New AN barrel & Emglo Pistons

RoadScholar · Jul 13, 2017

On the '72 (750) I blew up on a speed run I ran out of meat on the original barrel, 'couldn't go .060 over. I purchased a new AN 750 barrel of the 850 type (four long allen bolts) in standard bore and a set of standard sized Emglo pistons. I now have a skirt clearance of .006 (give or take a tenth); not what I was hoping for. The new pistons measure 2.870, I need pistons that measure 2.872 at the skirt; I'm looking for a skirt/barrel clearance of .0045

Has anyone come across a set of standard pistons that measure 2.872 or larger?

concours · Jul 13, 2017

Not surprising.

Option 1) go .020" right out of the gate

Option 2) send the jugs here, have them brought down to the correct diameter, and enjoy long engine life from the hard bore: http://www.mt-llc.com

I agree, too loose for a new rebuild, would not assemble like that.

jimbo · Jul 13, 2017

concours said:
Not surprising.

Option 1) go .020" right out of the gate

Option 2) send the jugs here, have them brought down to the correct diameter, and enjoy long engine life from the hard bore: http://www.mt-llc.com

I agree, too loose for a new rebuild, would not assemble like that.

+1 , they did great work on a two stroke cylinder I had

comnoz · Jul 13, 2017

RoadScholar said:
On the '72 (750) I blew up on a speed run I ran out of meat on the original barrel, 'couldn't go .060 over. I purchased a new AN 750 barrel of the 850 type (four long allen bolts) in standard bore and a set of standard sized Emglo pistons. I now have a skirt clearance of .006 (give or take a tenth); not what I was hoping for. The new pistons measure 2.870, I need pistons that measure 2.872 at the skirt; I'm looking for a skirt/barrel clearance of .0045

Has anyone come across a set of standard pistons that measure 2.872 or larger?

I have not seen this much variation in the JCC pistons so finding larger pistons is unlikely.

The bore coating process by millennium or bore tech is excellent and clearance could be brought to the minimum of .0045.

Or run them at .006. They will work fine even though they are on the loose side. It's much better than slightly too tight. Jim

grandpaul · Jul 13, 2017

Sounds to me like the Andover cylinders are out of minimum bore spec for a new replacement spec.

I have never heard of Emgo (JCC) pistons being "too loose" out of the box in 20+ years.

grandpaul · Jul 13, 2017

comnoz said:
The bore coating process by millennium or bore tech is excellent and clearance could be brought to the minimum of .0045. Or run them at .006.

If you do, make sure the ring end gaps are at .010-.012. You may need to try several different ring suppliers to get rings you can work with, as the Hastings rings should be pre-sized to the correct end gap for the JCC pistons; so the ring end gaps will be too big resulting in slightly lower compression and probable oil burning/fouling.

Triton Thrasher · Jul 13, 2017

grandpaul said:
comnoz said:

The bore coating process by millennium or bore tech is excellent and clearance could be brought to the minimum of .0045. Or run them at .006.

Click to expand...

If you do, make sure the ring end gaps are at .010-.012. You may need to try several different ring suppliers to get rings you can work with, as the Hastings rings should be pre-sized to the correct end gap for the JCC pistons; so the ring end gaps will be too big resulting in slightly lower compression and probable oil burning/fouling.

Bigger ring gaps shouldn't cause any problem.

RoadScholar · Jul 13, 2017

I spoke with the service that does my balancing, because I have a set of +.020 "Hepolites" (whom ever makes them these days). My plan was to go to the first over and be done with it, problem is that the Hep assemblies weigh 60 grams more (each) then the Emglos, which would put my balance factor somewhere in the low 40%s, and the lower end is fully assembled.

The service recommends trying coarse glass beading the Emglos, say that they can be made to grow .0015 to .002, which is what I need to be able to get a clean .0045 set of skirt clearances, with a very minor precision hone.

I'm a bit skeptical, but to do an overbore, on a new jug, means a complete teardown, and a new effort from the ground up. I am not opposed to the financial side of things, but this will set back the motorcycle's completion by at least a month and provide plenty of opportunity for distraction and interruption (add another month or two).

Any feeling about the wisdom/reliability of the coarse glass beading approach?

comnoz · Jul 13, 2017

I have used glass beading to make them grow a bit. Not so sure you could get .0015 or .002 though.

If you do glass bead them make sure the glass bead is fresh and do it at low pressure. High pressure will fracture the glass bead and you will end up with glass dust embedded in the aluminum.

You could always have the pistons lightly knurled if your worried about .006. Older machine shops will have the equipment to do it.

Believe me, you will never know the difference between .0045 and .006 in operation with full skirt pistons. Jim

xbacksideslider · Jul 13, 2017

Shot peen, steel shot, not glass, be careful, just a touch or two on the inside of each skirt. We used to return RD350 pistons back to spec when we'd re-ring em.

Dances with Shrapnel · Jul 13, 2017

I'll bet one of our long absent member of this forum would suggest a light tapping of the skirts with a ball peen hammer over the end of an anvil horn would do the trick. In all honestly, I'll bet a skilled tin smith or panel beater could pull this off.

I do remember the great illustrations of knurling the piston skirts to resize them in an old Chilton's Auto Repair Manual. This is when you pulled into place a replacement rear crankshaft seal with a kellum grip. This is when you could drop an oil pan with the engine in the car.

Brooking 850 · Jul 13, 2017

Dances, yes a slight peening up inside the skirts close to the gudgeon area will do the trick, there is a machine that can do that, my engine guru guy here has one.
Regards mike

Rohan · Jul 13, 2017

Heck, Phil Irving in his memoirs records squeezing a piston in a vice to get back the required size.
This somewhere on the road travelling across Canada on their BSA (?),
on his way (ultimately) to a design job for a crowd called Vincents...

acotrel · Jul 13, 2017

'Do it right first time'. If it means stripping down the bottom end and rebalancing - do it. It is not as much work as cleaning up the mess if the motor lets go. In the old days, the shot-peening inside the pistons was called Kotherising. From memory the increase in dimension did not last long after the piston was run in the motor and been through a few heat cycles .

BritBikeBoy · Jul 13, 2017

I had some pistons 'Kotherised' quite a few years ago. Unfortunately I never ended up using them, so can't comment on the results of running them in a motor. Definitely brought them back to specification though.
Be interested to hear if anyone has had success with this method.

Triton Thrasher · Jul 13, 2017

comnoz said:
Believe me, you will never know the difference between .0045 and .006 in operation with full skirt pistons. Jim

That's too obvious and easy.

Knurl! Blast! Bore! Balance!

And above all- worry!

Fast Eddie · Jul 14, 2017

comnoz said:
Believe me, you will never know the difference between .0045 and .006 in operation with full skirt pistons. Jim

That was my thoughts when reading this thread also. Plus less chance of seizure. Plus less fussy bedding in rules.

Seems like the solution is to just fit the damned pistons and enjoy the bike.

Sometimes, life is complicated enough, and the simpler option is all that's needed!

click · Jul 14, 2017

OK, I've a simpler option unless I havne't been following what's going on!!

You purchased new std barrels from AN.

AN sell std pistons, why not ask them what the tolerences are with their std pistons? I would presume (maybe incorrectly) that if they sell std barrels & std pistons the tolerences should be within spec? It might be worth contacting them about the tolerances and the balance factor on your crank?

I know this bypasses all the 'fun' you could have coating barrels and hitting pistons with small metal ballbearings to make them bigger

SteveBorland · Jul 14, 2017

Triton Thrasher said:
grandpaul said:

comnoz said:

The bore coating process by millennium or bore tech is excellent and clearance could be brought to the minimum of .0045. Or run them at .006.

Click to expand...

If you do, make sure the ring end gaps are at .010-.012. You may need to try several different ring suppliers to get rings you can work with, as the Hastings rings should be pre-sized to the correct end gap for the JCC pistons; so the ring end gaps will be too big resulting in slightly lower compression and probable oil burning/fouling.

Click to expand...

Bigger ring gaps shouldn't cause any problem.

The issue here is the lower radial pressure rather than the gap. I think it was AE who did some testing and found that even with huge gaps (1/4" if I recall) there was virtually no extra leakage. However, if the bore is too large (or the ring too small), then the radial pressure drops off, resulting in decreased sealing. What Jim is suggesting (I think) is that the OP needs to find rings that will work in his slightly oversized bores.

Having said that though, I would be inclined to contact AN and ask them for a set of pistons to match the bores, or to swap the cylinder block for one with with standard bores.

/Steve in Denmark.

Deets55 · Jul 14, 2017

This is more of a question than a suggestion. Since Jim said the piston clearance is not an issue, and Bill's concern is ring gap, what is wrong with fitting the next size ring set and file fitting them to specs? I would guess that the smaller expander ring should be used. I have seen this mentioned here before. I believe it was met with mixed reviews, but still might be a viable option.
Pete

New AN barrel & Emglo Pistons

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