MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - Scored No 3 bore on 1500

Hi folks,

Long time no see. It's been a while since I have done anything on the midget, probably why I have been in such a good mood of late! :-)

One of the things I wanted to do over the winter was do a bit of porting and polishing on the cylinder head. At the same time it would give me a chance to sort some small coolant leaks and investigate a slightly low compression on no.3.

Having whipped off the cylinder head, the low compression (I can't remember the numbers, but it wasn't *that* low and the engine still ran fine) looks to be due to some nasty looking scoring to the back wall of no.3 bore.

Your thoughts on it's cause and solution would be much appreciated. The block is already bored +40 thou, so I hope that at a hone and some new rings might be sufficient to patch things up whilst I sort a new engine in the next year or two.

Cheers,
Malcolm
Malcolm Le Chevalier

oops... Pic...


Malcolm Le Chevalier

Could be from a broken ring? Or if the compression isn't too bad it could be from a damaged oil control ring. They are commonly made up of 3 or 4 elements and I have found these can get snarled up - I believe when they are first assembled - and the edges would then cause scoring around a segment of the bore like that.

Next step is to remove the piston to check!
Guy W

Malc,
is it safe to say it wasn't like that when you put it together?
Difficult to tell with the flash on but I'm with Guy. As most of the marks don't come up the full height, I'm going with an oil ring centre section that's slipped during assembly.

Best of....
MGmike
M McAndrew

My money is on a broken top ring, I'm thinking the scratches are too high up for an oil ring problem
Looks a bit nasty- might hone out- or drastically improve it - bit hard to tell till you attack it

I suspect something is jamming the piston across in the bore as it has cleaned the carbon off the non thrust side------a piece of ring, or piston ???

willy
William Revit

Thanks Guy, Mike, Willy.

Just been out for a second look tonight, but can't drop the sump quite yet, so can't pull piston. But it's perhaps not as bad as I thought yesterday, I am going to have a crack at honing it. It can't make it any worse and will at least patch it up whilst I think about doing another engine (and upgrading the gearbox at the same time!) :-)

What do you mean about the carbon build up Willy? I just checked and it looks the same in each of the bores. A bit of carbon on the front/back of the bores, less so on the sides. I didn't think the rings rose that high up anyway.

Cheers,
Malcolm
Malcolm Le Chevalier

Hi Malcolm
Just going on what we could see in the pic.really--
Correct, the rings don't go to the top of the bore, but the top of the piston does
The top of the piston is slightly smaller in diameter compared to the rest of the piston and this usually results in a carbon build up in the top of the bore down to the level of the top ring
Normally ,the piston gets pushed over to the thrust side of the bore by combustion forces and depending on how much clearance / wear the piston / bore has the thrust side of the top of the piston 'can' wipe the carbon out of the thrust side of the bore
In your photo, which I presume is of the rear of the bore, the carbon has been wiped off the bore on the right hand side of the pic which is the non thrust side
This is what made me think something was pushing the piston over that way --------------BUT
If you are saying they are all like that I am happy to agree that it is a feature not a fault--
It will be interesting to see what comes out
Cheers
willy
William Revit

Sure looks like that will hone out okay to me

Just don't get wild with it...check out a few youtube vids before attacking it.

Depending on the hone it won't take much

Prop
1 Paper

Malcolm,

When you were building that engine, did you run the rods through a machine shop to check for alignment and roundness?

Charley
C R Huff

Thanks Charley.

I never checked the rods, but had no reason to think that they were "bad". The engine had clearly had a proper re-build at some point in the past (big bore, crank and rods had been balanced etc.)

The bores looked good until I came along, gave things a refresh and thrashed it for 3000 miles! So I am fairly sure it is something I have done wrong (likely) or a dud new part during rebuild.

Cheers,
Malcolm
Malcolm Le Chevalier

Ouch.... yes any time you pull rods they need to be checked for roundness

Granted i doulbt. Everyone does that bur its good shop practice
1 Paper

Managed to get the sump off last night (bit of a nightmare, but all my fault!) then discovered that the con rod bolts have a 12mm 12 point head. It's the one socket I don't have as it cracked years ago trying to remove some stubborn caliper bolts on an old Rover. Dammit!

Now equipped with a new socket and honing tool, ready to have another go tonight. I will keep you all posted.

Cheers,
Malc.
Malcolm Le Chevalier

I think those marks will hone out fairly well. On a road engine you don't need to go for a 100% scratch free finish. So good luck.

On a 1500, with the big ends undone I would as a matter of routine, replace the shells and especially the crank C thrust washers. You can do all of this in situ but will need to remove the alloy bridge piece to replace the front main bearing. If you are doing this then you need to remove the bottom 2 (or is it 3) of the timing chain cover bolts as they screw into that alloy bridge piece. Done carefully this is possible without removing the timing chain cover and without too much damage to cover gasket. It can always be recovered with some silicon gasket sealer. Alternatively, if the rear and centre caps are in decent condition you may choose to leave the front one undisturbed - it saves a bit of hassle!
Guy W

"the con rod bolts have a 12mm 12 point head"

12mm? Really?
Dave O'Neill 2

Could be aftermarket bolts mine where normal bolts


mark heyworth

12 point?...could be ARP HARDwear

Remember you will need motor oil (wieght?) To torque it into place

Prop
1 Paper

Malcolm,

I suppose it was clear, but just in case, my comment on the rods being checked was because the score being on one side only made me wonder if a slightly bent rod was overloading that side.

With respect to honing the cylinder, faced with the choice in the past, I have opted for the benefits of a good surface above that of not exceeding the recommended bore size. Maybe others can comment on this.

I did a cheap rebuild on a 745 cu in Cummins diesel, and I honed the cylinders (actually using a hand held glaze breaker) significantly past the recommended limit to get a smooth wall. Subsequently I drove it from Kentucky to California (about 2300 miles) and used only 1 quart of oil. I would have been perfectly happy if it had used a gallon.

Charley
C R Huff

Yup, 12mm 12 point. They are Ford Cossie bolts.

http://www.burtonpower.com/std-con-rod-bolt-ford-cosworth-yb-suits-original-rods-only-sc248.html

I am not going to get involved in thrust washers etc. as they were only changed 3000 miles ago. The scoring occurring so soon after rebuild is what makes me think it is something I have made an ar*e of.

Cheers,
Malc.
Malcolm Le Chevalier

Ouch... so the scoring is very resent ...under 3000 miles

Wow...im with chuck on this even if you find a broken ring inside..its cheap to have them checked



Prop
1 Paper

Hadn't realised it was so recently rebuilt. So delete my comments about bearing shells! - Unless they show any signs of damage when removed, of course! When I had a 1500 I replaced bearing shells as a routine "service item" every 25,000miles, but at 3000 miles they should be still perfect!

At only 3K, the degree of marks on the bore do suggest a trapped oil ring element.
Guy W

So here is the picture you have all been waiting for...

This is exactly as it came out the bore. Er... Looks fine to me.


Malcolm Le Chevalier

OK, so I was wrong. Not the first time and won't be the last! I assume that the blades of the oil control will compress fully into their slot?They are quite proud in the photo.
Guy W

Yeah, everything compresses fine.

I think I have sent everyone on a bit of a wild goose chase I am afraid. After just a couple of "primer" passes with the honing tool most of my supposed scoring came right off!

It did reveal some nasty bits in other places on the bore though.

(Note: yes, my honing skills need a quick upgrade to get nice even 45 degree hatchings!)


Malcolm Le Chevalier

Did you check the end gap on the rings?
Guy W

Good thought Guy
You should have a minimum of 9 thou
Maybe it just got some rubbish in there to mark the bore, but they are l o n g scratches
One little thing in the pic--
I doubt it is the cause but when you put it back together, the bottom rail on that oil ring appears to have it's join/gap right there where the join in the expander is. It's better to have them all seperated by 1/4 turn or more and the joins in the rails not over the gudgeon pin
I'll probably get nailed for this but--
When I assemble, I fit my gaps as--
Top compression ring gap---- to the thrust side
second ring----------------- to non thrust side
top oil scraper rail--------thrust side
bottom oil scraper----------non thrust side
expander------anywhere but well away from rail joints so over a gudgeon pin !!!

willy
William Revit

Willy

When i built mine thats how i was told (as i recall)

I think chuck has the inside track onthis one ..slightly bent rod ... not a big deal and easy to fix

Thats alot of scratching for nothing wrong with the pistons and rings

Did you reuse the trust washer 3000 miles ago or replaced it
1 Paper

Malc,

looking at the last pic it looks like the scores stop at the second ring hight. Have you measured from the top of the score to confirm which ring is the problem?

I guess you then check the ring gaps and for any unexpected sharp points around the ring or at the gap. I also seem to remember reading that de-burring the ring at the gap should be considered. I'll try and find the reference and confirm.

Best of

MGmike

M McAndrew

Thanks all. I did install the rings as per supplied instructions, that's all I can remember.

I won't get chance to investigate further until tomorrow afternoon.

It could be that they were already knackered and I didn't notice when I re-built it, it is 40 years old after all. Who knows?!

Cheers,
Malc.
Malcolm Le Chevalier

"Yup, 12mm 12 point. They are Ford Cossie bolts."

Oh yes, I forgot you had 'funny' bolts ;o)
Dave O'Neill 2

Better funny bolts then funny nuts....hhmmmm


Prop
1 Paper

Had a rummage in my photo archive. I don't think the bores were in that good nick to start with. I don't have a picture of 3, but no. one certainly had some marks (see pic).


Malcolm Le Chevalier

And I have had a go at no. 3 with the honing tool. I wanted to do as little as possible, so I have got to the point where the scratches are just starting to disappear, what remains of the scratch must therefore be imperceptibly shallow. I certainly can't feel it with my fingernail any more.

I am going to rebuild and live with it for now. Should see me good for a few years at least.

Thanks for all your help and suggestions everyone. You have been a great help, as ever.

Cheers,
Malc.
Malcolm Le Chevalier

pic


Malcolm Le Chevalier

Looks OK now. Has it overheated at all in the last 3000 miles? The scratches could have been caused if it had a partial seizure, There is some scuffing of the piston just above the top ring in that earlier photo. No harm done, but that could explain the bore marks.
Guy W

Malcolm,

It is hard to evaluate in the photo, but are the hone marks at a bit of a flat angle? It is my understanding that when they have enough angle, the sharp edge of the new rings meet the metal of the bore with a scissor action. But, if the angle is too flat, the rings meet the honing marks of the bore head on so that they gouge the metal out of the bore.

Regarding the initial cause, besides my rod theory, could anything have happened to damage the rings during installation? Also, perhaps the scoring was caused by grit?

Charley
C R Huff

Scratches go too far down the bore to be ring issues.

Probably dirt or carbon trapped between piston & bore.

The old ridge is still at top of bores - I guess you didn't remove that when fitting new rings?

Honing should be done at slow drill speed with the hone being moved up and down the bores quickly to leave a cross hatch pattern with an angle between up and down strokes of around 60 degrees.
Chris at Octarine Services

I've no idea (as usual) but I was surprised at how much carbon(?)/muck/dirt appears to be the piston top in that photo after just 3k miles but perhaps it's loose wet stuff or the engine wasn't apart to be cleaned 3k ago (I've missed the story).
Nigel Atkins

I seem to recall and maybe im wrong bit wast there an issue with the fuel system running rich this past summer..

Prop
1 Paper

Wow
You've showered that little engi ne with lots of love...I hope it shows you the same in return.

I looked at all your pictures..I didn't see anything that looked really bad.

Cold it be the valves were causing your slight compression issue?
S

Yeah, it gets waaaaaaaay too much love!

Prop has a great memory, I was running way to rich recently, so carbon build up is likely due to that. I will have a good decoking session whilst I can. The middle two cylinders are worse than the outers, due to the manifold design they must run richer.

I do miss the car running. Although the roads are regularly being gritted at the moment, so I wouldn't want to take it out anyway.

Malc.

Malcolm Le Chevalier

This thread was discussed between 04/12/2016 and 12/12/2016

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.