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MG MGB Technical - what size rings

Just took the pistons out of my engine and every piston has a broken top ring. The engine appears to be a replacement item as it has MGOC stamped on just about every part. So I have no idea as to whether the bores are standard size or bigger.
The pistons are MGOC on have stamped on them 17078 and a number 3
any ideas what they could be.
I relise that I cold just measure them but then I'd have to buy a micrometer!
P I Barnes

Sorry, correction they say MOWOG 17078
P I Barnes

Actually, you can take the whole mess to your machine shop. They'll measure the cylinder bores as part of the diagnosis and planning. Then they should discuss with you the options at hand - including whether to replace or reuse the pistons. They should also be able to tell you what size rings (and bearings, etc.) to buy, and even discuss with you what types are available. (My machinist prefers to order them himself so he knows what quality of parts his customers use.)
Matt Kulka

Only problem there is that I don't have a machine shop! I live out in the sticks and a long way from a shop. Anyway the bores look fine. I just want to replace the rings.
P I Barnes

PI - about the only alternative would be to wrap a piece of small copper wire around a piston so that it overlaps on one side (and I'm talking SMALL, like one strand out of a twisted 16 gauge wire - stretch it slightly to get it perfectly straight), then cut both strands where they overlap with a knife. You then tape the cut piece to a metal ruler, and subtract the two readings to get the length, convert to decimals, divide by 3.1416, and you'll have a pretty good idea of the piston's diameter. Add about two or three thousandths, and you've got your bore. Not particularly elegant, but close enough to see if the bores have been reamed out.

The proper way is with a micrometer or caliper, but the wire trick expands the diameter by over three times (which is pi, 3.14159265 etc) and gives you a bit more accuracy, and also works well on items like crankshafts that you just can't measure with a ruler.

Now I own several micrometers, as well as digital, dial, and vernier calipers, but I still remember.... Cheers - - Alec

Alec Darnall

Every time I check the pistons of an engine I clean the top and they are stamped STD or +20 +30 or so on. That tells you that it is a standard or 10thou 20thou or so oversize piston and the bores should be the same.
Clive Skelhorn

No size marking usually indicates it's a standard size piston. All top rings broken? I would want to know why they broke before installing new rings on the old pistons. Are the pistons in good condition above the first ring, any signs of heavy detonation?There has to be a reason, rings don't break under normal operating conditions unless something is excessively worn or parts were not correctly fitted on initial assembly.

Clifton
Clifton Gordon

I'm not so sure that you can find out the bore size from the above but it is certainly worth a try. At the end of the day it is not a big deal because you will check the ring tip gaps. The pistons are stamped MOWOG which is a genuine British leyland part so there is a chace that the bore is original size unless of course your engine number starts with 48G which is a factory exghange unit, will be linered and could be any oversize. The number 3 is the bore size grade number and is intended for selective piston fitting to individual bores. Place your chosen size of new top ring at the very top of one of the bores and square it up from below with a piston. Measure the ring tip gap with feeler gauges which should be .003/.004" per inch diameter. If you guess the size wrongly you will soom find out this way !! Move the ring about half an inch down the bore and measure again. The difference divided by 3.14 is an approximate measure of bore wear in thousanths of an inch. If this is .005" then it's rebore time.

Now you say that every top ring is broken which interests me. Maybe the rings have already been replaced and were either incorrect or wrongly fitted. Replacement top rings should be stepped to provide a "ridge dodger" which will prevents them being puunded by the wear ridge or more likely they have been broken by either preignition or heavy running on. It is very important that your engine is not overadvanced and despite the fact we say a couple of puffs of running on is fairly normal on occassions serious and heavy running on on a regular basis is very damaging on an engine. This is a prime cause of broken rings.
Iain MacKintosh

If you want to be sure of the size, I'd buy this:

http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=37173

For $49.95--- not the best but it's good enough.
CDD Dewey

If the top ring is broken this can mean someone in the past has renewed the rings without honing the wear lip off the top of the bore.
I've never come across an oversize piston that didn't have the oversize stamped on the crown however it's very easy to measure the bore with an inside mic or a set of snap gauges and an outside mike or vernier.
Ron
R. Algie

Ron, its quite frequent that the lip isn't honed off. PIB doesn't have any measuring equipment that's why I suggested the method. I agree however that if the lip has been removed then it will be useless. I like you susoect that the bores might be standard size.
Iain MacKintosh

Thanks Guys. I think that I'll buy a Mic. I'm not sure how the top rings got broken. The car had been left standing outside for five years before I got my hands on it. On the day I took it away the PO had "got it started" for me without checking anything first. Could the rings have broken then?
Ian, you mentioined a "ridge dodger" top ring. Where do I get such a thing? I'm going to put a honing tool through the bores but to be safe a stepped top ring sounds like a good idea!
P I Barnes

To check for standard or oversize pistons you don't need a micrometer, it can be done with a dial caliper that you can buy for about $15 at discount places. You want to know if the piston size is standard or over-sized, oversizes are in .010" increments or greater. A dial cheap dial caliper with .001" accuracy is all you need. I used one for years to make measurements on racing go kart engines I built, my engines always passed tech and we frequently won races. FWIW,

Clifton
Clifton Gordon

PI, The rings and cylinders may have rusted while the engine was idle for five years. Starting the engine with taking measures to free the ring from the cylinders could very well be what caused the breakage. Since you have given the additional information it would be best to remove and disassemble the engine, you could have cylinder bores with enough wear or damage to need re-boring. It sounds like you still have the crankshaft in the engine. It difficult/or impossible to keep honing grit out of the lower engine parts.

Clifton
Clifton Gordon

PI Barnes-

I think the 'ridge dodger' that Ian spoke of is better known as a ridge reamer in North America. Rings frequently wear a step in the bore- if the new ring is runs into the step, it will break the top compression ring.

a ridge reamer is a relatively imprecise tool, but it will cut the ridge to the same diameter as the bore below it, so stepped rings are not required.
greg fast

OK, bought a Mic. What sise are the origonal pistons meant to be?
P I Barnes

PI - you make it hard when you don't say what engine you have, or even the year of your car, but here are the standard sizes of the bores, straight from the Moss catalog, and verified by Haynes:

948cc - - 2.478"
1098cc - - 2.543"
1275cc - - 2.780"
1500cc - - 2.900"

To get these in millimeters, multiply by 25.40

Best of luck - - Alec
Alec Darnall

Well, that's pretty stupid of me - I gave you the sizes for midgets - sorry about that - - Alec







.
Alec Darnall

And... for the MGB 1798cc engine it's (drum roll) 3.160" or 80.26mm

Hope this helps - - Alec
Alec Darnall

No, a ridge dodger is a top ring that has a very small step cut out of the ring face all the way round it's circumference. This is a common inclusion on ring sets certainly in the UK. The ring will likely be marked "Top" but in any event the top is the surface adjacent to the step. When fitted and the piston comes to the top of the bore the ring edge will therefore not come far enough up the bore to hit the step.
Iain MacKintosh

Thanks again for all your help. Engine should be up and running in no time!
P I Barnes

Clifton,


I Agree with your comment about the rusted in cylinder walls, able to break the top rings on all fours pots. Added to the lack of oil, that may explain the failure.
Renou

Clifton
Are micrometers expensive in the US? I bought a Moore and Wright set in a wooden box that does from 0" to 4" on Ebay for about the equivalent of $20 and a 0 to 1" for about $10. I've got a 12" vernier caliper but having originally been trained [a long time ago] in a machine shop with mics I prefer them.
By the way in the old days any half decent toolmaker could measure to less than 5 thou with a steel rule!! I wonder how many could do that nowadays.

PI If you buy a cheap bore hone for use in an electric drill it will bust the glaze in the bores before you install the new rings and it will also remove the lip so you don'
t need stepped rings.
Ron
R. Algie

Ron,

If you use a cheap hone in the cylinders for more than a very quick glaze bust it will go out of round - if you try taking out the ridge then it wil go inverse barrel shaped too!

More importantly, you will take away the only truly unworn part of the bore that the machine shop will use to rebore on the centreline!!
Chris at Octarine Services

Chris,
In an engine that has seen any amount of wear the bore already is out of round, all I'm suggesting is busting the glaze with a bore hone and removing the ridge at the top of the bore, two techniques have been done successfully by professionals and amateurs since Pontius was a pupil pilot.
Ron
R. Algie

This thread was discussed between 11/10/2005 and 15/10/2005

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