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Piston weight balancing - Out of the box way off! #777699 07/01/19 5:12 pm
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Denis J Offline OP
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Hi All
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

[Linked Image]
[Linked Image]
- I used a sharpie to mark the wider skirts
[Linked Image]
[Linked Image]
- 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
[Linked Image]
- 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

Last edited by Denis J; 07/02/19 6:08 pm.

...no idea what your doing
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Re: Piston weight balancing - Out of the box may off! [Re: Denis J] #777704 07/01/19 6:54 pm
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triton thrasher Online Content
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Who sold you odd pistons from different batches?

Who said they were forged?


Amateur Loctite enthusiast.
Re: Piston weight balancing - Out of the box may off! [Re: Denis J] #777746 07/02/19 6:07 am
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Allan Gill Online Happy
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I’ve had NOS hepolites which were different weights. Had no fears of taking any of the skirt of the match them up.

Not sure how much these pistons compare to A65 or A70 or B44 even but all of the above I have are JE pistons. The A65 being around 375g from memory complete with pin/rings and circlips the B44 being around 325g complete.

I usually match the pistons only in weight then weigh them as a set, there’s often a difference in pin weight too, with careful machining this can be reduced also.


beerchug
Re: Piston weight balancing - Out of the box may off! [Re: Denis J] #777749 07/02/19 7:03 am
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Mine were very close but came in one box, you have 2 boxes so batches mixed up ?

Re: Piston weight balancing - Out of the box may off! [Re: Denis J] #777795 07/02/19 6:11 pm
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Denis J Offline OP
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ON JCC's website they claim they make forged pistons. On the phone with them it was the same. Regardless they seem to be good. I have run them flat out in a few hi comp bikes with no issues, but it has always been a habit of mine to check them carefully

I have been lucky on the last 2 engines (A65 and TR6) where I was able to swap pins and rings to get the piston kits to match exactly. 12.9g is too far out

I can only assume the batches are different because of the appearance of the underside. This is an assumption of course.


...no idea what your doing
Re: Piston weight balancing - Out of the box way off! [Re: Denis J] #777796 07/02/19 6:18 pm
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Denis J Offline OP
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Please excuse the in progress grinding:
[Linked Image]
[Linked Image]


...no idea what your doing
Re: Piston weight balancing - Out of the box way off! [Re: Denis J] #777895 07/03/19 6:58 pm
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13g is a lot to shave off. I’d be looking at the band around the bottom of the inner side of the skirt. Knife edging the skirt on the inside again (although I’d end up doing this on both).

Have you placed a micrometer on the wall of the piston?

Often a manufacturer will make one piston to cover at least 2x oversizes, then the finished product is Machined to size.


beerchug
Re: Piston weight balancing - Out of the box way off! [Re: Denis J] #777896 07/03/19 7:10 pm
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triton thrasher Online Content
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They look like cast pistons. That might imply a smaller skirt clearance than for forged pistons, so I’m told.


Amateur Loctite enthusiast.
Re: Piston weight balancing - Out of the box way off! [Re: Denis J] #777932 07/04/19 11:09 am
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pushrod tom Offline
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Yes, the Jcc are cast. I think I would have contacted the supplier and insisted on a more closely matched set. PRT

Re: Piston weight balancing - Out of the box way off! [Re: Denis J] #777938 07/04/19 4:49 pm
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Denis J Offline OP
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New set on the way
- I had to be reminded by a friend that a weight diff that big is really only manageable when building a small block Chevy or bigger (4" bores on up)


...no idea what your doing
Re: Piston weight balancing - Out of the box way off! [Re: Denis J] #777949 07/04/19 8:07 pm
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Allan Gill Online Happy
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Have you convinced the supplier to take them back or have you had to shell out again?


beerchug
Re: Piston weight balancing - Out of the box way off! [Re: Denis J] #778388 07/11/19 6:11 pm
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Samsmc1 Offline
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I always thought that JCC and all Asian made pistons were cast and not forged.
That said I don't understand how they can suggest that the clearance on the current Hepolite (JCC) pistons for an A65 I just bored last week require .004.
I always thought that cast pistons were in the .002 range for skirt clearance.
Speaking of balancing British motorcycle engines, can anyone tell me where to look for recommended balance factors?
There was someone on the east coast who specialized on Nortons maybe 20 years ago that gave a friend of mine a balance factor to use on an Atlas.
That motorcycle was as smooth as any 90 degree Ducati V Twin or Japanese four at 75 MPH. In fact, he ran the yearly solstice run across Texas that year and made it with 20 minutes to spare.
British 360 twins can be made to run smooth in narrow RPM ranges if you know the correct balance factors.
Sam

Re: Piston weight balancing - Out of the box way off! [Re: Denis J] #778398 07/11/19 7:19 pm
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Allan Gill Online Happy
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My A65 is balanced to 62% “dry” it’s a real smooth motor! However other factors like mixture, ignition timing and cam all seem to make a difference.

Rule of thumb on pistons is 1.5 thou per inch. I wouldn’t use anything less than .004” on an A65, that’s with JE forged pistons...


beerchug
Re: Piston weight balancing - Out of the box way off! [Re: Denis J] #778876 07/16/19 10:32 pm
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Samsmc1 Offline
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Allan
When you say dry, are you saying no oil in the sludge trap?]
I set these JCC pistons to .0045 last week when I bored this pup. BSA's always scared me.
Same with the timing side bush - I set it at .002.
I had a BSA someone supposedly set to spec get tight on me when riding it after it was just assembled. - that taught me a lesson.

Re: Piston weight balancing - Out of the box way off! [Re: Denis J] #778911 07/17/19 6:11 am
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Allan Gill Online Happy
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Hi Sam. Yes that’s correct no oil in the trap,

I agree also on the end float, they will nip if too close as I’ve found myself (all be it with the end fed conversion)


beerchug
Re: Piston weight balancing - Out of the box way off! [Re: Denis J] #778913 07/17/19 6:26 am
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NickL Offline
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2 thou and sometimes less is common on water cooled engines for piston clearance
but on an air cooled engine as Allan says 1.5 thou per inch of bore is a pretty good guide
for cast pistons unless specified by the manufacturer.
I have never seen a set of JCC/Emgo pistons that are forged, the ones i have fitted are all cast.

The timing side bush clearance is ok at anything from 1-2 thou, go any bigger and you'll suffer oil pressure losses.
I doubt you would seize one if the clearance was 0.5 thou though, as long as the crank runs true.
End float can be 1 thou unless you start the thing up and rev the goolies out of it straight away, the cases
will grow faster than the the steel crank. They all have a few thou more end float when they are warm.

Re: Piston weight balancing - Out of the box way off! [Re: Denis J] #778915 07/17/19 7:11 am
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triton thrasher Online Content
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Originally Posted by Sam
always thought that cast pistons were in the .002 range for skirt clearance.


Your bike will seize if you use 0.002 inch.

Last edited by triton thrasher; 07/17/19 7:13 am.

Amateur Loctite enthusiast.

Moderated by  Allan Gill 

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