JCC Std Piston Size for 750

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wrecks

wrecks

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As part of my rebuild, I'm replacing my original 1971 pistons with the slot behind the oil ring. I had a local machinist check my cylinder bores and they measure 2.8750 (the bike only has 13,000 miles). I asked a guy in a parts shop over the phone to measure the standard sized JCC pistons he had in stock and he reported that they were 2.867 at the skirt. This would give me 0.008 clearance which is way more than the 0.0045 I've read that I should be shooting for.

Does this sound right? Perhaps he measured wrong? Does anybody have the actual manufacturing spec for JCC pistons - I looked around but couldn't find it. If that is the actual size of the JCC pistons that would be too loose...right?
 
Good idea, replacing those time bomb pistons.

Back in 2013, Jim (Comnoz) had this to say about it:
" I fit JCC pistons at .0045 to .0048 inch. They need to be slightly looser than the old original Hepolites.
At less than .0045 you will start to see rub marks and they will require a long break- in.
Getting a person with Norton experience is not a bad idea. Depending on who is doing the measuring the results may differ. Jim "

Personally, I wd not go less than .005

FWIW, stock bore 73mm is 2.374"
M y guess is that the fellow measuring those pistons is not using a micrometer and is not getting a reliable number.

Since you have a local machinist, I'd show him the JCC parts and ask him if they are really what you need to get the .0045 to .005 clearance.
 
I think Mr Rick has a typo. I have 73 mm bore = 2.874 inches. Otherwise, I agree with Mr Rick, and would go with no less than 0.005" clearance.

Slick
 
Okay, even going for a target of 0.005", I'd still have to be concerned with ordering those std JCC pistons (unless the guy in the shop was just sloppy with his measurements - which is entirely possible.) I guess I could order them and simply return them if they are actually that much under sized.

This is a little off topic, but was the standard bore for the 750 originally measured in inches or mm? The workshop manual says "2.875in (73mm)". I'm assuming the 73mm was an inaccurate conversation and that the original measurement was in inches and the conversion should have been 73.025mm. The manual also shows Grade A as being 2.8746/2.8750 and Grade B as 2.8750/2.8754.
 
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Okay, I have my JCC pistons in hand. In figuring out the proper piston to bore clearance, where do I measure the pistons? I've read that the measurement should be made on the piston skirt. Does that mean that the measurement should be taken all the way down at the bottom of the piston where it is longer at 90 degrees to the wrist pin? The area indicated with a yellow arrow in the below photograph...

 
Wherever the diameter is biggest, at a right angle to the pin. That's somewhere near the bottom of the skirt.
 
You might find this advice helpful, from a Triumph guy with a lot of experience with JCC pistons.

JCC Std Piston Size for 750


Ken
 
wrecks said:
Okay, I have my JCC pistons in hand. In figuring out the proper piston to bore clearance, where do I measure the pistons? I've read that the measurement should be made on the piston skirt. Does that mean that the measurement should be taken all the way down at the bottom of the piston where it is longer at 90 degrees to the wrist pin? The area indicated with a yellow arrow in the below photograph...

Click to expand...


That is correct. Go for .0045 to .0048 clearance with a new bore at your arrow.
You will be ok up to about .006 on a used bore if there are no scratches. Just give it a light hone with a ball hone.
If you come out looser than that I would suggest a rebore to .020 OS pistons. Jim
 
comnoz said:
That is correct. Go for .0045 to .0048 clearance with a new bore at your arrow.
You will be ok up to about .006 on a used bore if there are no scratches. Just give it a light hone with a ball hone.
If you come out looser than that I would suggest a rebore to .020 OS pistons. Jim
Click to expand...
This is really helpful, thanks!
 
Cylinder bore on Nortons have always been measured in mm as are the pistons, and have been since the late 40's. The conversion is done for the imperial markets and those at home who could not comprehend mm.
A piston manufacturer should always make to bore size - the maths is not hard and the dimensions needed not hard to achieve, small production runs usually are to dimension, though this not the case with mass manufactured pistons, they are measured and the measurement coded then delivered to the boring machine and the codes read and then the block machined to suit the piston - thus no grade A or B, or rejects, just pistons that fit the assigned bore perfectly. I doubt if any Norton pistons these days are 'mass' produced. I doubt if wassel even get 10,000 of one size made in a production run.
 
So please tell us, wrecks, what they measure. I searched like hell to find the answer to the question you originally posted, and cd not find it, either.
Seems ridiculous for JCC to produce a product so sensitive to size, and then not publish the spec!
 
In the four engine production plants I worked the bores were measured after honing and the grade stamped on the block. When the block was delivered to the engine assembly line the pistons matching the grades were fitted, the onus was put on the piston maker to supply the correct number of pistons in the right grades leaving the block line the looser tolerance followed by measurement. Later when I worked for a bearing shell maker they supplied bearings in grades of 3 microns difference to forecast order, they were colour coded, as the cranks were final ground and measured the matching bearing grade was fitted, if the crank finishing wandered to one side of the tolerance band we made and shipped the outside grade on an overnight for them. The customer was always right even when wrong ;)
 
Mr. Rick said:
So please tell us, wrecks, what they measure. I searched like hell to find the answer to the question you originally posted, and cd not find it, either.
Seems ridiculous for JCC to produce a product so sensitive to size, and then not publish the spec!
Click to expand...
I agree, it was frustrating that I couldn't find a size specification for them anywhere. I'm going going to bring them by the local machinist's shop on Monday morning. I have measured them with a digital caliper but I don't trust that figure so I don't even want to publish it here and run the risk of confusing the matter.
 
Good idea! Maybe he cd measure them with 2 different tools, as an accuracy check? And record the temperature?
Also, if your own number and his don't agree, it might be useful to find out why...
 
What i find a little strange is Madnortons comment nortons cylinders / pistons have always been measured in MM since the late 40`s ??

then why are all Hepolite pistons marked with size in imperial 020 /040 not .25 /.50 / .75 /1.00 etc like all japanese pistons , UK produceers used the imperial system metric size was quoted as a secondary measurement check your OEM piston suppliers catalouge imperial measurement first them metric

best policy is to let your machinist measure the piston skirt then add the clearance to achive the correct finished bore size
 
Mr. Rick said:
Good idea! Maybe he cd measure them with 2 different tools, as an accuracy check? And record the temperature?
Also, if your own number and his don't agree, it might be useful to find out why...
Click to expand...
My machinist measured the std. sized JCC pistons this morning and they were 2.8690. He measured them with his micrometer which he checked on a test block prior to taking the measurements. My bores are 2.8750 so that will give me 0.0060 clearance.
 
So initially your machinists reported a size 2.867. now he reports that they are 2.869. Generally, digital vernier callipers are ok but a micrometer is the weapon of choice OMHO. And a proper bore gauge for the cylinder.
 
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