Piston Question

Please direct me to a similar topic if already covered.

We have JCC Pistons fitted marked 020;

with a bore size of 3.052"
piston measures 3.047"
Clearance .005" which seems appropriate although with aluminium barrels feel this could have been reduced to around .003"

STD Bore for 850 Commando 3.040"
If pistons as marked are correct we should have 3.060" (or thereabouts)
To give us .020"
We have a differential of .012"? (assuming marking on pistons is correct to std 850 specs)

Would anyone have any idea of where this piston has come from or what it is out of, clearly the std bore size is different to that of an 850.

As the condition of the bores and pistons are acceptable, i'm chasing a set of rings, preferably with the "Hastings" type oil control which has a tighter tension against wall than whet had been fitted.

regards Danny

A standard 750 bore is 73 mm or 2.875", so you can rule that out.

I have had JCC pistons mis-marked. In fact, the pair in my Atlas are mis-marked R and L.

Slick

texasSlick said:

A standard 750 bore is 73 mm or 2.875", so you can rule that out.

I have had JCC pistons mis-marked. In fact, the pair in my Atlas are mis-marked R and L.

Slick

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Thanks Slick, yes i'd come to the same conclusion, it's bad enough ordering in correct parts as it is to Wetern Australia so wanted to make sure what I was purchasing was correct. I'm none the wiser so far.

Hagarr said:

with a bore size of 3.052"
piston measures 3.047"
Clearance .005" which seems appropriate although with aluminium barrels feel this could have been reduced to around .003"

STD Bore for 850 Commando 3.040"
If pistons as marked are correct we should have 3.060" (or thereabouts)
To give us .020"
We have a differential of .012"? (assuming marking on pistons is correct to std 850 specs)

Click to expand...

The standard 850 cylinder bore dimensions were between 3.0315"- 3.0320" ('A' grade) and 3.0320"- 3.0325" ('B' grade).

Hagarr said:

Would anyone have any idea of where this piston has come from or what it is out of, clearly the std bore size is different to that of an 850.

Click to expand...

The cylinders should have been rebored to match the supplied pistons plus the specified clearance for the cylinder barrel material and not simply bored + 0.020" oversize.

Yep that is totally correct L.A.B. the problem is I need rings and to purchase based on dimensions only can be tedious, so seeking if anyone may have some insight as to what model piston the previous builder has used.

Hagarr said:

if anyone may have some insight as to what model piston the previous builder has used.

Click to expand...

If the JCC pistons are marked +.020" then I can't see them being anything other than JCC +.020" 850 Commando pistons.

Hagarr said:

Please direct me to a similar topic if already covered.

We have JCC Pistons fitted marked 020;

with a bore size of 3.052"
piston measures 3.047"
Clearance .005" which seems appropriate although with aluminium barrels feel this could have been reduced to around .003"

STD Bore for 850 Commando 3.040"
If pistons as marked are correct we should have 3.060" (or thereabouts)
To give us .020"
We have a differential of .012"? (assuming marking on pistons is correct to std 850 specs)

Would anyone have any idea of where this piston has come from or what it is out of, clearly the std bore size is different to that of an 850.

As the condition of the bores and pistons are acceptable, i'm chasing a set of rings, preferably with the "Hastings" type oil control which has a tighter tension against wall than whet had been fitted.

regards Danny

Click to expand...

Are you measuring the pistons with a calibrated micrometer? Are you measuring the skirt, pin area, crown, or what? Are you measuring parallel to the pin or perpendicular to the pin?

I have found JCC 750 pistons mis-marked for left and right, but I've never found 750 or 850 JCC pistons made or marked the wrong size.

L.A.B. listed the possible standard bore range. Add to that the 0.020 realizing that everything has tolerances so the 0.020 won't be perfect. Add to the bottom of the A range and top of the B range and you're close:

3.0515-3.0525 Std Bore + 0.020
3.047 + .005 = 3.052 (Piston + Clearance)
So, nothing wrong.

marshg246 said:

Are you measuring the pistons with a calibrated micrometer? Are you measuring the skirt, pin area, crown, or what? Are you measuring parallel to the pin or perpendicular to the pin?

I have found JCC 750 pistons mis-marked for left and right, but I've never found 750 or 850 JCC pistons made or marked the wrong size.

L.A.B. listed the possible standard bore range. Add to that the 0.020 realizing that everything has tolerances so the 0.020 won't be perfect. Add to the bottom of the A range and top of the B range and you're close:

3.0515-3.0525 Std Bore + 0.020
3.047 + .005 = 3.052 (Piston + Clearance)
So, nothing wrong.

Click to expand...

Yes on all counts, i'm a Fitter & Machinist with experience in the Automotive machining area. the problem i'm getting is the std bore size seems to be the issue, some say it's 77mm others say it's 3.040", at 3.040" the bore does not reflect being machined to .020" or it would be 3.060" whereas i'm measuring (with micrometer and bore mic) and getting 3.052" and pistons are marked .020".

As the manual says 3.0315" - 3.0325" is the range of the standard bore. 77mm = 3.03149" Anyone saying 3.040" is the std bore size is wrong. (where do these people get their info from?) from your measurements with a .005" clearance piston to bore you do not have a problem. Have you stuck a bore comparitor down the cylinder to see if the bore is straight and round (not tapered or oval)?

Hagarr said:

some say 77mm others say it's 3.040",

Click to expand...

Hagarr said:

Yes on all counts, i'm a Fitter & Machinist with experience in the Automotive machining area. the problem i'm getting is the std bore size seems to be the issue, some say it's 77mm others say it's 3.040", at 3.040" the bore does not reflect being machined to .020" or it would be 3.060" whereas i'm measuring (with micrometer and bore mic) and getting 3.052" and pistons are marked .020".

Click to expand...

Neither 77mm or 3.030 is exact and I've seen 3.040 nowhere but this thread. L.A.B gave the actual ranges. When the bikes were built, the barrels would be bored, measured, and classified. Then they could classify the pistons as well and match. When later needing a bore job, get a set of pistons, measure and bore to fit.

Hagarr said:

Yep that is totally correct L.A.B. the problem is I need rings and to purchase based on dimensions only can be tedious, so seeking if anyone may have some insight as to what model piston the previous builder has used.

Click to expand...

When you buy piston rings, it is normal to adjust the end gap by filing when you are fitting them, so the size is not critical. They are often slightly oversize. Even if the end gap is slightly too large when the rings are in the bore, it does not really matter.
Some people worry too much about bore clearances. A loose motor is usually faster than a tight one. If the rings are miles too large and you need to file the end gap too much, the shape of the ring in the bore can permit gas leakage. Otherwise there usually is not a problem.

The Norton workshop manual actually lists the bore as ' 3.030" (77mm) ' under the specification heading. But under the cylinder block heading goes into more detail as the finished sizes 3.0315" to 3.0325" (a difference of 1 thousandth of an inch)
The Haynes workshop manual is where the incorrect 77mm = 3.040 " figure rears its head

dobba99 said:

The Norton workshop manual actually lists the bore as 3.030" (77mm) under the specification heading. But under the cylinder block heading goes into more detail as the finished sizes 3.0315" to 3.0325" (a difference of 1 thousandth of an inch)
The Haynes workshop manual is where the incorrect 77mm = 3.040 " figure rears its head

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Thanks , now corrected in my Haynes manual .

dobba99 said:

As the manual says 3.0315" - 3.0325" is the range of the standard bore. 77mm = 3.03149" Anyone saying 3.040" is the std bore size is wrong. (where do these people get their info from?) from your measurements with a .005" clearance piston to bore you do not have a problem. Have you stuck a bore comparitor down the cylinder to see if the bore is straight and round (not tapered or oval)?

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Dobba, I know never the best to work from a Haynes Manual but they are stating 3.040"
I still don't see how a British bike built in the 70's would have used metric values for their spec.

Hagarr said:

Dobba, I know never the best to work from a Haynes Manual but they are stating 3.040"
I still don't see how a British bike built in the 70's would have used metric values for their spec.

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There's more where that came from.

Hagarr said:

I know never the best to work from a Haynes Manual but they are stating 3.040"

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Haynes copied the data from the factory manuals and not always that accurately so 3.040" is a simple Haynes typo.

Hagarr said:

I still don't see how a British bike built in the 70's would have used metric values for their spec.

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Traditionally, bore and stroke measurements (and cubic capacity) were given in metric units even from the earliest times...


...even going so far as to stamp it on engine cases (ES2 79 x 100).

Hagarr said:

I still don't see how a British bike built in the 70's would have used metric values for their spec.

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Its only a measurement, either inches, mm, cubits, grains, etc etc, don't get hung up about it.

Hagarr said:

Dobba, I know never the best to work from a Haynes Manual but they are stating 3.040"
I still don't see how a British bike built in the 70's would have used metric values for their spec.

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They were selling worldwide, not just to the UK and USA. They listed both as did Triumph, BSA, and others. The oldest Triumph 650 Factory Workshop manual I have (covers 1963-1970) lists both. In the technical section, Triumph lists the metric as the actual value and the inches is metric/25.4 rounded to three decimal places - Norton does the same. In the body of the manuals, both companies list the actual range in detail.

The general public could not buy the factory manuals, so Haynes made Owner Workshop Manuals. There are errors in the Factory Manuals and even more in the Haynes manuals. Haynes saying 3.040 is simply wrong. What does Haynes specify for the inch measurement (did they at least get that right?)? Do they list the actual detailed ranges in the body of the book?

Regardless of all that. You have a set of pistons and if you want to be anal about it, bore and hone to fit. Otherwise, bore to the bottom of the range and hone to fit and don't get hung up on exact unless you are building a race bike - these are engines designed long ago in a galaxy far, far away! I often find Combats with standard bores, the original pistons that like to lose their tops, and very low milage. I get a new set of JCC standard pistons and hone until I have reasonable clearance and of course, taking out and slight taper. While they were available, I used Hastings rings - these days I use Hepolite sets which include JCC pistons. pins, and clips and come with UK made rings (same company that made rings for Hepolite many years ago unless I've been lied to).

Mark Savage said:

Well , they would have been 45 or 51 CI not 750 or 850cc if not metric.

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That's actually a very good point, Thanks.