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Sunday, 14 March 2010

Death of a Triumph engine

I've just been sorting through some old photos and the one that's attached here still makes me wince!  In my early motorcycling career, I had a pre-unit construction Tiger 100 500cc twin . (See: The passion really starts to get hold)  The previous owner had lavished a lot of care on tuning the engine, with 10.5:1 compression pistons, E3134 sports cams, tuned exhaust and so on.  After owning it for a couple of years, I discovered a small crack at the bottom of one of the cylinder liners.  The crack didn't look recent but to be safe, I decided to have a new pair of liners fitted and a local company undertook the work.

Carefully put it all back together and commenced carefully running it in.  A week or two after the rebuild, I was going up a steep hill a few miles from home and it felt sluggish so decided to pull over and have a quick check.  Peeked in my rear view mirror before stopping and saw an awful lot of smoke - oh sh*t!  Got towed home, stripped the motor and holy heck.....

The resolution of the photo isn't all that good (you can enlarge it by clicking on the photo) but the arrows mark the obvious damage - it's an excellent example of cause and effect.

The cause is that the new steel liners had gradually rotated in the alloy block.  Each liner has 2 cut-outs at the bottom to give clearance to the con-rods when the crankshaft and rods are at half-stroke.  As the liners rotated, they gently shaved the con-rods, depositing fine alloy powder throughout the lubrication system until the motor seized (the effect!).  The broken piston is testament to that!!

I wasn't confident that I'd be able to get every trace of the powdered alloy out of the oil galleries and wasn't about to have it happen again, so gave the motor away for basic parts and got an inexpensive bog standard second hand replacement which ran perfectly until I sold the bike a few years later.

Putting new liners in an alloy block involves chilling the liners and heating the block to ensure a good interference fit.  I've got no proof, but did wonder if the people undertaking the work perhaps skimmed the bore in the alloy block to make fitting a little easier.  Of course, this was denied and I had to wear the problem.  

However, it's incidents like these that ultimately give us a better understanding of things mechanical, even though we don't see it like that at the time!



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