Big End Bearing problem (TR6) Any ideas?

[dasadrew]

A mate has bought a TR6 a while back and we're trying to get a few of its problems sorted.

Car came from USA and has not been driven except in and out of its garage for about 15 years One cylinder was just not firing and was continuously oiling up. Also a lot of oil was being lost.

Head off, and we discover piston #5 was completely shot. Rings were flapping about in the breeze and piston seemed to be a used one taken from another car where it had previously melted. (all this probably took place about 15 years ago - before Internet and Rimmers so to speak, so use of an old donor engine probably was the best repair which could be affected those many years ago.)

The way ahead for our quick repair seemed clear. New piston duly bought and put into place. Bearing shells and crank seemed OK for a few more miles with very moderate wear pattern. Everything reassembled in correct order with all original parts in the same position as before. Car fired up first time and ran sweet as a bird. 6 cylinders at last.

Mate drives about 70 miles without a hitch. Then something seems to start jamming in the engine because the starter has difficulty turning it over. 20 miles later: massive big end banging.

Just stripped down bottom end down:

Bearing shells and journal on the affected cylinder worn and grooved beyond belief on one side of the journal, leading to over 0.5mm ovality.

New crank is now needed, but we're hesitant to install before knowing why this failure happened so suddenly after fitting a new piston.

Any ideas? What could cause such rapid and catastrophic wear on one journal?

It almost seems like the engine was living with a dormant bearing wear going back to a problem on cylinder #5 which had led originally to the previous ower replacing that piston. It almost seems that with a new perfect piston, the loads placed on the big end by correct compression and ignition pushed this dormant wear over the brink. Am I talking nuts?

Cheers

Drew

[Andy Rundell]

oh dear.......you sure you didn't substitute the removed shells for ones made of cheese or not torque up the big ends correctly - cos thats what it sounds like ...............Andy

[dasadrew]

Unfortunately - we were hoping to find some such "obvious" mistake! No luck - big ends were still tight and snug, no bolts sheared or stripped, right way around, even bearing shells installed in the same orientation as we removed them. Even the journal wear is "even" across the width, so it doesn't look like a foreign particle got in there either.

The shells have an even wear pattern but are themselves also 0.15 mm thinner than their brothers on other big ends! I can't think that all this wear could have taken place on just one journal over 70 miles, but for some reason, the big end knock didn't manifest itself with the old knackered piston, but does with the new one.

[jleyton]

I did work occasionally on 6's but (in Manuel speak) I remember nothing.

However, although I think this is very unlikely, a lot of modern cars have a deliberate oil feed sparayed onto the underside of a piston to assist cooling. There couldn't be something like that on the 6 that failed, could there?

Cheers

Julian

[KOY 23]

Sounds like an oil feed problem to me, also that No 5 crank journal was not hardened. Had the crank been ground previously?
John.

[V Mad]

When you fitted a new piston, the clearance between piston and cylinder may have been insufficient, casing pick up and tighness; this would explain the difficulty in cranking. The extra load then caused the big end to fail; unlikley I would have thought but possible?

[dasadrew]

Thanks for the replies so far guys.

The crank is still on its original diameter.

When I took out Piston #5 to examine and then replace it, I took out Piston #2 as well to have a comparison. Since restart, Piston #2 has no problems. Actually, my mate finished off doing all the ancillary bits and made a cock-up with the oil filter bowl seal. He said that oil spewed out when he reversed out of the garage but he noticed it straightaway and switched off. All other journals seem to have been unaffected.

We still can't work out why Piston #5 had been replaced with a used donor many years ago. All cylinder bores are in top condition and are also original size.

The idea that that new piston was "sticky" sounds intriguing. Maybe the head will have to come off to look at that bore....

In the meantime, he's found an eBay seller ga_classics who do complete recon short blocks for £395. That sounds too good to be true, doesn't it?

Drew

[richardthestag]

In the meantime, he's found an eBay seller ga_classics who do complete recon short blocks for £395. That sounds too good to be true, doesn't it?

IIRC from reading books and mags over the years the 6 weak spot is the thrust bearing on the crank. when this lets go you do not have long before you are gifted an expensive boat anchor. Or did I dream it?

[Andy Rundell]

i think ga_classics are the firm who used to supply the recon Stag engines to Rimmers

..........Andy

[Andy Rundell]

IIRC from reading books and mags over the years the 6 weak spot is the thrust bearing on the crank. when this lets go you do not have long before you are gifted an expensive boat anchor. Or did I dream it?

the thrust washers are an area of potential wear on 6 and 4 pot engines - usually easy to spot by pull/pushing on the crankshaft pulley or watching the pulley when depressing the clutch.

Personally i doubt this would be the issue unless the thrust-washer has self-destructed and shrapnel has blocked an oil feed - but if the washer has failed you should be able to see the movement without too much effort

...........Andy

[Marrone Cervo]

hi have you checked for a bent con rod. i had a gt6 engine bored out and ballanced they never picked up on a bent con rod. Its either that or the block might need line boring. Was it tight to turn over when it was first assembled? because that can be a sign that the block has a twist in it.

Peter

[dasadrew]

Hi Peter,

the block turned over OK by hand but, because it's impossible to get a spanner on the front pulley, my mate was doing it by pulling on the fanbelt, so it was difficult to say if there was more or less resistance.

The more I think about it, and the history that for some reason piston #5 had gone stateside in the past, my thoughts are drifting to a very subtle geometry problem on the con rod which resulted from that original failure (whatever it was). As the replacement donor piston was pretty rough, that conrod never saw real loaded action until we put in the brand new piston.

Drew

[Giles]

I would still think a oil feed problem is likely. If it was a block issue would that not show up on the mains rather than the big ends? I would think the feed in the crank is blocked, starving or partly starving the journal and the oil spray from the con rod to the underside of the piston /bore. This is likely to have caused the original and this problem? Just my thoughts. Have never worked on a Triumph staight six though.