Piston---Bore Clearance

I am set up to fit .20+ JE forged pistons into my '72 ,750 (a former Combat engine) and have gotten conflicting clearance numbers from acknowledged experts. The supplier says .0055 to .006, the machine shop says .002 to .0025.

I'd be greatful to hear what experiences forum members have had with this issue.

With best regards,

RS

.003 - .004 with .005 the max is what I use.

I WOULD NOT, UNDER ANY CIRCUMSTANCE, BORE A NORTON (or Triumph, or BSA) TO .002 CLEARANCE!!!!

Find another machine shop!

FORGED pistons maylar, not cast so 0.005-0.006 is more likly correct, if the piston are made correct the clearance should be built into the pistons, ie, std bore size + 0.020" should give you the correct clearance other wise you end up with excess ring gaps.

I use .0045 piston clearance.

FORGED PISTONS :!: :!:

You might get away with .0045" on a street bike, but I wouldn't recommend it. Steve Maney and I both sell the same Norton pistons, made by JE to our spec, and we recommend and use .005" clearance. Guaranteed to seize immediately if you use .002" or .0025". That's what JE typically recommends for their forged pistons in Japanese bikes, and that's what they size their pistons for in a standard bore size. The instruction sheet that comes from JE with the pistons calls for .002" or .003" (depends on bore size), and that might be what the machine shop is reading. I cross it out and change it to .005" on the sets I sell. If you have their pistons made for a standard Norton bore size, you will need to hone the bores out another .003" or so for proper clearance. On special order, you can have them make pistons sized for .005" clearance in standard bores, but you have to specifically ask for it.

Jim Schmidt just went through the process of sorting all this out for his lightweight rod/piston sets, so he might have something to add here, if he's watching this thread.

Ken

lcrken said:

You might get away with .0045" on a street bike, but I wouldn't recommend it. Steve Maney and I both sell the same Norton pistons, made by JE to our spec, and we recommend and use .005" clearance. Guaranteed to seize immediately if you use .002" or .0025". That's what JE typically recommends for their forged pistons in Japanese bikes, and that's what they size their pistons for in a standard bore size. The instruction sheet that comes from JE with the pistons calls for .002" or .003" (depends on bore size), and that might be what the machine shop is reading. I cross it out and change it to .005" on the sets I sell. If you have their pistons made for a standard Norton bore size, you will need to hone the bores out another .003" or so for proper clearance. On special order, you can have them make pistons sized for .005" clearance in standard bores, but you have to specifically ask for it.

Jim Schmidt just went through the process of sorting all this out for his lightweight rod/piston sets, so he might have something to add here, if he's watching this thread.

Ken

Click to expand...

But you get them made for a specific purpose a NORTON or are the allowing for the fact most racers may be using alloy barrells :?: :?:

splatt said:

lcrken said:

You might get away with .0045" on a street bike, but I wouldn't recommend it. Steve Maney and I both sell the same Norton pistons, made by JE to our spec, and we recommend and use .005" clearance. Guaranteed to seize immediately if you use .002" or .0025". That's what JE typically recommends for their forged pistons in Japanese bikes, and that's what they size their pistons for in a standard bore size. The instruction sheet that comes from JE with the pistons calls for .002" or .003" (depends on bore size), and that might be what the machine shop is reading. I cross it out and change it to .005" on the sets I sell. If you have their pistons made for a standard Norton bore size, you will need to hone the bores out another .003" or so for proper clearance. On special order, you can have them make pistons sized for .005" clearance in standard bores, but you have to specifically ask for it.

Jim Schmidt just went through the process of sorting all this out for his lightweight rod/piston sets, so he might have something to add here, if he's watching this thread.

Ken

Click to expand...

But you get them made for a specific purpose a NORTON or are the allowing for the fact most racers may be using alloy barrells :?: :?:

Click to expand...

JE does not make off-the-shelf pistons for Nortons, so they don't have a tailored instruction sheet. They will make custom pistons to anyone's specs. To my knowledge they have made them to specs from Steve and I, from Kenny Dreer, and from Jim Schmidt, and probably several others by now. When we started doing Norton pistons with JE a bit over 10 years ago, the JE engineers wouldn't make a recommendation for clearance for use in a Norton. They were quite upfront about saying they didn't have any experience with them, and that if we had found a clearance that worked from experience, that's what we should use. The instruction sheets that JE supplies with them are pretty generic. Our 750 pistons are made from the same forging blanks they use for some Kawasakis, so those are probably the same instruction sheets they supply with the corresponding Kawi pistons. I'm sure we could get JE to print some just for us, but it's simpler and cheaper to either change the instructions, or supply our own to our customers.

Ken

Yep Al barrel with water jackets can get away with tighter gap.

On this note, what about Maney Al barrel, Jim's pistons and 50% more
fuel burn heat in a boosted engine?

What about ring gap or does that matter with Jim's TotalSeals?

hobot

Thanks folks.

The JE tech rep that I spoke with did , indeed, mention something about Kawasaki, and the lable on the box has what's left of the word Kawa... on it; a liquid cooled engine will, generally, always have tighter clearances.

The machine shop in Tennessee mentioned Porsche/BMW when talking about the tighter clearance and forged pistons (Leo is out with a bad back this week)

Jim Schmidt's instructions mention .0055 to .0060.

I was hoping Ken would chime in, thanks! I don't intend to race this engine, but I do have high expectations for its ability to sustain high revs over short bursts while providing complete peace of mind. The pistons will be attached to forged rods and a one piece forged crank that spins in "C" spec SuperBlends.

Based on the information gathered here I am going to specify a .005 bore to piston clearance and do a methodical break-in with lots of green oil.

Again, many thanks forall the assistance.

RS

From what I have gleaned, the C2 crank bearings are the ones to use. I got C3 before I got all the info, but don't expect any issues, I'm not racing.

Yes, Leo has my head and am awaiting his work on it too. Got a call tonight from MMW about his back.

Dave
69S

A couple of cents that may save some bux.

Ya might what to do some searches on clearance fudge factors.
The 005" is for moderately run long enduring moderate tune engines.
Throw in extra fuel burn and hi rpm and bore clearances need to open up.
A prior post explained the main issue is not so much the total expansion
but the oval shaped bore distortion and effects on rings and piston bind.

If this is a competition engine, where scoring is more important
than interval between over hauls, might consider the 006" gap
and knurl the pistons to take up slap and self burnish the ridges
to best fit on its own, with some gain in oil film function.
Special device does this and lasts pretty well in street use
but gets worn down if pressing limits much.

I suspect the same for the crank bearings with extra temp and rpm
than standard. Past Norton dragster racers preferred the thinner
early and Combat cases flexible to tolerate better the hi rpm
crank jump rope. Too tight may cancel the superblend forgiving
geometry.

hobot

IMHO (from personal experience ...)

For the street with cast pistons

Seizure at .0035"

Slap at .0065"

Pick your poison, I'm with GP on this one.

I use .0045" and have had no problems, but "built in clearance" from the piston manufacturer is usually somewhere around .005" to .0055".

Don't forget to check for piston to cyl head/ head gasket clearance before assy after an overbore (bore concentricity). (I dry fit the barrel, gasket and cyl head and look UP through them for possible fouling). Piston top edge contacting a gasket or edge of a combustion chamber is not unheard of if piston height exceeds barrel top. Deleting the base gasket may cause this problem. $.002

Regards, Tony

When I first started racing a Commando in 1972, I was using Hepolite Powermax pistons. Axtell showed me how he honed cylinders, and told me to use .004" on the right cylinder and .0045" on the left, because it was at the end of the pressure chain and didn't get as much oil cooling as the right. I did that for 20 years with Powermax pistons and never had any seizures, scoring, etc. Only problem I ever had with them was the oil ring grooves would eventually wear out. The forged pistons expand more with heat, and really do need the extra .001" or so of clearance.

Ken

Great to see so many chiming in on this one. So I'll throw in something. .005" is the minimum in my opinion and that's because I noticed some light scuffing and scoring at .005 with alloy cylinders. I'm talking about JS forged pistons - not other designs which are close but not the same (JS pistons are short and lightweight). This was after 1/2 hour break in at partial throttle and then with about 1 or 2 seconds of full throttle. I then polished off the scuffing which was minor and lightly honed the cylinders to clean up scoring scratches, ending up with .0055" clearance. See photo below. In the real world some people are going to twist the grip within the 1st hour because they can't help themselves. Racers simply don't have the time to blast up & down the street pissing off their neighbors with open pipes. So I recommend .0055" for street and .006 for racing. There is no objectionable piston slap because the forged pistons expand more than cast pistons and take up the slack. On the down side I have heard from several people who purchased forged pistons on their own and ended up siezing them or severly scoring them when running inadequate clearances at .0045" or less. By the way - forged pistons are stronger and longer lasting than cast pistons and can be made with a lighter design structure.

Photo below is after polishing the pistons to .0055" clearance and running & testing for one year. Note that I only drill pistons for my own bike. The 750 pistons in photo have been drilled, undermilled, and chamfered down to 170 grams (and the pins taper bored). This makes for a smooth running Atlas that cruises at 70 to 80mph with tall gearing (see photo). Note the Keihin/PWK 32mm flat slide carbs (also for Commandos). see them at http://www.jsmotorsport.com





Jim

Racing flute builder Jim,
I'm paying close attention to this for Ms Peels special boosted needs.
Little vital details like forged vs cast pistons fudge factors is what
I've also come across in boosted applications.
But I've no back ground to apply hot rod car data to 920 Maney block.
I've come across a scaling factor of clearance vs bore size to complicate this.
Any clues on that bore size aspect?
I'll dive back into references now you'all have given some baseline
to juggle from.

Glad to learn clearance can be gained some off your pistons instead of bore.
I sure like those signature air cooling vents in your sets.

hobot - no gear head prono photo's of this quality to share- yet.

Reviewing references on air cooled engine builders found a formula
for piston clearance used by one fella that breaks his engine in
by electric motor to get a tad closer gap and not seize up later.

0.001 x bore in inches + 0.001" = gap to shoot for.

So 3" bore should like .004" clearance with cast pistons.
Forged would like another .001" more and boosted &/or
extra high rpm add another .0005" clearance.

hobot