****

Austin Works wrote: This often happens when the rings are too loose in the grooves causing the ring to chatter or flutter up and down. The brittle ring material won't take much flexing at those high speeds. The answer (if the grooves are too wide and you want to keep the pistons) is to recut the faulty grooves and use wider rings.

Doug Dexter
Austin Works East

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Maggie Shapland
Web page: http://www.cse.bris.ac.uk/~ccmjs/
1925 Lanchester 21, 1925 Talbot 10/23, 1929 Peugeot 190S, 1986 Moss Monaco


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