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MG Midget and Sprite Technical - Engine refurb thoughts

Gentlemen, and friends, for your entertainment and advice:
I have for well over a year been intending to check out my engine to decide how far to go with a light refresh to see if I can reduce a blue smoke problem. The blue smoke is continuous (not an overrun problem) and seems worst when pulling away in slow moving town traffic. Bad enough to cause some embarrassment and apologies by me to nearby pedestrians.

Engine was last fully rebuilt with a +0.03 rebore, new pistons etc about 80,000 miles ago, so it has done OK. I usually do a compression check every 3 to 6 months just to keep an eye on condition. For a long time I was getting a consistent 210 psi across all 4. In the last couple of years of use this has dropped to around 170 to 180psi so not bad but the lower numbers on the 2 middle cylinders. It has a skimmed MG metro head, nominally giving 10:1 compression ratio, so quite high for standard unleaded. 276 cam; MG metro electronic ignition; single HIF44; Titan manifold and LCB exhaust.

So this morning I finally got round to taking the head off for a look-see. Bores actually don't look too bad. There is no discernible wear lip at the top but there are some fine vertical scratches visible on all cylinders. Too fine to feel with a finger nail, other than one on the rear face of cyl 4 which is more pronounced.

Piston tops have soft, oily carbon deposits, but its not even and has flaked off in places. Plus are sooty and a bit oily, but this isn't much of an indication as I have been starting and running the engine up to warm every couple of months for the last year. Its 12 months since it was regularly used on the road.

I think I know what I want to do, but expert opinion is always welcome! What would your diagnosis and remedy be for this engine?

Here is a photo of the head just as it came off the car:

Guy W

And here is one of the lower half, again uncleaned other than to wipe oil and water off the surface of the block.

Guy W

Here's another of the block & #4 piston top. You can just see the vertical scratch near the top of the bore.

One other thing, with pistons at tdc I can get lateral movement of the piston just by pressing 2 fingers onto the piston and pushing quite lightly from side to side. Is this normal, or would that suggest tired rings?

Guy W

Did you mean + 30 thou rebore?
I'd go + 60 and use a different rebore shop from last time to do the machining and honing to suit the new pistons.
David Smith

80k miles on a rebore is pretty respectable I'd think. Especially given the compression figures you recorded pretty consistently until recently.
But David has a point - maybe time to visit the boring bar again.
Greybeard

Yes, I am pretty happy with the 80K, its just what I do next.

David, can you elucidate?
The pistons are marked +.03. Is that not 3 Hundredths =30 Thou?
And why do you suggest going to a different machine shop? The one I have always used is the only one I know of within 50 miles and they are A series specialists. They did the machining and supplied the pistons to match last time. No reason at all to think they are not up to the job, so why go elsewhere to an unknown shop?
Guy W

Yep +.03 = 3 hundredths = 30 thou.

I agree the last machine shop did alright - it went well for a long way after all, even with the extra stress of the high compression head.

Do you have an internal mic? An interesting comparison would be between the ovality at or near TDC with the ovality at mid-stroke.
Greybeard

Just unusual to mark / talk of oversizes in hundreths, not come across that before. Is the smoke down to fair wear then? (I must have imagined that it had been doing it for a time, maybe mixing you up with those that suffered oil breathing / smoke issues, Lawrence and others.)
If you're happy ewith the rebore place, no need to look elsewhere. But I don't see any sensible alternative - I suppose you could hone and re-ring but that seems to only be putting off the inevitable.
David Smith

Re: the blue smoke at low speed.

Valve guides/seals maybe? Esp as your carbon deposits are soft and oily, suggesting it's maybe happening more when the engine is not very hot. As you have the head off anyway it a good time to look and between that and a rebore I'd imagine the problem will go away.
Greybeard

I don't have an internal mic, but perhaps I can borrow one. The photos don't really show it but he bores are really in very good condition, bar that one scratch on #4. But you are right, they could be oval or wider at the mid / lower end. There's no discernible ridge at the top of stroke though.

The next step is to drop the sump and remove at least one piston at which point I may try the trick of pushing a loose ring down the bore with the piston crown and measuring the ring gap at 2 or 3 places. That would identify bore taper wouldn't it?

Pretty sure that the blue smoke is not from the valves or guides. I did try replacing the seals (all 8) but it didn't significantly improve on the blue smoke problem. And it isn't really smoking on overrun. I think it is from cylinder / ring wear.

I am shying away from a rebore just yet. Partly because my garage is so chocker with the frogeye and a vast quantity of "stuff" that it - and my lifting crane - are simply not available at the moment. I am holding back on a final decision until the pistons are out but I was thinking along the lines of new rings or possibly a set of cords might give me another 20K or so? Any view on that?

And what about the lateral piston movement? Is that normal or is it a suggestion of either bore wear or even of broken rings?
Guy W

"There's no discernible ridge at the top of stroke though."

I've had bores worse than that, and then put a set of rings on them, and smoke all gone. Then I've used the engine for another 100k.

If the wear ridge is so slight, you could even get away without a ridge dodger top ring. But better if you can find a set.

You can't really decide without seeing the rings. You may have a broken ring, and you may find the oil controls are gummed up and partially stuck in the grooves.

As for the vertical score, again, my spare engine has one more visible than that, and I've just re-ringed it, and put it back in the Sprite again. AND, that's AFTER using a set of cords on it for the last circa 60 thou miles. Gawd knows how many miles that engine has done since i last had it bored back in the early '80's, but it's pulling like a train now.

How many rings on the pistons? 4 or 3?



Lawrence Slater

>>The next step is to drop the sump and remove at least one piston at which point I may try the trick of pushing a loose ring down the bore with the piston crown and measuring the ring gap at 2 or 3 places. That would identify bore taper wouldn't it?>>

Yes it would show taper. But not ovality.

You can probably do that without an internal mic if you have a good internal caliper and an external mic or a good vernier. And a steady hand. Don't forget to measure the pistons accurately as well as the bores. You might need some info from the manufacturers on thrust face lobe wear tolerances.

IIRC back in the day there were places that did rebores with the block in situ. I don't know how good the procedure was - I never had it done and I don't know if anyone still does it. It sounded a bit chancey to me, but it would mean a tow or a trailer rather than pulling the engine, always supposing the crank is okay and doesn't need to come out.

However - if you pull the pistons and don't find anything truly horrid you could probably get away with re-ringing for a while. But from what you've described I have the feeling the rebore is in the post. Sorry about that.
Greybeard

Hi Lawrence. I remembered that you had travelled this route before!

There are 3 compression and one oil control ring. Pistons are by TRW (?) 71.34 grade, for a 71.37 bore
Guy W

Just sent you an email, and as it's a 4, it could be just the ticket.
Lawrence Slater

Greybeard,
I am kind of hoping that the crank is OK. Oil pressure is still good and the engine just sounds right! When I rebuilt it last time I had the block line bored and had the crank ground and the whole assembly balanced, along with a lightened flywheel and harmonic damper pulley, all as a unit. It has always run very smooth and revs nicely.

You may be right about the rebore, but Lawrence's comments give me hope!
Guy W

Have you got it Guy?
Lawrence Slater

No, but l did email you back in case you have the wrong @ddress.
Guy W

Guy, just a quick reply as I'm just delving into a problem that my Sprite has just developed and I need to get ordering parts!

The piston movement is normal, even on a new engine. It just looks more alarming on an old engine because the rings are not as tight. It's because the pistons are a smaller diameter at the top to allow for greater expansion as there is more material there. They are measured at the skirt.

john payne

>>Lawrence's comments give me hope!>>

Yep, but he's just admitted his bodges and he's trying to sell us a used car!!

Just kidding Lawrence!

Anyway, easy to measure your crank journals while the sump's off and the pistons are out, but it sounds like you're right about the crank. Happy days!
Greybeard

Guy, I replied to your email. Have you got it yet?
Lawrence Slater

Yes Lawrence, we should be on the same wavelength now!

Any views from anyone on the photo of the head?

And what about the way the carbon looks on the piston tops. I don't recall seeing soft carbon build up quite like that with patchy areas where it appears to have flaked off, leaving almost clean piston top showing. Might this be an indication of pre-detonation?
Guy W

Im not a fan of moving pistons... mine did not move when I built it, but id think because you can move them with just a finger nail, that would be a problem

How much longer will you keep the car, or should I say go to an elderly nursing home... Im thinking instead of .60 over, go all in and make it a 1340...(.120 over) maybe even stroke the crank for a 1420.

If you rebore over the .30 you have you wont be able to bore to 1340

At 80,xxx scence last rebuild, id say its getting tired and would enjoy a complete rebuild... or your at a point of just chasing your tail...if thr rings are slopy then id think the main and con bearings are not to far behind, and you can see where this is going

Then there is the K series

In for a penny in for a pound

Prop
Prop and the Blackhole Midget

It may have just been.the photo quality.. but did it blow the HG at 2/3 on the head ?

Prop
Prop and the Blackhole Midget

No Prop it didn't blow the gasket, but well spotted as that is one concern of mine. There is just an indication on the head of slight leakage across there, although no corresponding evidence on the block. But it is very slight. I was wondering about trying the copper wire trick.
Guy W

The pistons MUST move laterally. If they didn't, they would expand with heat, and lock solid in the bores. That's why you have expanding rings that the pistons move against. At least that's my understanding of it.

As regards the head gasket, was oil seeping across to the centre chambers, or did it just spill there as the head was lifted? It's all one side, so how did the oil get up to the divider between 2 and 3?
Lawrence Slater

Obviously I'm looking at the picture, and you have the head, but that looks to me like burnt oil between 2/3.

Lawrence Slater

Yes Lawrence, its not the oil discolouration on the head, which I think occurred as I broke the head gasket seal by spinning the engine on the starter. But at the narrow neck between 2 and 3 there is a bit or carbon build up, suggesting at least incipient blow through.

I am waiting for Peter to chip in. I would really value his experience on this!
Guy W

Im with greybeeard!

Blue smoke at low speed.

Valve guides/seals
Steven Devine

Steve blue smoke isn't just at low speed. What I meant was that it is embarrassing, - even offensive, to pedestrians, when I am trickling along in town traffic. At higher speeds on the open road the blue is still there, but much dispersed by the air turbulence and there is no one there to choke and cough on it, just the sheep!

I don't think it is valve guides and seals, as those have fairly recently been attended to, and the smoke only appears on acceleration, not on overrun

I will let you know what the rings are like when I get the sump dropped, but it will be a few days now as I am back at building work again now.
Guy W

This thread was discussed between 31/08/2015 and 01/09/2015

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