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MG Midget and Sprite Technical - A-Series Timing Belt Conversion

Just had a new belt break (~3,000 miles and 6 weeks old). We are in the middle of France, on the way home from the Euroclassic.

I have had the kit (MiniSpares) for a number of years and decided to replace the belt as a precaution a few weeks ago. We are due to come back tomorrow and the belt broke on the way to a visit to a champagne 'cave' earlier today, so mighty fed up!!

Car is now going to be repatriated and we have a hire car to go up to Calais tomorrow.

What are others experience of the belt life? I have had no problems at all previously, and have replaced the belt with less than 10,000 miles before.

Think I might replace it with Peter May's vernier chain drive?
Richard Wale
page 6

It is in Dutch but the pictures say it all
New engine new belt disastrous failure including cam hitting big ends valves hitting pistons etc.

There seem to be some bad belts around
This was a minispares kit there might be a quality diffrence though minispares usualy offers good quality products
Onno Könemann


I dare not think about what has gone on inside the engine - I have just fitted 4 new Omega pistons as part of the rebuild that included the new belt!

Not sure about quality parts though - I have had a broken HT head stud (clear metallurgical failure), broken heavy duty competition rocker shaft and now a broken timimg belt. The first ones were some time after they were first bought.

Richard Wale

Well since yours broke it might just be fine

Belt brakes
No drive to the camshaft
Valve springs stop the cam rotating

it all depends on how long the crank stil rotated and if the angle in which the cam stoped was in interferance with the crank

But the pistons should be fine and a crank can survive a ding.

The belt in the story above striped a few teeth and kept drive to the cam
Onno Könemann

Oh Jez Richard,

Im gutted for you, Onnos last thread does make good scence about the cam stopage and limited disastor

scence my old mazda pick up from years back broke its timing belt, Im a lover of steel chains over rubber belts.

I Hope the rest of your trip is un-evenful and a pleasure in the ways you had intended, have fun!

If its any constalation, im sure you will have your engine up and running before mine. LOL


I have a timing belt on my 1420 and not had any problems luckily, although the engine hasn't been used recently, and have generally observed the change intervals. One time I changed it I got the replacement from the local BSL depot, I just gave them the belt size and didn't mention the application. A few days later I had a box labelled Pirelli Isoran and a label on the side saying Innocenti mini, I think I still have the box for my records even though the car hasn't been on the road for some years. Maybe the OE makers aren't doing them anymore and the quality is slipping from the stand in suppliers.
David Billington

I have never had a belt drive on an A-series, but I do remember a fellow A35 racer having one slip and having several bent pushrods.

I did have a timing belt break on a Ford CVH engine about 25 years ago.

I am a BIG fan of timing chains!
Dave O'Neill 2

Sorry to hear your bad news. I put a lot of mileage on a Mini Spares belt drive kit without problems.

The new engine had a Titan belt kit along with an oil leak right into the cover that led to the belt getting oily so a while back I converted to chain drive.

I don't suppose this is the thread to say I have a little used Titan belt kit for sale that needs a new belt is it?
Daniel Thirteen-Twelve


thats a good point... I was thinking possiable a sharp bur that was catching the belt, but oil would deterioriate rubber fairly fast, and probably would break down in 600 miles / 3 months, Still id think the belts would be oil resistant rubber. Then agian we are taking about the high quality standards in china manufacturing


Got the car back yesterday and took the head off to find out the extent of the internal damage after the 3,000 mile old cam belt failed. On removing the rocker cover, the first thing that was obvious was that some of the pushrods were bent, which was a bit worrying.

After taking the rocker shaft off and then the pushrods out, all of them are bent!! See picture. I have yet to strip the rocker shaft to check for straightness - it was also new 3,000 miles ago, but I think I will replace it to be sure (but that's what I did with the timing belt, and look where that got me!).

Taking the head off revealed that all four pistons have hit the valves, but appear to be ok with a light clean and deburr? No marks in any of the bores. What do you think? See picture.

All eight valve stems are bent, but the guides look perfectly OK. The head itself looks fine, the test will be whether the new valves lap in without any trouble.

Turning the camshaft on its own felt absolutely smooth and normal, so don't think there is any problem there.

It is a new Piper Billet BP270, with cross-drilled lobes for improved lubrication, which was fitted during the rebuild 3,000 miles ago. Following another BBS post I have just checked the timing with the standard chain set-up and it is perfect! I fitted the timing belt conversion when I had a reprofiled Kent 276 that needed 6

Richard Wale

Second picture

Richard Wale

Bad luck Richard. I think it would probably be prudent to strip the bottom end and check everything over and check the rods. Regarding the pistons I'm reminded of the advice given to a mate by a piston expert at Ricardo when he noted the Ford X-flow piston he was shown had been hit by a valve sometime in the engines past, don't reuse it as the piston can be damaged by it and fail later.
David Billington

Those images did a good job of inducing nausea. The pistons should be turned clean on a lathe in order to preclude cracking. As for your bottom end, as long as you have the engine out, you may as well inspect the bearings.
Stephen Strange


The pistons are also almost new Omega's, 3,000 miles old! Trying to avoid replacing them unless really necessary?? Just hoping that the pushrods took the brunt of the contact shocks and in effect protected other things.

Wishful thinking??

Richard Wale

I've had similar dilemmas where lack of money has prevented from doing what I really wanted.

If it were me I'd try and get some solvent to clean off the carbon from the piston tops and then look at them very closely. In fact maybe you can get some crack testing dye? If there are NO cracks running from the indentations you have the choice of taking a risk or not bearing in mind one risk is piston failure. I suppose a lot will depend on the rpm you are going to run the pistons too, CR, etc. I suppose with a Piper 270 you are not likely to use more than 6.5K and CR will be around 10:1?

I'm not an expert in these matters and don't claim to be. Why not ring and ask a couple of engine builders for a view?
Daniel Thirteen-Twelve

this whole thread sounds like a cautionary tale to just stick with a timing chain!!

Norm "scared of belts, now" Kerr
Norm Kerr

Did you give the supplyer of the belt a ring?
I think you could try to claim (some of) the cost since they should not break!

But i would pull the botom end and check every thing.
To bad it did this mutch damage
It realy cures me from the idea to ever fit a belt...
Onno Könemann

<<Did you give the supplyer of the belt a ring?>>

I have emailed twice, but no reply. I did ring them about previous problems at the time, but complete refusal to acknowledge any responsibility.

I too will not be reusing the belt kit!!
Richard Wale

Yes, wishful thinking indeed. You have to face up to the fact that the force that bent the pushrods was imparted by the pistons through the valves, so they will all need your attention. Although the valves may appear to be OK when inspected by the naked eye, they should be checked by colleting them in a lathe and using a dial gauge to check them for bended stems. Be prepared to be disappointed. The pistons should be refaced by grinding (not turning!) on a lathe and afterwards checked for cracks by professionaldye penetrant testing. This sounds expensive, I know, but a set of Omega pistons are worth the effort to try to salvage. After all, if they were humble County pistons, I wouldn't bother, I'd just scrap them. Look on the bright side, this is now the perfect opportunity to justify the purchase of a set of lightweight tubular chrome-moly pushrods to go along with you higher-lift camshaft!
Stephen Strange

Concerning that all 8 valves have contacted the pistons - if the belt just broke I would not expect all the valves to hit... perhaps it stripped some teeth first? Any pictures of the broken belt?

I have re-used Omegas after they have contacted valves - are yours cast or forged? (forged are much tougher...)

James B

Thats tough,

I certinly wish you all the best of luck, as always Im sure it could have been worse, Not sure how...

The upside is you wont find yourself commenting on how bored you are anytime soon...LOL



All the valves are bent to some degree or another - the dish in the piston has ensured that! In some ways maybe not a bad thing, as it absorbed some of the impact as well by bending the valve stem, instead of a full face flat engagement with the piston crown.


Yes, the belt has a number of teeth missing, but is still in one piece as such. Although there is no 'well known' manufacturer's name on it, apart from 'Made in China', it is identical to the one it replaced that had done about 6/7,000 with no problems.

The pistons are the cast version.

Am about to convert wishful thinking into practical investigation - dropping the sump and inspecting all 4 pistons and rods. I have just had a meeting deferred, so now have time to do it before catching my weekly flight to Norway later this afternoon.

Richard Wale

Out of interrest what make is the belt, im only aware of Dayco being available and made specifically for this kit!


Mark Turner

Anyone know where to get lightweight tubular chrome-moly pushrods ???

d brenchley


When you get the pistons out, check the ring land clearance and see if it is the same below the impact area as it is the rest of the way around. If it is the same, my guess is that the pistons are okay.

If you get them cleaned up first, I would not expect crack checking to be too expensive. I seem to recall some do-it-yourself kits, but I don't remember if they were for aluminum.

C R Huff

I now have the pistons out and all but one look ok - no visible cracks (need a 'proper' check as well). The rings and grooves are all fine on all 4 pistons.

No. 1 piston (the one in the photo) is tight on the gudgeon (wrist) pin to piston bearing and has marks on the side of the piston rubbing the bore. The piston has been distorted, so is definitely scrap! There is a witness of rubbing in the bore itself, but no actual damage.

It appears at the moment that Omega pistons only come in sets of 4, which is what I fitted a few weeks ago! Peter May is trying to get a single one.

Question (to avoid the need to run in, as I can't go to Goodwood in 3 week's time and do 500 miles running in beforehand) - if I fit the undamaged rings from the nicely run-in No. 1 piston, to the new one, will it then be give-or-take run-in as well? Seems worth a try?

Mark, The belt is the standard one from Mini Spares, no maker's name just 'Made in China'.

Richard Wale

>>>>>>Anyone know where to get lightweight tubular chrome-moly pushrods ???<<<<<<<<<<

Yepp I sure do...And vary nice one also...and Not vary expensive... They have to be custom made, but they are not vary expensive I think I paid $80 USA and that included shipping ... I asked them to keep the specs on you might be able to call them direct


No there not bent... its an optical illusion cause there not sitting flat but at an upward angle on one end...thus the curved look

Sounds like substandard belt to me (washing machine maybe). I know Dayco are the only genuine belt manufacturer to tool up for this application. I have always used DAYCO with no problems (and I work for a competitor belt manufacturer). Peter May stocks these.

Also reference rings I have bedded new omega rings in on the rolling road, takes about 20 mins and you can see the breathers stop steaming, then Ive gone racing (read 8000rpm) with no problems!!

Best of luck.

Mark Turner

Forgot to ask, what piston size and dish, look like 10ccish to me, 1380??
Mark Turner


I mentioned earlier in the thread that some years ago I popped into my local BSL and ordered a replacement belt for my A series cam belt set-up, IIRC I just gave the belt designation from the existing belt, I got back a boxed belt labelled Pirelli Isoran and Innocenti mini. Is it likely the application is unique with that size of belt and the quality supplier(s) have dropped out of the market to be filled by sub-standard replacements.
David Billington


Yes, they are Omega cast 73.5mmm, with the 11cc dish. Chris May has got hold of a single piston, so that is good news - I think I will try the 'make it work' school of running in this time!

I've decided to go back to chain on the basis that it can happen, it did happen and that was once too many times! I only fitted the adjustable kit to overcome the need for a 6° adjustment away from dot-to-dot on the Kent reprofiled 276 - with the new Piper billet cam now fitted I don't need any correction at all, it's spot on with the standard chain set-up.


Richard Wale

Hi Richard

Glad all is coming back together now.


I checked and it seems Pirelli power transmission Belts were bought by Dayco quite a while ago, so most likely the same belt, just re-branded, which means its should stay in one piece.

This of course all highlights the problems of gettering good quality properly designed and quality controlled parts now!!


Mark Turner

Think I have found the basic cause of the belt failure, by stripping teeth and bending all the valves and pushrods.

As part of the strip down, I decided to look carefully at the Piper x-drilled billet cam to see where the oil feed came from etc.

The answer is from the centre cam bearing, down into the hollow centre of the cam and out through the x-drilled cam lobes. All fine so far. Then looked at each end of the cam - the oil pump end is blind, and the front is drilled and tapped with a socket head grub screw plugging it. Problem was that the grub screw has a parallel thread and was not tightened, which would explain how oil got into the centre recess of the cam sprocket (seemed a bit odd to find oil there when I first took it apart).

Oil and cam belts obviously do not get on very well, so after ~3,000 miles a tooth broke off, causing the chain reaction that damaged the engine.

Now have all the replacement parts, will seal and tighten the grub screw and hopefully get the engine rebuilt this weekend in time for the MG's on Track day at Goodwood on Monday!

Still going back to the 'good ole chain' though!!

Richard Wale

Final update - engine rebuilt and run on Sunday for the first time since the belt failure in mid-September.

New cam run in for 30 minutes @2,500+ rpm, then head re-torqued, tappets adjusted and trial run of about 50 miles. All OK.

So finished just in time for the MG's on Track day at Goodwood on Monday. Absolutely superb weather and full 6 sessions completed. Fantastic circuit and highly recommended!

Richard Wale

This thread was discussed between 16/09/2010 and 27/10/2010

MG Midget and Sprite Technical index

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