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MG TD TF 1500 - Petrol

Just found out why the TF was useing so much oil. There is a hole in one piston and elsewhere broken rings. The bloke who stripped it down for me suggests that it could be related to useing standard unleaded rather than a higher octane. The engine was just out of a complete rebuild when the problem developed. Does anyone have any comments or advice on the this kind of effect?

Bill McGee
Bill McGee

Bill,

I would suspect that because it started as a fresh rebuild that the engine hadn't gone back together properly. When the engine was done, did it get bored out, new rings and pistons? If it wasn't bored there may still be a ridge at the top of the cylinder that was worn into the cylinder by the old rings. When you put the new rings in, they will break when they hit the ridge and part of the ring can make it to the combustion chamber and get hammered into the piston. Any photos of the piston with the hole in it??

Does the rebuilder have any experience with TF engines? The rings may have been broken when the pistons are installed if installed from the bottom of the cylinder rather than the top.

As far as the fuel goes, I use the cheap stuff in my lawn tractor without any problems. Not much different than the TD/TF engine other than the cost of parts.
Ron Boisvert

Bill,
My old but very low mileage rebuild runs perfectly on U. S. 89 Octane no lead crusing at up to 65 MPH. Has not used a quart of oil in three years I have owned the car. The rebuild is about 12 years old.

Jimbo6
1953 MG TD
J. M. Haskins

To hole a piston with cheap fuel it would likely been pinging really loudly or super-lean and running very poorly. A loose or dropped /bolt/nut/screw, or lack of valve-piston clearance can cause a hole. I did exactly what Ron said with the ridge at top of the cylinder- on my first rebuild when I was 18 years old- it breaks the rings nicely. Otherwise there could have been no ring gap, too loose side clearance with the pistons, or installation damage.
George Butz

Ron:
Thanks for the idea or the ridge. It had new rings and pistons but no rebore. I shall have that checked. I asked about scouring but they said the bore was clean. Will have a recheck. The head was replaced in the rebuild install ation from the top was at least possible. These fellows have little experience of TF's but are sports car specalists with plenty of experience in B's and Sprites and other like cars - occasional TC's etc. Prepare a lot of such cars for racing. They should be up to the tricks, but... Does your tractor have the sex appeal of a TF?
Jimbo, George: The fuel I used was standard - I think it is about 92 unleaded. The damage must have occurred very early after the rebuild and I dont recall any prticular noises; it was also in for tightening so such should have been picked up. Note though "should" - there "shouldn't have been a problem anyway. Subsequently the motor actually ran quite well - ignoring frequent stops for oiled plugs. No strange noises. Perish the thought of something left in there - I pay these blokes as professionals. This would not explain the multiple cylinder problem, though.
So far ridge or defective rings sounds the best.
Thanks
Bill McGee
Bill McGee

Hi Bill, the ridge in the bore will break only the top rings, but it will do it every time if the ridge is substantial.

The oil control rings should not be broken so there is still the oil burning question to be solved. Of course the hole in the piston would not do much to prevent oil burning. Were rings other than the top ring broken?

The fuel used should depend on the state of the head and the ignition timing. If the head has increased compression as lots of them have, standard 91 octane ULP will not be the best. Most of our club guys use 98 octane in our cars. As George stated if the car was pinging this would cause piston damage.

I also find it a bit odd that the pistons were replaced without reboring the block. I mean why go to all the expense of a new set of pistons and put them in worn bores? Maybe second hand pistons were used. Dunno.

Cheers,

Paul.
Paul van Gool

Paul:
Thanks for your comments. The head that went on was a reconditioned one I brought in from a supplier in Holland. It was about twice the depth of the old one which had been shaved so much it was starting to cut into the plug slots. This would have reduced the compression ratio considerably, but it also ment the change to unleaded. the new ratio would be much closer to standard. I did not think particulally about octane when I changed, just vagely followed the general recomendation of the the petrol companies that older cars ran happily on standard. It never crossed my mind that this could have potental for damage. The fellows are going tomeasure up the capacity and CR. I looked briefly at the rings in the workshop yesteday but didn't pay too much attention beyond noticing breakages. I was too taken by the hole in the piston. I will recheck. I am off to NZ tomorrow (trying to forget TF's) so it will be a few days befor I get back to the problem.
Thanks.
Bill McGee
Bill McGee

Oz standard ULP should be 91/92 octane, which is pretty decent by Asian-Pacific standards... some countries survive on 86/87 octane as their base fuel.

PULP should be 96, other Premiums like Optimax should be up to 97/98.
Will

Is the octane rating measured differently here in the US? 91 or 92 auto fuel is the most we get here. Most cars run on the lowest (87. Aviation fuel is now standardized at 100 octane is called 100LL (low lead). It has twice the lead content as the old leaded 87 octane auto fuel.
D Clark

Hi Guys, in OZ we use the RON system of measuring octane ratings.

I am unsure whether this is the method used in the US. Maybe someone from the US can answer this for us.

Cheers,

Paul.
Paul van Gool

The US uses the (R+M)/2 method. Reasearch octane plus Motor octane divided by 2.

John
John Progess

Hi Guys, I found some info re this octane question on the net. This should clear up any confusion.

Cheers,

Paul.



There are two basic octane measuring methods, the Research and Motor methods, plus some derived versions like AKI (Anti Knock Index) and Pump Posted octane numbers.

* The Motor rating came first, is a more severe test and represents engines under high load, full throttle conditions. Itís usually abbreviated to MON (Motor Octane Number).
* The Research method was developed to represent engines under lighter load, part throttle conditions more like those commonly found in road vehicles. Itís abbreviated to RON (Research Octane Number) and is usually a higher number than the Motor rating.
* The AKI or Pump Posted number is the average of the MON and RON ratings for a given fuel. The AKI is a better indicator of the fuel characteristics, when only one octane number is advertised.

Fuel type
MON AKI or PP RON
Unleaded:

82 MON

87 PP

91 RON
Premium unleaded:

86 MON

91 PP

96 RON

You need to know which octane rating is meant when the manual says "use 95 octane". To reduce consumer confusion over different octane numbers, the USA passed laws so that AKI is the octane rating shown on pumps at gas stations. Commonly-available fuels are 87, 89 and 91 AKI octane
Paul van Gool

So if I understand this right a stock compression engine would be ok with USA PP87 regular. I've been using that but also mixing to about 80% 87mogas / 20% 100LL avgas since it's handy for me. Never heard a ping. My engine was rebuilt to use unleaded but I thought the avgas adding lead couldn't hurt anything and could help valve lubrication.
D Clark

My TF88 (ie 88bhp; can't beat 'em, join 'em) seems to run fine on our 91/92 RON.

But it diesels/run-on a lot less on Optimax 98 so the little extra expense is worth it.



Oh yeah, Oz seems to dye our fuels too. ULP is usually dark red to purple. PULP is often a light green or yellow (Optimax, BP).

imho PULP leaves less staining on cream paintwork. The red does leave ugly smudges. :P
Will

The experts who have looked at the damage, particually the hole in the piston, are sure that the problem arose from fuel detonation. Suggestion is that I got a batch with ethanol or paint thinner. Anyone have thoughts on this?
Bill McGee
Bill McGee

Bill,
No opinion on the ethanol but to further clarify the octane question I read from the pump last evening the formula for determining octane number displayed on pumps in the State of Florida. It is the Research octane number plus the Motor octane number divided by 2. In other words the average.

Safety Fast!

Jimbo6
1953 MGTD
J. M. Haskins

Bill -

Ethanol blended fuel is required in my section of the northeast part of the US. All fuel contains 10% ethanol and I have not had a problem with my TD's or MGB.

Jan
Janson Hurd

Gentlemen:
Thank you one and all for your comments.
Bill McGee
Bill McGee

Have been warned that ethanol in fuel could affect the older batches of diaphragms used in the SU fuel pumps.

Don't know what % before that becomes a major issue, but for sure the authorities ain't testing that "10% safe" limit with 50yo materials technology in mind.
Will

Will

All of the fuel pumps that I have rebuilt in recent years have had a plastic sheet over the rubber diaphragm where it is in contact with the fuel. Also make sure that all fuel lines are safe with the fuel. My flex lines between the pump and the carbs are a teflon tube with a metal braid.

Jan
Janson Hurd

I just got a restored 53TD from one of the best known
collectors. He told me to add lead additive to the fuel but I am still unclear if premium is better or worse or same as "regular" What is best??
S Hess

Best is what works for your engine and what you are willing to pay for. If your car runs well on regular, why waste the extra money for premium.

If your engine pings on regular, I would first check things like carburetors, plugs and timing. Then if it still pings,I would try a different brand of gas. If it then still pings, move up a grade.
Bruce Cunha

Refering back to my original post about my engine problem, I have just had an engineer from the local moterists association inspect the parts. He says that the wrong octane explanation is (pardon the expression) hogwash. He has no doubt that the cause was fuel wash - excess fuel washing away the oil causing very rapid ware of the rings. Subsequent ring disintergration caused the hole in the piston. His comment was that it was like driving with the choke right out. He didn't look at hte cause of the fuel flooding but suggested carby ware as a possibility. i havn't loked at the carbs yet but this is unlikely as they were running well previously. My first thought is that on reassembly after the rebuild, the carbs and intakel manifold were put on as a unit incorrectly so that the choke cable clamp fouled the body strut. This could perhaps hold the choke open. I would appreciate thoughts on the topic.
Bill McGee
Bill McGee

Hi Bill, check the needles. Standard needles for a TD should be ES. Start with this. Any engine using excess fuel like that would blow lots of black smoke.

Cheers,

Paul.
Paul van Gool


Getting back to the subject of cylinder top ridges. If you are going to fit new rings without reboring, the ridge needs to be removed with a special ridge reamer that is made for this purpose, they are normally adjustable to various cylinder bore sizes. It is also a good idea to hone the bores to remove any glazing.

Harry
H.E.W. Walker

Bill:- do you notice your dipstick oil level *rising*?

If you have excess petrol washing your cylinders, some of this will get into your oil and dilute it = more volume.

Plugs will also go sooty black very fast.
Will

re: carb tuning "rule of thumb"

Some oldtimer told me many yrs ago... "if your engine can be started from Cold without needing to use the choke, your SUs are running too rich".

Idea seems to work - still haven't had time to study SU anatomy.


But have been fiddling with my choke. Have now got it sweet; its now got a spot where it richens the mix, pull harder and it also raises the idle at the same time.

Handy when starting the car without sitting in it.
Will

This thread was discussed between 25/10/2004 and 29/11/2004

MG TD TF 1500 index

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