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MG Midget and Sprite Technical - Tired engine?

With the midget back on the road, I've now done around 500 miles and have been keeping an eye on the engine - the one thing I didn't do major work on. And I'm thinking now maybe I should have.

It is a recon engine, it's been balanced at some point and I was told it has an unleaded head. That's the limit of the knowledge passed on by the PO.

It runs and drives just fine, but I keep leaving blue smokey clouds behind and in the 500 miles have used about 1.5 litres of oil (Halfords Classic stuff 20/40). The engine was using some oil before, but seems to be using more after it's two-year lay up.

I did a compression test this weekend and the figures were 80 - 90 - 85 - 90. Wet figures 100 - 105 - 95 - 105. So, looks like the bores are quite worn?


I have a couple of questions as I consider what to do next...

Oil pressure is good (about 60-70psi at speed and 20 at idle when hot) so am I right in thinking these figures suggest that the head and crank are probably not too bad?

I'm thinking my next move should be to take the head off and get the bores checked for roundness. Then it would be either a new set of rings or a rebore?

If I need the block rebored, I think I'll take the opportunity to warm the spec a bit - probably an SW5 cam and deck the block to take the CR up a bit. But having only just finished paying for the rebuild, I'd like to delay major expense for 12 months if possible.

If you have any advice on keeping the old engine going for a while allowing saving time, I'd be grateful?

Many thanks
Mark
MarkH1

Hi Mark

Smokey engines can last a long time. Your compression seems on the low side but consistent - points to bore/ring wear, as you say.

When is the smoke at its worst? Over-run or WOT?

Are you losing oil from the timing cover or bellhousing?

By all means remove the head and inspect the bores. You can even remove a piston and check rings, too (drop the sump...). Check esp to ensure minimal lip at bore top - this can be a breaker for new rings.

Fitting new set pistons and big-end bearings with engine in situ is relatively straightforward. I'm a bit lazy and will normally take the easy option...

Mind you, if the engine's coming out in the next 12-24 months, you might keep going as you are... you don't want to fit new pistons and then rebore later...

A
Anthony Cutler

Thanks Ant,

The smoke is at it's worst when pulling away from the lights, particularly if I've booted it from one set of lights to the next and then stopped and pulled away again. Also a bit of a puff on up-shift.

I've got a set of stem seals kicking around. I've not checked whether the head currently has seals fitted on all valves or not, but if I'm going to take the head off anyway I guess I may as well renew them while I'm at it and see if that has any effect.

Would the general rule be that if the bores are not too oval replace the rings and if they are to get the bore done (eventually)?
MarkH1

I understand that if the bores aren't too bad that it's advisable to hone them lightly to break any glazing that might be present. This will also help new rings to bed in properly.

More knowledgeable bods will reply soon, I'm sure!

-:G:-
Gryf Ketcherside

Hi Mark - I had a MkIII 1275 that smoked, used a litre of oil a week (250 miles) and carried on like that for 40,000 miles. Didn't get worse and, being car number three on the 'importance list', ended up being sold like that too!
Regards
Andy
A S Duff

Mark,
Another thing worth checking is the condition of the rocker shaft. A worn rocker shaft can result in oil splashing out too fast and then it accumulates on the top of the head, gets onto the valve stems and produces blue smoke. Noticeable after a period of tick-over preceded by a fast run (hot oil).

Guy
Guy Weller

My '73 had a USA-spec block with dry liners. The block was fine at 110K but the piston rings were so worn they were barely present... engine was running fine but a little somkey at times.

Yours sounds like worn guides as pointed out.

Old engine builders used to say a smokey engine lives forever; and the engine blows up when it's running better than ever!

A
Anthony Cutler

I got agree with the others....if your talking a year from now to do a rebuild, id drive it till then...just remember to treat her like the grand mother she is, if you go on a long trip...certianly get good road side delivery insurance

If something starts going bang! bang! bang!,,,,shut it down FAST...that piece of advice saved me a bundle of lettuse.


Prop
SheBoyGan5 from Prop

>>>>>>>>>and the engine blows up when it's running better than ever

So, so true, my 1301 engine went bang at the 'Ring while holding on the tails of cars putting out 20 - 30 bhp more. Was just thinking "she's going well..."

Max max-at-midgetmax-dot-com

For me it was...It sounds great, but why is my oil pressure flucuating every now and then.....

Oh wait, Now I know...BAng BANG BANG!!! WOW, I thought the crankshaft was 1 piece unit, not 2...DUH!

LOL.

Prop
SheBoyGan5 from Prop


Thanks all,

I'm tied up for the next couple of weekends, but think I will try and get the stem seals swapped over after that, hoping it will improve things, and not massive expense if it doesn't.

Guy - Is there a quick and easy way to test the rocker shaft?
MarkH1

Mark-
It's worth checking the cam timing, especially since it's a recon. It could be expected to have less miles than most, but there is always the chance that it wasn't assembled correctly, and/or the timing gear wasn't renewed in the "recon". Late cam timing can cause both low compression, because the exhausts close late, and oil consumption/smoking, be because the inlets open late. If this is the case, you will also be pleased on both power and fuel consumption too.
I was once pleasantly surprised at improvements in both compression and oil consumption when a customer asked me to replace the rattley chain as a "make do" until rebuild time. That was 20 years ago; it's finally time for the rebuild!

FRM
FR Millmore

Hey FRM,

Are you infering that his cam timing is retarded and needs more advancing?


Sorry no humor implied, Just curiuos If Im following correctly.

Prop
SheBoyGan5 from Prop

Prop-
Not "inferring", which is done by the listener, or "implying", done by the speaker, as a way of "not quite saying" something. Rather, I think I said it outright, at least as a possibility.

Does the damn engine run yet? Get thee to thy work, oh slacker - We await all ears!

FRM
FR Millmore

So now everyone has helped on the engine I will do so about the oil you are using. The 20W40 may be named as classic but that will probably mean its without modern additives and with old times additives BUT... real classic oil is 20W50 or even 20W60. Unless you are in an artic state its best to use classic oil with the 20W50 or if the engine is old the 20W60 viscosity.

Bas
Bas Timmermans

Im almost there FRM,

I had to take a week off from the engine...work got in the way (that thing that puts money into my pocket and burns up all my time)...tough week, lots of wall paper to strip...So Im back on the engine tomarrow.

Im thinking Ill wait till sept 28 at 1:00 pm to fire the engine up for the 1st time....that way it will be one year to the exact moment the engine blew up...sort of a "Heres Blood in your eye" to the machine Gods that be


Prop
SheBoyGan5 from Prop

This thread was discussed between 15/09/2009 and 18/09/2009

MG Midget and Sprite Technical index

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