Piston mystery

[handi_andi]

Hello all

My damaged piston mystery has just doubled on removing all four pistons for new rings and big end bearings.

Originally I found damage on the top of piston three on the intake side, as shown in the picture below. On removing the rings on piston 2 though I found that a section of the piston between the two compression rings was missing and giving the black nature of the surface, as shown in the picture below, it looks like it has been cracked for some time. Also, all four pistons have light scrap marks and surface dulling on the fore and aft sides of the piston skirts. Naturally I am concerned that I have two broken pistons and am now wondering whether there is some common cause.

By the looks of it the crankshaft has been reground to 10thou undersize, based on markings on the big end shells, which isnt that surprising. More surprising is that one of the con rods looks like it is newer than the other three. All four conrods are marked 125E Front C on one side, however, on the reverse side the older three are marked SDF4, where as the new looking one is marked SDF6, does anyone know if this is significant?

In addition I was surprised that the little ends could not be regarded as finger push fit given that they required a piece of plastic pipe and a mallet to knock them out.

Anyone got any ideas as to what the problem could be, as am feeling a little uneasy at just putting two new pistons back in without some idea as to what has gone wrong.

When I brought the car the ignition was far from near being correct and the engine was knocking a far amount. In addition, when the car was last running I had been having problems with overheating as I couldn't get the mixture correct due to dirty carbs so both of these things could have contributed. One amusing thing, and only a twincam could manage this, well ok maybe an old 2 1/4 landrover engine could too, was that the compression before I removed the head was equal on all cylinders but a bit in excess of the book figure.

All thoughts well and truly welcome after a long weekend of pulling the engine, disassembling and cleaning up.

Cheers

Andy

[Craig Elliott]

Andy

Were the missing bits from the pistons still in the engine?

Craig

[handi_andi]

Craig

The piston two section was still in position, i saw a crack and took another a closer look and out dropped the segment. Piston 3 segement has never been found and there is no damage on the head either. There were also no metal bits in sump when i cleaned it earlier.

Cheers

Andy

[twincamman]

I hope you didn't use quick start or ether on it > That will crack pistons ---ed

[handi_andi]

I believe I may have been guilty of using it in the past yes. No obvious damage on pistons 1 & 4 though.

Andrew

[Craig Elliott]

Seems odd that the broken piece from the top of the piston in the first picture has gone AWOL - could someone in the past have realised that something was up, lifted the head to have a look, didn't like what they saw and hastily stuck it back together and passed it on...

C

[handi_andi]

Craig

When I first saw the damage on piston 3 my instinct was that someone had damaged it fitting it, shoved the head on and tightened up. It looks like it has been recently rebuilt given the lack of any wear lip on the piston bores. Now the damage on piston 2 and i am very suspicious about the whole thing. Am loathed to do a complete rebuild, however, if I can not get to the bottom of this mystery then it is looking more likely I will have to take that route. Something just isn't adding up with the whole thing.

Cheers

Andy

[1964 S1]

Hi Andy, it looks like a "rebuild" to me, how complete is up to you. I'd have the crank and block measured at a competent machine shop. If things are true, buy the bearings, pistons, rings, gasket set and put it back together if the valves look good. Was it running when you took it apart, if yes, it's more evidence that our engines are near bullet proof? The connecting rods are the question mark in my thinking. Good Luck, Eric

[twincamman]

welll if you don't do a complete motor rebuild you will never have complete confidence in the car -----bite the bullet and do it right so you can enjoy the car ---new pistons bearings and water pump -gaskets- VALVES AND SEATS etc -mine was $3500 in parts complete but I know it wont let me down FOR 50000 MILES - -ed ---- as to which pistons get the overload of ether and crack when it fired is a crap shoot-------

[handi_andi]

I am coming to the conclusion that a rebuild is in order, just don't want to take it further than necessary, although as pointed out without doing everything myself or getting a competent engine shop to do it will I ever have confidence in the engine. I was originally thinking that I could have a bent crankshaft, however, given that there are 5 bearings in the engine i couldn't square that in my brain. This morning though I had a brain wave and weighed the pistons and conrods. All of the conrods are within 2grams of each other. The pistons though are a different matter:

Piston 1: 484grams
Piston 2: 508grams (excluding missing piece, cos its sat outside somewhere)
Piston 3: 520grams (excluding missing piece, cos its AWOL)
Piston 4: 488grams

Could it be a simple case of vibration fatigue on the pistons given that the difference in weights are well outside the allowable differences?

Cheers

Andy

[GrUmPyBoDgEr]

Hope you don't mind Andy but I've sent your piston pictures on to a man who earns most of his living identifying piston problems.
If he honours me with a reply I'll post it here.

Cheers
John

[handi_andi]

John

Thanks for that, don't mind at all, all information is information into the melting pot. Want to get to the bottom of the problem before I do anything.

Cheers

Andy

[handi_andi]

Just measured the minimum distance between the conrod holes and they are all within 20thou of each other, which doesn't seem too bad? Will measure the bores later but am expecting them to be near spot on for size, however, have no easy way of meausing them for trueness unless anyone has any good tricks.

Cheers

Andy

[handi_andi]

Evening all

Have managed to do some measuring this evening with mixed results.

Firstly the good news, I measured all four bored in four different directions and all the measurements came in at between 18 and 19thou undersized, which given there is some glazing fits with it being bored 20thou overbore. So think a quick hone and they should be fine.

The crankpins though, which were measured using a vernier calliper and again in four directions, varied between 1.925" and 1.930". The measuring could be slightly out so will try and get a suitable micrometer in the near future, however, I don't believe I would have measured that far out. My copy of the workshop manual states a cranpin diameter of 1.9370" to 1.9375". So the crankpins are between 7 / (12) and 12 / (17) thou undersized based on minimum and (maximum) base sizes. The big eng bearings are 10thou undersize which fits with the measurements, however, am concerned that the crankshaft isn't close enough and should be reground, would like to avoid it if I can as don't really want to have to strip the engine further if I can help it. Hence any pointers on the alloable measurement range would be good thanks

Cheers

Andy

[types26/36]

[quote=" The big eng bearings are 10thou undersize which fits with the measurements,Andy[/quote]

I think that would be "bearings are 10thou OVERSIZE" .....the crankpins are undersize, why dont you get some PlastiGauge to measure the crank/bearing clearence.
http://cgi.ebay.co.uk/Plastigauge-Engin ... 286.c0.m14