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MG MGB Technical - New Rings in Old Bores?

I recently bought a '77 Roadster, with good bodywork.

When I got it, the engine was running very rich at idle, uneven, hunting, and smelling rich- black plugs obviously. The PO had fitted K&N filters, but left the ACD needles, so I guess he had richened it up a lot at idle to avoid over leanness at speed. I have restored the original filters and housings, and reset the mixture.
I have put in new plugs, and set the points gap. The vacuum capsule is working, and the timing is correct, with it disconnected. The plug leads are not tracking, but the idle is still uneven- the engine rocks side to side.

So I did a compression check. Dry I have about 140psi in 1,2&3 but only 110 in #4. Adding oil brought #4 up to 200! (maybe I put in too much!). (I have checked the valve clearances too)

My conclusion is that #4 has a problem with bore and or rings. The engine has done 80,000 miles.

My plan is to take off the head and sump, and push the pistons out.

I read that .005" of wear is ok with new rings, so assuming the bores are within this, I intend to fit new rings.(and big end bearings)

Now for the advice please!

1) How much bore wear can I get away with? Is it .005" on the diameter? (I'm not burning a significant amount of oil BTW)
2) What do I have to do to the bores to remove the glaze? Can I just emery cloth them, use a flap wheel in the drill, or do I need a 3 legged stone "glaze buster" tool?
3) I hope I can get stepped rings, If not I will have to bevel the edge of the top ring. Are replacement rings stepped?
4) I assume I have the con rods with the bolts and 12 sided nuts- 18V847 engine. Do I need new nuts, or do they work again? (Do I need a special tool to undo them?)


Until I remove the pistons I don't know what the problem is, unless it is obvious when I take the head off. I may just have a stuck ring in #4.
My intention is to get the rest of the engines useful life- it should be good for 100,000 without a rebore surely?

If it needs a rebore, then it will have to be done, but I want to keep the engine in the car if possible, as the costs will spiral with all the extras like new camshaft & followers, clutch, timing chain and so on.

Cheers

Martin
M Layton

If you're expecting to do all that when you do pull the motor, why not live with it until you decide to do the whole job? The way I see it, you'd be wasting your money doing a half assed job now. If it's a stuck ring, maybe it will free itself with regular use and oil changes, or a penetrating soak if you intend to store for a while?

You can pull that piston if you want to, but I'm not convinced it would solve your uneven running. If you have a badly stuck or broken ring, you probably will have some bore damage, too. #4 may have worn more than the others from heat due to rusty debris around the rear of the waterjacket, it seems to collect there. Another reason to pull the engine and have the block boiled out.

Hehe, I think you raised your compression ratio with the oil..too much.

If it was running rich for a long time, I'd expect quite a bit of carbon in the combustion chambers, most of which may burn out on a long trip or after an "Italian tune up" (run it up until it coughs, keep it there for a while, repeat). And after it does, your engine may run more evenly. I've seen guys pour water in the carb while running, but I wouldn't recommend that. It _does_ remove carbon, chunks may fly out the exhaust.

Tom

All I intend is to whip the head off, and clean it up/ check & grind valves (Which I need for a short engine anyway)
Then drop the sump and pull the pistons. If the bearing shells are thin, change them.

Estimated cost

Head set = £10
Rings = £40
Sump gasket = £1.50
Oil pump gasket = 0.25

Total £51.75

Then an extra £20 for shells and £12 for the con rod nuts if I must change them.

A lot cheaper than a recon engine, and 20,000 more miles is 7 years worth, much longer with the weather we have now!


The work is easy enough, I just haven't done it for 20 years, and then you just put Cords rings in and screwed everything back together, with no question about reusing parts as everything had tab washers.


Martin
M Layton

Just replacing bearings and re ringing an engine used to be a common practice when I was growing uA lot of folks could do that and get more milage out of their car without the cost of a full rebuild.

Problem as I remember it was if you did one part of the engine, it normally put more stress on other parts that were not rebuild and frequently ended with another part failing.

Doing wht you planned would probably extend the life for now, but may be more costly later.

If you do replace the one ring, I woudl get a bore hone and checker the bore.
Bruce Cunha

I agree with Bruce. However I quibble with his "extra stress" causes later failures. This is/was a common statement, back when this was frequently done.

In fact the new parts probably result in higher oil pressure and less stress on the old. The real problem is that other systems in the engine are already near failure, and they would have failed in due course in any event. Pro rebuilders, refuse to do half a job, because the customer won't remember the cheap price for half a job, but instead will remember the failures.

On the other hand, I had a Triumph GT6 in which I installed a 2.5 liter 6 from the TR6. I bought the engine from a junk yard. It ran great, except it had a rod knock in journal 3. I pulled the pan, carefully cleaned the jounal, installed a new bearing shell. It measured too loose. Installed tapered shims (you can't find these any more) set it up with about .001" clearance and very carefully drove the car for about the first 500 miles. The car ran fine for many miles therafter. I sold the car a year or two later. About 3 years after that I encountered the buyer. He advised he had the trans go out once, the rear end twice, but the engine "ran like a truck and was bullet proof."

The exception doesn't necessarily prove the rule.

Barry
Barry Parkinson

Further thought - sorry to ramble on.

I once rebuilt a 350 chevy that had over 100k miles.

The bores were worn, but I didn't expect to keep it long so I removed the ridge, honed the cylinders and installed new stock sized pistons with soft iron rings.

It ran great as long as I kept it below 3500 rpm. If I got above 70 mph my oil consumption increased dramatically.

Loose con rod bearings will also increase oil consumption as more oil is thrown out from the loose bearings up into the cylinder area.

Barry

Barry Parkinson

From my experience with MGB engines, the cylinder bores wear more than in some other engines, but the crankshafts wear far less. I've not been into a late 70s engine, but late 60s and early 70s came out of the factory with chrome moly rings which were harder and wore the cylinder walls quicker.

At the mileage yours has, you can do the old fashioned overhaul, but like others have commented, it would be preferable to do it right and totally tear it down, tank the block and cylinder head and rebuild all the way. But I certainly understand your point of view and have done the same. One thing I would do after inspecting the pistons and bores is consider having the pistons knurled and honing the cylinders before reinstalling. A good shop should be able to do that for you.

Tom mentioned the "Italian overhaul". I never heard a name for it, but MG engines have a remarkable knack for tuning themselves with a good, hard drive. In another thread, someone mentioned using "Seafoam" to free a potentially rusted and siezed ring. I used it in 80 LE when I got it to clear the fuel system and such and it worked great. There may be other products out there like it, but adding this to a tank of gas and the "Italian tuneup" might just do the trick if a ring is stuck. If that's the case and the compression comes back even, you might get by with just a valve job.

By the way, since you mention doing the head, be sure to go back with a quality copper head gasket and check the head and have it planed if needed. Watch out for the cheap head gaskets. You'll just end up with a blown head gasket in a few thousand miles.
Rick Penland

One thing not mentioned is that often the bores are no longer truly cylindrical on higher mileage engines. Often there is a certain amount of ovality to them, depending on the metalurgy of the block and the stresses the engine has be subjected to during its life plus other factors. Simply trimming the ridge off and honing the cylinders may not be enough to "true it up" for new rings.

I did as Barry on a Datsun 510 ages ago but because its bores were incredibly true even tho high mileage (a testimony to the improved cast iron they used in the early 70s) another 80K miles were easily put on that engine without blowing smoke out the tailpipe. But that doesn't always happen, and accelerated wear can occur on blocks which don't have as good quality as the Datsun or BMW or Mercedes or Toyota. Our blocks may well fall into the softer metal category, so checking for ovality can give you a good idea whether your re-ringing will be successful or just patch work.
Bob Muenchausen

It's a good idea. You'll get another 40k miles out of it.

I did the valves, replaced the rings, the rod bearings, and the oil pump.

All this can be easily done with some helpfull advise from someone with the tools and know how. And all with the engine in place. For the head have someone do the whole thing and you do the rings etc. with engine in place.

Other than what was already stated, make sure you hone the cylinder walls, and as stated above make sure you remove the ridge in the cylinder wall before removing the pistons.
werner haussmann

I did it once. It was time for a new throw-out bearing and I'd used 'crome moly' rings at re-build. After 35000 miles they'd not seated and compression was low, with high oil consumption.

I did a valve job and popped the sump, pooped out the pistons and HEY! The bores were still just fine. The rings looked like new (unworn).

New rings with a stepped upper one, to avoid the minimal ridge, bolt it back together and,,,,,, success!

I got 11 more years of daily use out of it, including a coast to coast in 2002.

That engine's truley worn out now. 500,000 plus miles, crank @ .060 under and bores that are now very uneven in compression pressures.

I'm doing up an 18V for 'next'. It's crank is still within tolerance for 'new'.



R.S.C Caskie

Thanks for the replies.

I can neither remove the ridge nor hone the bores at home, (I understand honing to be machining the precise criss cross marks in that hold the oil, rather than scuffing up with emery cloth-that I can do!), so if the block has to come out I might as well have a rebore done.

The best plan is wait until winter, then pull the engine to get it inside in the warm and get a proper job done.

I suppose I can just put a few ccs of diesel in #4 and let it sit overnight to try and free the rings off if stuck- then I won't have to do anything!
M Layton

At the risk of swaying you from the more righteous path, you could add £10 to your budget for a honing tool from any hire shop and Plan A might still work.
Steve Postins

You can hone the cylinders, and remove the ridge at home with the engine in place. That is what makes all this worth while. Check out some local car clubs. You might find someone can loan you the tools and has some experience to guide you. You only need them for a short time.
werner haussmann

Martin, I was discouraged from re ringing my B engine at 115,000 miles by the sages on this forum. I went ahead antway as I had no means of getting the engine out. I honed with one of those 3 stone gadgets, but wasn't too happy with the result, so I resorted to rough wet and dry paper. I got some new AE rings without the step, but slight chamfer on top ring. When I checked the ring gaps they were towards the bottom of the tolorance. The result has been brilliant, oil consumption has stopped (1/2 pint in 2000 miles)I should add that when I got the pistons out originally I had broken top rings on the first 2 cylinders, although the compression readings wern't that bad (cyl 3 was the worst @ 140 psi). I fitted a Burgess econotune head with proper stem seals too. performance and economy better as well. Also fitted new BE shells, oil pump and camshaft. Just come back from a 1200 mile french odessey with no probs
Paul Hollingworth 1

I copied a hone I noticed some where. Take a smooth steel rod about 18 inches long. Mine current one is 1/4 inch diameter. You can use whatever your drill can accept.

Drill two 1/8 holes about 1 inch and 3 inches from one end. Get two pieces of rubber or neoprene gasket material. 4 inches x?? (you can increase the size to suit what you are honing by making longer pieces of rubber) drill holes to match those in the rod. I made two small metal stips 3/8 by 3inch again with holes to match. Cut emery cloth to length to fold over and completely cover the rubber with some overlap. Put holes in the emery to match everything else. Use the shortest machine screws you can to hold it together. One emery cloth covered rubber flap on each side of the rod.

Goes together..... metal strip .. emery/rubber...rod..emery/rubber...metal strip.

I dip the whole thing in a pot or varsol, kerosene or diesel fuel. Fold the rubber/emery to the left drop in the cylinder and spin with an electric drill at slow speed moving up and down the cylinder.

The varsol etc lubricates and keeps the emery cloth from clogging up. Replace the emery cloth when worn.

Yes it is kind of hokey but it works. It has been used on everything from lawn mowers to a 350 Chev. Not everyone has a machine shop or engine rebuilder near by.
Kelvin Hawkins

Martin

Unsure if it is the same where you are, but in the US, many of the parts shops have tools you can rent. Things like ridge reamers and hones.
Bruce Cunha

Everone is assuming that this is a ring/piston problem, but if you filled the dish in the piston with oil, it would raise the compression significantly. This is a common failure of folk doing the wet test - the oil has to be on the cylinder walls/rings, AND not in the dish. You may well have a bit of a burned valve in #4, accounting for the low comp, and within limits a gob of oil splashing up there will help to seal it, just as it seals up worn rings. You can get an idea of where the loss is by pressurizing the cylinder when on the compression stroke with compressed air, and listen for the leak - either in the crankcase, the exhaust, or the inlet manifold. Be sure you have the car in gear, handbrake ON, and wheels blocked when testing. The pressure will try to turn the engine, eating stray hands and the like, or running over you if in gear.
If you do re-ring it, by all means borrow a ridge reamer if you can't get the stepped rings. If the ring hits that step, you WILL need pistons, since it will break the ring and possibly the land on the piston.
FRM
FR Millmore

If it is a piston problem (FRM's post) - the rings may only be part of the problem - particularly if the 80K is actually 180K on the speedo - the piston grooves also wear and it might pay you to fit a new piston with the new rings.

In any case, I would start by soaking the cylinder in redex for 48 hours to see if that disolves enough tar for the cylibder to work better.

FWIW - Roger
RMW

Thanks for all the input.

I think the best plan is to run it for the few weeks left of the summer, then take the head off for a decoke and investigation.
Martin Layton

On my trips to the States i've seen a honing tool in Sears for $21.99. I'm thinking of using it to hone a V8 block. What is the best method of operation?
ph barton

Get rid of the step at the top of the bore. Oil the stones, insert into bore, run the drill motor kinda slow while pulling and pushing bottom to top. Don't hit anything at the bottom. If I remember right, the crosshatch pattern should be about 30 degrees this way ><.
Tom

When honing the cylinder in place, place a rag at the bottom of the cylinder to catch the oil and grit. Don't let any get into bottom end of the engine. Clean really well after the hone with an oily rag.
werner haussmann

This thread was discussed between 24/08/2006 and 04/09/2006

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