May I revisit, FWIW, this Q of metal spraying of journals, big ends in particular?
Last week the drive side mainshaft of my A was sprayed and ground to size to give an interference fit for the inner race, and at the same time the thrust face on the outer side of the crank web was built up to compensate for wear caused by the inner race's having spun at some point on the shaft and damaged but not broken (luckily) the shims. Crank end float was north of 20 thou but the engine was running perfectly when I decided to take it apart. In fact it had done 300 km the day before I grasped the nettle (August 16th) and decided to have a look, just because I felt a look was overdue.
The work was done by my favourite engine shop, T&L Engineeering (with whom I have no connection apart from as a regular customer for awkward jobs I can't do myself). The net of it was that after their careful ministrations and measurement there is now no need of any shimming of the bearing, the race is a modest interference fit as it should be, and the end float is exactly 2 thou. This job cost £109 excluding bearing and seal. Other jobs done to get the crankcases back to 'good as new' cost somewhat more, but it was interesting that even after 50K+ km in my hands, and who knows how many km before I got the thing as a dirt cheap unknown bitza beesa, the timing side bush and journal were still round and excellent at max 1.5 thou, and the big ends were unworn on -30 and really didn't even need new shells. All in all, testimony to religious 1000 mile oil changes I suppose - and more controversially a tribute to modern multigrades. No oil filter, no air filter, no mods at all, just a standard iron head large journal thick flange flat-top piston GF spec engine which has been treated semi-kindly most of the time, but has had very little attention in years.
In a long discussion about metal spraying, I raised again the question of building up cranks, especially big ends. This was something I had previously talked to them about in relation to a very tired 650 AMC crank from a defunct engine from one of my other bikes which I am raising from the dead. T&L would have been happy to spray it back to standard had it been necessary (in fact it took a grind to -60) as the radii were still there, and they say they have done a steady stream of these awkward and hard to find three bearing cranks over the years. Loads on big ends (as opposed to things that rotate concentrically) don't seem to be part of their equation.
They have built up 'thousands' of m/cycle cranks, almost always including big ends, and have had only one known failure, where an oil pump problem caused instant death. They say that they won't go more than 100 thou down on standard to get to a good starting point, and that there have to be no signs of lack of structural integrity. If those things are OK, they say that with a thin layer of molybdenum properly applied first as a base / key, and cast iron (or whatever is required) applied afterwards, there should be no problem. Technology advances have helped, they say, but the number one thing is the skill of the person on the tools.
The critical path is engineering expertise not marque knowledge, and I have to say that I will only ever send any of my engines to these folk for tricky repairs, even though it's a long way from France where I live. Were I in the position of needing to salvage a crank I couldn't replace more cheaply, then I'd have no hesitation at all in going there for advice and, probably, a solution. This week they have replaced a set of valve seats, and welded up and skimmed true a pair of AMC cylinder heads with gouges on the mating surfaces etc etc, all at a fair price, quickly, and well.
I take these guys at their word, so my 30 year old experience of disaster mentioned somewhere above is, I am now pretty sure, unrepresentative of what is possible in the right hands today.
Cheers, Bill
