Emgo Pistons Mismatched

I bought a pair of Emgo + .020" Pistons for my 750 rebuild, not because they were inexpensive, but because I'd heard of a few positive experiences with them and no issues. And being JCC, the quality was good.
Well, I've got an issue now. Crank balancer just told me there's 14 grams difference from right to left so he's balanced @ 54% to the lightest side (R). I confirmed it on digital scales at home. My mistake for not checking first and "Ya get what ya pay for".
Anyone come across such a large discrepancy - 223g (R),237g (L) ?
Where to go from here ?
Try and find one to suit ? - Sounds near impossible.
Or start again and back to the machinist ?

Why did the machinest check and then do nothing to rectify the issue. Take the heavy piston down to the light ones weight, then both sides will based on the same 54% he calculated based on the lightest weight.

I'm with Kommando, that's normal practice IMHO, although its not an excuse and is not a good sign from the piston manufacturers perspective!

Use a dremel or similar to remove metal from the inside of the heavy piston, and / or, if the weight is too much, machine tapers into the gudgeon pin on the heavy piston.

Fast Eddie said:

I'm with Kommando, that's normal practice IMHO, although its not an excuse and is not a good sign from the piston manufacturers perspective!

Use a dremel or similar to remove metal from the inside of the heavy piston, and / or, if the weight is too much, machine tapers into the gudgeon pin on the heavy piston.

Click to expand...

agree, but 14g is quite a lot of material to remove. It might be worthwhile spending 15 min with a vernier caliper (well, they are all digital these days but you know what I mean..) and try to see if there is a significant difference in skirt thickness & length, crown thickness, gudgion pin boss size, etc., then concentrating on these areas with your trusty Dremmel.

/Steve.

The weight difference between a straight and a tapered wrist pin is around 20 g so it should be possible to reduce the total weight by grinding the inside of the wrist pin.

I'd send the pistons back and ask for another set.

In nearly 40 years, I've NEVER had JCC / Emgo / Cycle Craft pistons vary by more than 2 or 3 grams.

I have not seen much variation with the JCC pistons. 3 or 4 grams and not more than a couple tenths in size.

The worst I have seen was the last set of GPM pisons I got. 21 grams and .004 difference in size. Jim

Q; Does the weight include the two wrist pins?
If so does one of the wrist pins have a taper on the internal bore?

kommando said:

Why did the machinest check and then do nothing to rectify the issue. Take the heavy piston down to the light ones weight, then both sides will based on the same 54% he calculated based on the lightest weight.

Click to expand...

I wish it was that simple. In this neck of the woods it's hard to find a machine shop that's prepared to take on vintage motorcycle work. I found one to do my rebore but, unbeknown to me, he doesn't do crank balancing and sent my crank out to a third party that does his cranks. This bloke's a secret squirrel type and doesn't communicate well so I assumed he wasn't interested in lightening the piston because it was too much of an ask.

SteveBorland said:

Fast Eddie said:

I'm with Kommando, that's normal practice IMHO, although its not an excuse and is not a good sign from the piston manufacturers perspective!

Use a dremel or similar to remove metal from the inside of the heavy piston, and / or, if the weight is too much, machine tapers into the gudgeon pin on the heavy piston.

Click to expand...

agree, but 14g is quite a lot of material to remove. It might be worthwhile spending 15 min with a vernier caliper (well, they are all digital these days but you know what I mean..) and try to see if there is a significant difference in skirt thickness & length, crown thickness, gudgion pin boss size, etc., then concentrating on these areas with your trusty Dremmel.

/Steve.

Click to expand...

I've got a Dremel, but no practical experience in taking any weight out of a piston let alone 14 grams.
I think it would run the risk of compromising the pistons structural integrity.
But I'm keen to take a few measurements when I wake up (just finished night shift).

grandpaul said:

I'd send the pistons back and ask for another set.


In nearly 40 years, I've NEVER had JCC / Emgo / Cycle Craft pistons vary by more than 2 or 3 grams.

Click to expand...

I should. Then I'm back to square one. Then there's shipping costs - OZ to U.S. and back.
I'll send him an email tomorrow - he should at least know it's a F. U.

comnoz said:

I have not seen much variation with the JCC pistons. 3 or 4 grams and not more than a couple tenths in size.

The worst I have seen was the last set of GPM pisons I got. 21 grams and .004 difference in size. Jim

Click to expand...

I found it hard to believe at first so I dropped into the shopping centre on the way home and bought a set of digital scales. Less than a gram difference between our figures.
Interestingly the pistons have 2.894 written on top of one and 2.894/5 on the other, indicating acceptable size differences at least.

Bernhard said:

Q; Does the weight include the two wrist pins?
If so does one of the wrist pins have a taper on the internal bore?

Click to expand...

The weights are pistons only.
The pins have a straight bore.

Heavy piston on light rod, light piston on heavy rod........perfect!

I've also heard of them coming with mis-sized pin clips. Emgo pistons are from Taiwan or China aren't they? Better pistons are available for the heart of your motor. Cheaper parts are OK for the exterior where you can keep an eye on them and replace them easily if necessary.

jseng1 said:

Cheaper parts are OK for the exterior where you can keep an eye on them and replace them easily if necessary.

Click to expand...

Well.......when you put it like that. I haven't even bought cheap parts for the exterior, and this IS an area of the motor that I haven't thrown money at. I think I'll pack my bags - I'm goin' on a guilt trip :wink:

Didn't take long to spot the difference. I didn't even turn my digital caliper on.
Definitely poor casting on the heavier piston.
Image 1 shows much more clearly defined contours inside the head of the lighter piston (R), especially around the pin housing gussets.
In image 3 it appears the gussets are thicker on the heavier piston.
I'm confident now that there's plenty of extra meat that can be trimmed without causing any structural issues.
My problem now is jseng1 has planted the seed.

I have a spare set of JCC 0.020 over pistons with Hastings rings that I am willing to sell at decent price.

Happy to weight them to check weights are close to the same.

Just PM me if interested.

regards,

JS

Well, far be it from me to nurture the seed that has been planted... but...

These are the JS Motorsport pistons I recently fitted after the first build 'nipped up' and scored one piston. These are the high comp 850 versions, I had them treated with a 'Diamondyze' hard anodising process. I have no previous experience using this treatment method, so we'll see how it pans out:

freefly103 said:

I have a spare set of JCC 0.020 over pistons with Hastings rings that I am willing to sell at decent price

Click to expand...

Thanks for the offer freefly103, but I think if I decide to stick with Emgos I'll try and lighten the problem piston as I've already forked out 500 bucks on machining and balancing to suit this pair.

Fast Eddie said:

Well, far be it from me to nurture the seed that has been planted... but...

These are the JS Motorsport pistons I recently fitted after the first build 'nipped up' and scored one piston. These are the high comp 850 versions, I had them treated with a 'Diamondyze' hard anodising process. I have no previous experience using this treatment method, so we'll see how it pans out:

Click to expand...

I think you mentioned to me once before Fast Eddie - something along the lines of "if your going to keep a bike the extra cost of 'doing it right' is nothing when amortised over years". Dunno about nurturing - more like fertilizing and watering.
I have a feeling this seed will germinate - and my wallet vacuum will morph into a black hole

So the pistons, rods, rings and pins were not carefully weighed before balancing the crank?

(just asking)

Emgo pistons are normally sourced from JCC (JCC own brand marking is cycle craft), as are the new Hepolites also.

ELMZY said:

Didn't take long to spot the difference. I didn't even turn my digital caliper on.
Definitely poor casting on the heavier piston.
Image 1 shows much more clearly defined contours inside the head of the lighter piston (R), especially around the pin housing gussets.
In image 3 it appears the gussets are thicker on the heavier piston.
I'm confident now that there's plenty of extra meat that can be trimmed without causing any structural issues.
My problem now is jseng1 has planted the seed.

Click to expand...

Those look less like a good and a bad piston than two reasonably good pistons from different factories.

Like a Bradford-made Hepolite and a Taiwanese piston.

Anyway, what the hell is the balancer playing at? Odd-weight pistons will not do and you cannot completely balance a reciprocating mass with a rotating mass.

grandpaul said:

So the pistons, rods, rings and pins were not carefully weighed before balancing the crank?

Click to expand...

I don't know how careful the balancer was - I wasn't there.
But I'd like to think the rods/shells/bolts/nuts/pistons/rings/pins/clips that were supplied with the crank were all weighed before he calculated the simulated weight to include the weight of the lightest piston only.
I didn't weigh anything before I dropped the gear off at the machine shop - but I will next time.
He was probably thinking what a dickhead I was for dropping off dodgy pistons.