I am very concerned. I sent my cylinder block (1970 TR6C) to a well-publicized guy on the west coast to be bored and fitted with new pistons and rings.
It is the new pins that are my concern: the walls are much thicker, the pins weigh 8 grams each more, and they are a much looser fit--in both the pistons and the small end bushes. (I did not renew the small end bushes, as they seemed fine.)
Removing the original pins when dismantling required heating the piston crowns, as did removing and installing the last set I did on my 1968 TR 120 back in the eighties.
Although I do not detect any play with my fingers with the new pins, they are an unnervingly easy slip fit, cold, into the pistons. In fact, they will fall right out of the pistons of their own weight if you hold the assembly on its side..
The micrometer sees this difference, between old and new, as 0.0001 in
The old pins are still a slip fit in the new pistons, but at least they don't fall out.
I'm am thinking about reusing my old pins. So two questions: should I be concerned with less than an interference fit between piston and pin; and is 18 grams of additional reciprocating mass asking for trouble?
Anyone ever heard of stock pins breaking? Why increase the wall thickness and weight? Oops, I'm up to four questions, now, aren't I Oops, now that's five... I'm stopping...
As I posted to Triumph Rat Forum Just some thoughts. a. When measuring tenths of a thousandths the difference in the diameters can often be explained. One has to be extremely careful to make sure both the pin and the micrometer are all the same temperature. Holding the micrometer too long, or even leaving it on a bench exposed to a bright light can cause differences in the measurement you will get. The same goes for the pin by either holding it too long or exposing it to a warm heat source.
b. Since a lot of pistons manufacturers have gone from boring the piston pin hole to roller burnishing it, with the exact same hole and pin dimensions, the pin went from where you had to warm the piston to where it became a slip fit to where it became a slip fit of its own weight. It has to do with the improved surface finish and accuracy of the hole and modern methods of finishing pins.
c. the difference in weight of 16 grams (8 grams x2 pins) means the difference in the reciprocating weight is less than 4% (top end of rod, piston, pin and rings) of a stock assembly and less than 1% difference in the bob weight used when balancing the crankshaft. The piston, and thus the pin, most likely is from JCC. These piston assemblies have been used in these motors, as you received it, without any concerns for almost 40 years. The JCC pins are more than adequate for street use, and have been used in quite a few budget racing motors. If I was looking a get some real HP (60hp plus) I would be looking for a better pin.
d. Comparing REAL AE Hepolite (not Wassell JCC - Hepolite) Standard vs plus .020" pistons there is a difference of 390 grams (195 grams x 2). That is a big difference from 16 grams and these engines are typically bored without rebalancing with little, if any, difference in vibration. The weight difference for JCC pistons also increases with each oversize.
Quote: Anyone ever heard of stock pins breaking? Not in a street engine!
The balance factor is 85% of the weight of the piston assembly and a portion of the conrod, call it 350g (rough guess) X 0.85 = 297.5g, 16g is 5% extra making the balance factor 80%.
the pistons are around .320 grams x 2 = 640 grams the top end of the rods are around 106 grams x 2 = 212 grams TOTAL 640 + 212 = 852 grams. 16 / 853 = 2% of reciprocating weight (both rods and pistons). But if it matters Robert has all of the kit so he can measure all of the bits and do the math. The stock 650 engine was balanced with two 689 gram weights, one on each rod.
BUT the new oversize pistons are going to weigh some where around 20 grams (x2 or 40 grams) for each oversize over standard. So if we are to worry, and few do, the increase in the weight of the piston is going to have a much bigger effect upon the balance factor than the 8 grams difference in the pins.
I tried several scales. They all agree: the stock Hepolite removed from the bike (marked inside as such), with rings (de-coked, no pin) weighs 262 grams, while the JCC .020 over weighs less than a gram more.
With pin and circlips included, I measure a shade under 317 grams for one Hepolite assembly, though I can imagine a rough estimate of 320 per complete assembly is within bounds.
But no way am I finding anything even remotely like a 20 gram difference between the stock Hepolite and +1 oversize JCC for the piston with rings alone. Perhaps the discrepancy is due to comparing two different brand pistons/rings?
However, I find it fascinating to learn that modern piston manufacturing methods have resulted in pins that no longer require an interference fit. Thank you very much for that reassuring information!