Hitchcocks Forum

Just had the bores on the Ducati 860 bored out to fit new oversized pistons. The interesting thing (for me at any rate) was the fact that the bores were honed to the oversize 87mm exactly and the piston was designed to fit in there with a 2.5 thou clearance. This seems so blindingly obvious as a way of avoiding miss communication with the engineer that I'm surprised it hasn't always been the system. I was also told that because of the modern ring profiles and metallurgy and the fineness of the bore finish to assemble the motor with a minimal wipe of oil as slathering them with lubricant would result in rapid glazing and a prolonged period of oil burning and very slow running in. The times it would seem are a changing!

Hi SIMON,------ actually nothing new there... In Mechanical Engineering, when it comes to quantifying the types of a "Fit" of one item into another then you can either take the shaft, ie the round solid item, as the datum size and vary the hole diameter to get your clearances etc. ( This is known as "Hole tolerance")..., or you can take the hole as the datum size and vary the shaft sizes to get the required fit. This is known as "Shaft tolerance"...There is also overlap areas as well... I must stress that I have simplified for illustration only. ..Have a look at this}------ http://en.wikipedia.org/wiki/Engineering_tolerance ------ Your pistons are not perfectly round as such, but are usually slightly oval and tapered to allow for the different rates of expansion between the crown (especially on a high compression piston), the gudgeon pin area, and the skirt.....On our RE Bullets, have a look at the differences between a standard piston, our hosts 8.5:1 forged piston and the Electra X piston with its slipper-type design... To para-phrase }--- we are trying to fit a semi-square plug into a round hole !!!, and accurately at that !! Incidentally, my Classic 500 with an 8.5 forged piston, is running with 8 thou skirt and 10 thou head clearances (Cold) in an Aluminium barrel...and straight 20W/50 oil...Sweet as a nut when hot, at 60 mph and uphill, but "slaps" a little on start-up in this cold weather...... I can live with that. PS---- You can imagine the childish smirking that went on as this item was first taught to us 16-year old Apprentices back in the mid 1960's ??

Yes I've played a bit with cam grinding using a four jaw in a myford with some success in the past but the point i was making was simpler (as befits my nature) the fact is most bores are piston size plus clearance so you have a 80mm piston measured at the bottom of the thrust faces and you add 3.5 or 4 thou or whatever to the bore. This set of pistons came with the simple instructions bore to 87mm which to me seemed genius as there was no room for error or engineer preference.