I wanted to share my current build information.
I am rebuilding a 72 Commando engine back to stock-ish and was performing a routine check of the components to make sure they are balanced to one another.
Like a good o.c.d. mechanic I try my best to get all the pieces to balance within 0.1 grams
These JCC pistons are a 12.9 grams out!
- The color of the aluminum looks the same, the external markings look the same and the machining look very much the same - at a closer look the skirts are different finishing widths and the underside "forging" is extremely different - but hidden by the pin boss.
I have been installing our eastern friends pistons for a very long time now. Their quality control has been good to me and I have never lost a piston even in my most aggressive attempts to do so. They are often a very nice "forged" piece and if you call them their customer service is excellent. It is almost as if they are telling you want you want to hear
- The most I have ever seen these pistons out-of-matching is 2 full grams and they were suspect out of the box (different color metal, forging/casting differences etc)
A closer look revealed that the heavier piston has a very different pattern..almost as if it was the last of that run or pistons in the dies.
The lighter piston has much "crisper" lines and markings. Very obviously made on a different set of dies
JCC claims these are forged - I am beginning to think these are cast. There is much evidence to back up a casting argument.
It is visually apparent there is a major difference on the underside.
Moral of the story
- At a glance even a seasoned builder with intent to overlook the balancing of the engine would not notice that these pistons were made very differently. It would take a moment of comparison to see
- A 12.9 gram difference would make this bike a lemon. A novice getting into the sport of British bikes may assume "it is what it is" and quickly jump on to a modern bike or a Japanese counterpart missing the best of what these machines have to offer.
- It's not just our British parts! I regularly refresh Honda and Suzuki motorcycles - A big LOOK OUT moment when rebuilding these is to carefully heat cycle any N.o.s. pistons you may use as there is a 100% chance you will "soft" seize them in the first few minutes no matter how hard you try not to. The old cast pistons were terrible then and now that they had time to sit on the shelf for 40+ years they are still just as terrible - like a bad wine.
The pictures may help the visual. I hope this information passes on to you before you bolt the engine together in haste
- I used a sharpie to mark the wider skirts
- You could never tell at a glance these were so completely different (my machine markings are present on the heavier piston) but closer look revealed the first tell tale - the oil hole for the piston pin is not chamfered
- Much wider skirt on the right - BUT this was not enough material to help. I was able to get 1 gram closer when I first matched the skirts
I am trying to collect a better picture of the supports under (above when in the installed position) the piston pin boss. The bridge tower type beams that support the pin boss to the piston crown are approx 30% larger and rounded/organic looking as a crude casting may be. The support beams also have a bridge between them that connects to the piston pin boss - all of this on the heavier piston
The lighter piston has very pronounced forging/casting lines as if the supports are almost measurable as a rectangle section of beam that would be approx 3x5 mm - much smaller than the other and much lighter in appearance