It could be a degenerative condition......It has to do with the side play and rocking of the rod little end, coupled with having a piston whereby the little end rod to piston clearance is too much......That is the gap that you can move the piston sideways on the rod with the pin in place.......If the rod supported by its big end does not rock too much from side to side, then the issue is not so bad.......If this condition worsens due to wear, or more clearance given due to racing specs (verses a road engine).......In service, the rod can and will favor rocking to one side.......This in effect jams the pin in the little end......This then causes the friction to go skyward.......Thus the pin and little end of the rod get hot and turn blue.......With our new pistons we had made (at a huge cost) by CP in the USA......we had them make the inside of the pin boss on each side much narrower.......so we now have a side to side clearance of about 30 thou total......we also got them to move the pin to the rear towards the thrust face.......we also got them to move the pin much higher up towards the crown to offset the normal huge "Top heavy" pistons you end up with when running high compression......All these features should hopefully stop the "Also very common" issue of these pistons suffering from bad scuff marks on the front and rear skirt faces......I have ran the engine for about 1 1/2 hours total with this new piston and so far it all looks very promising.......My earlier hand finished piston suffered major cracks around the pin boss on both sides.......16.5:1 Comp will do that.......Too high, so back down to a better 14.8:1 now.......It is my belief that this excess side clearance between the rod and the pin boss on the underside of the piston causes this excessive heat build up.......And you should not run too tight a fit of the pin in the little end bush as well.........Especially on a race engine.
on that note, having seen both authentic factory racing Pantah (TT1/F1, TT2) and beveldrive (NCR900) and modern era efi (851SP1/888SP4/955) Ducatis they all have the same browning on the top of the conrod; and I suspect its not brand related.
the 851SP1 and 955 with titanium conrods even had oil channels in the conrods to the small ends to minimalist friction and excessive heat build-up.
some useless information:
as all conrods no matter polished or forged or welded in combustion engines they are all exposed to the same: the heat produced form combustion. and the most modern pistons have the small ends very near the underside of the deck so more exposure.
given the heat exposed surface of the conrod I expect engines with long conrods to be more discoloured than short conrods as the conrods oscillate a larger angle and thus a larger exposed area; long conrods oscillate a smaller angle so the heat is concentrated to a smaller area.
before, engines had the crankshaft dictate where the small end is located in the piston and the gap between the piston bosses and the small end are large to accommodate the location of the conrod. that effectively meant, however small, the conrod centreline could be offset the piston centreline and impose a tilting force on the conrod, usually leading to the bigend/conrod running surface loaded on an edge rather than over a surface.
nowadays, with plain bearing bigends being the standard on almost any multicilinder combustion engine, the piston now dictates where the conrod will be: the conrod is much more confined to the inside of the gudgeon pin bosses, less than 1mm in fact, and the conrod is allowed to slide axially along the crankpin thus giving it a degree of freedom on the crankpin to find its position without the worry the crank may force it over to a side or rub shoulders with the conrod beside it. this is based on the last 1200 Panigale V2, and I suspect this design practice is common amongst many brands now.
coming back to greg's angle:
I think its not only the offsetting of centrelines piston/conrod, but also the longer conrods have the heat much more concentrated on the same area with the smaller oscillating angle it makes. a bit like aiming a flame torch on one spot of the conrod versus moving the torch sideways.
Vincent pistons have the gudgeon pin say halfway the piston, whereas the Royal Enfield piston has the gudgeon pin approx ¼ total piston height from the bottom.
and the running clearance between the gudgeon pin and small end and piston bosses is a balance between being free enough to get any oil mist between for cooling but confined enough to avoid hammering and gripping, over a range of temperatures but notably in the running temp range.
had a mate's Husqvarna SMS630 in the garage once for an overhaul, have titanium valves and very soft springs, and never ever do any lapping of titanium valves it said in the manual, because you will grind off the coating (mentioned earlier before?) and expensive shit happens very soon after running the engine. used dummy valves to lap off sharp edges after dressing the seats, clean properly and put in the titanium valves. engine still running 5 years later and close to 80.000kms now.
titanium is nice stuff to use for components subject to oscillations to get the reciprocating masses down.
using aluminium brons for valve guides is in my book a definite no-go! phosphor bronze and lead bronze and cast iron is fine, but no way aluminium bronze! have seen many valve guides being pulled out of heads and top-end components seriously deformed/destroyed due the guides sticking on the valves and things getting squashed into places they shouldn't go. also had a trial with alubronze plain bearings for 50mm mainline diameters, went fine as long as the revs did not exceed 3500rpm. its tough material with high load carrying capacity, but
not high surface speed friendly and particle bedding friendly as the other PbBronze and PhosBronze. used them for Brons / Waukesha gas engines in the camshaft bearing caps not a problem in the world, then again these engines don't go above 1500rpm (750rpm camshaft speed) and the mechanical injectors and heavy springs and valve train components could wear out these cams and bearings. but 4000rpm on 50mm dia mains, yep that locked the engine on the dyno with an oil pump capacity (pressure and volume) on par with a tuned Ducati 1198S Corse race spec (200bhp and approx 160Nm torque at the wheel).