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MG TD TF 1500 - Blow by

Blow by

Driving between 70 and 90 kph in fifth gear caused severe damage. The forth piston had a blow by and aluminum got stuck on the cylinder wall; it was not at all in a hilly area. The other three pistons were slightly pitted. Fortunately the cylinder wall was not damaged. After drilling and new pistons installed, everything is ok again.

The engine is equipped with a supercharger, but the pressure in the manifold never exceeded atmospheric pressure and the timing and mixture was correct.

The problem was most likely caused by pre ignition heating that can happen at low rev. and high torque, in combination with oil - in my case being drawn in via the inlet valves. It is well explained in this very interesting video. https://youtu.be/Wadmzv1Vs0Q

This was definitely something new to me.

YS Strom

Picture of piston.
Surprisingly,no damage to the cylinder wall.

YS Strom

The highest octane fuel should be used to help prevent this. Also ceramic or thermal coating of the piston crowns.
Len Fanelli

What model of car? What engine? Bud
Bud Krueger

It sounds like you are expecting too much power in fifth gear at low RPMs, and are causing pinging/detonation which you may not be able to detect or hear over the general car noises. Given the displayed results, I would also question that your timing is set correctly, and/or that the distributor is functioning properly.

At how many degrees fully advanced is your timing set? And yes, as Len notes, ALWAYS use premium fuel.

Which supercharger do you have?

Tom Lange
MGT Repair
t lange

It is a TD 52 XPAG

Ignition was set on mark at idling and mixture set rich with lambda sond at 60 kph.

Distributor looks ok and new cond installed recently.

It is an Eaton 45 supercharger with a homemade manifold.

As it probably was the oil content that ignited the fuel mix I doubt higher octane would have helped. However, it is certainly a good idea to use high octane fuel anyhow.

As this was more like an early ignition of a plug than an explosion caused by too high compression for the fuel, no pinging could be heard. But as said, it is very difficult to hear at that speed.

Coating of the piston top would probably have helped.
YS Strom

Image of the supercharger with homemade manifold.

YS Strom

It could be that your ignition timing is advancing too quickly because the springs in the distributor are too weak - especially if it was a new off the shelf distributor and not set up by a specialist. Setting at idle is not a good guide.
Dave H
Dave Hill

YS what is the compression ratio?
What camshaft are you using?
With a stock cam & a supercharger you may have too much cylinder pressure.
What engine speed did this happen at?
Len Fanelli

I do not know the history of the distributor and how to check the springs.

Will check ignition with strobe.

The compressor ratio set to maximum o,5 bar - but very very rarely used and not at all when the problem ocured. What max pressure can be recomended? The crank is of good quality an expert told me.

Cam was very worn. So vere the plunges. All has now been changed.

I guess the engine speed was arond 2 000 RPM.

Yngve

YS Strom

Its been many years since I owned my blown TD, but I had a Marshall 35 with a Downs Engineering kit.

The instructions were to keep boost to .5 bar (abt 7 lbs). I achieved this by playing with a boost gauge, and the S/C drive pulley of which I had several made of varying sizes, 'till I got 7 psi boost at abt 4000 RPM.

I never had any problem even with .025" shaved off the head, and sold the car with 62,000 miles on it, still running well.

This may be an unfair comparison because in the 1950s, octanes were generally lower than today, but I nonetheless, always used what in those days was called "Red" gas and don't recall any pre-ignition.

Gord Clark
Rockburn, Qué.
Gord Clark

Yngve - 5th is an overdrive gear, and expecting your car to have enough torque to accelerate at 2,000RPM in 5th gear is asking a lot. My car pulls like a freight train, but the power only starts to come on at about 2500rpm. If I were in 4th with a stock transmission at 2,000, I would absolutely downshift into 3rd to accelerate without lugging.

Given what you know, I would also replace your main bearings, as they take a beating when the engine is lugging.

Tom Lange
MGT Repair

t lange


Lugging the engine is generally not good for the transmission either, especially fifth gear.

Jim
James Neel

This thread was discussed between 28/08/2016 and 31/08/2016

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