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Crankshaft sludge


This morning I pulled the crankshaft apart on my barn find Domi 99. In a previous post I mentioned that a large chunk of the drive side oil seal support face in the left-hand crankcase was missing. Well I think I found a lot of it today - the sludge in the crankshaft was solid and much of it sparkled more than Fools Gold. The ‘best’ bit was a chunk of aluminium 5mm long and 1mm thick (photo below). It should make a good doorstop  :-)

The nuts holding the crank parts  together were really tight and on the first couple of turns of each nut a small bit of the thread broke away as a thin ‘wire’, but still attached to the nut. Are they special nuts or does the ‘wire’ thing indicate something else? The nuts weren’t centre-punched but maybe the bolts were - it didn’t look like it but maybe the thread got smoothed out as I was undoing the nut.





The photos looks like the lumps of clinker you would expect to find in the bottom of a steam loco's fire box. It looks like an amalgamation of alloy swarf, tar from old oil and possibly what's left of old oil seals that has turned to concrete over time. If you are really interested you could try to dissolve it in petrol or thinners to find out what it is. Whatever the 'clinker' is it shouldn't have been there.

What's more alarming is the fact that there is alloy present and, from your photo it look like there are some fairly large bits of swarf. You could expect to find some fine metallic particles in the oil from the initial running in of a new engine but not what look like pieces of swarf, When you have cleaned the inside of the crank case a general inspection will soon show where any pieces of the crank case have suffered damage. .

Regarding the thin wire on the crank bolts/nuts. This sounds like the threads have stripped, the wire being the actual threaded part. Whether or not the nuts were punched or not isn't an issue as undoing them would more than likely remove the marks anyway.

Best of luck and I hope there is no serious damage.




Hi John,

Thanks for the reply. The sludge is as hard as stone. The engine has had a very hard life - the bike’s got 91,000 on the clock and there’s plenty of evidence that it suffered a major catastrophe at some point. The barrel skirts were smashed and there are a couple of large gouges out of the inside of the crankcases. Popular opinion is that one or more conrods let go at some point. The bike was then chucked in a shed and that’s where it rested until I came along and paid silly money for it. Oh well, it’s all part of the joy/madness called motorcycle restoration.



No problem, touch wood nothing like that has happened to any of my bikes over the years but I have come across similar problems in my former working life and in present volunteer engineer role at a local heritage narrow gauge railway.

I 'inherited' a load of Domi bits and pieces from the former owner of my 1960 99 Slimline. One piece of 'spare' is a crankcase which has seen some damage, cracks in the case which I've repaired and some damage to the to the top rear joint, the obvious cause was the alternator side big end bolts let go and chewed a groove in the case.

Best of luck with your project.


Sludge is not surprising, as that is what trap is for, but before it can get there it has to pass through oil pump to tank and then back through gauze filter and pump to crank, So I would investigate tank too, is there a filter gauze in there, and how much sludge has settled at bottom over the years, you may be surprised. I certainly was with a commando which has a paper filter in tank return, It sprang a leakin the tank where bottom fixing stud is, so needed taking off and brazing up. Once well cooked a quarter inch layer of sludge peeled off the bottom (inside).

good luck with your cleaning



My first big bike was a '68 Bonneville, that I bought off a student friend of my brother for £220.  It was in very tired Slippery Sam style cafe racer trim.  I put it back to standardish condition and used it to travel between home and my RAF training base.  Being new to 'big' bikes, i was fairly cautious and kept the speed down at first.  The first time I took it up to 80 (on the Tring by-pass) it partially seized.  After letting it cool down, it started okay and I road home to NE London.  A top end strip, revealed heavily scored pistons, so I replaced them (the bores were okay) and checked for overheating reasons, but nothing was obvious and putting oil pressure gauge on, showed decent pressure.  I continued using the bike and all seemed okay for a couple of months while running in the new pistons and commuting home at weekends (100 mile round trip).  Thinking everything was now fine, I took the speed up to 80 again and, yes, the same thing happened - doh!  

This time, I stripped down to the crank to see what as going on.  When I removed the sludge trap plug, I was taken aback by not having the expected sludge trap tube; in fact there did not even appear to be a bore where it should have been, just a solid, hard surface.  It took some minutes to realise that the hard surface was in fact 'sludge;'.  I got a tee-handled bradawl and started screwing it into the hard surface; this was not easy - it was like screwing into hardwood.  I eventually dug out all. the sludge and uncovered the sludge trap tube.  The entire bore was packed with this hard sludge deposition.  To this day I am amazed that any oil had managed to get through that mass, never mind enough to lubricate the bores sufficiently to be able to sustain moderate road speeds - it says something about Triumph plunger pump efficiency!  I should add that after that rebuild and more running in, the seizures did not occur again, thankfully. 

I am about to split the crank from my Mk3 Commando, a bike bought as a tired, butchered wreck off eBay; it will be interesting to see what is inside this one.



I was recently reading Ricardo writing about engines in the 1920s. Oil filters were discussed and declared not to be effective since they could not remove the smallest particles which were most numerous and therefore created much wear. Hence the centrifugal 'filter' provided by the sludge trap. But of course they don't clean themselves when full.  I guess that modern detergent oils which hold particles in suspension until they reach a modern filter should make the trap redundant. Modern vehicles don't have them (do they?)


How effective an oil filter is depends on the rating of the filter medium, most auto filters are 30 microns but as they collect particles their effectiveness increases as the gaps close up. You can also get 5 micron filters for hydraulic systems that will fit but they are more likely to clog up, my preference is to have a strong magnet in the system to collect the ferrous particles of any size and run a normal auto filter for the rest.  


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