Understanding piston dimensions when re-boring oversize

[malmac]

Well team, I need some educating.

I have two sets of new R69s pistons of different brands.

One set of pistons has 72.92 stamped on the top. Given the clearance is .08mm, I would assume rebore cylinders to 73.00mm.

The second set of R69s pistons (for a different set of barrels) has 73.450 stamped on top. If I add .08mm to that measurement I get 73.53mm.

Is this the right way to think of it, or are the second set of pistons less subject to expansion with heat and only require .05mm clearance - so the barrels only need to be rebored to 73.50mm?

Any thoughts or advice? Please educate me.

Mal

[schrader7032]

What are the piston brands? My limited understanding is that the cylinders are sized to the measurement of the piston...if you're having a machine shop do the work, you need to give them your pistons for their measurements.

[caker]

Hi,

I was recently stuck (no pun intended) with my R71 project on a too-tight interference fit with the replacement cam and timing gear set and spent some time noodling on thermal expansion, as I galled the first set The solution was to used dry ice to freeze the cam -- the hot gear slipped on no problem. I learned this from one of your old posts, Mal - so thanks. It was a breakthrough moment.

Piston to wall clearance needs to increase as the piston/bore gets larger ... more piston = more expansion!

I don't know why those numbers don't add up. Good question. Could it be that the pistons are different alloys/coefficient of expansion and thus have different recommended wall tolerances?

The piston set I bought (for the R71 project) came with specified tolerance requirements.

-Chris

[malmac]

What are the piston brands? My limited understanding is that the cylinders are sized to the measurement of the piston...if you're having a machine shop do the work, you need to give them your pistons for their measurements.

Kurt

Three brands -= Nural, Mahle, KS.

See the photos. I assume the SP followed by a number is the recommended minimum clearance - so that number has to be added to the stamped number to get bore dimension - but what happens when BMW manual stated .08mm and yet the piston states .05mm.

Nural - fine just add the .08 and you get a nice round number for the bore dimension.
The K.S. would only get .05mm - sounds a bit tight when BMW claim .08 +.01 (so bit bigger is OK but smaller is a no, no.)

That is what has got me foxed.

Mal

[malmac]

Hi,

I was recently stuck (no pun intended) with my R71 project on a too-tight interference fit with the replacement cam and timing gear set and spent some time noodling on thermal expansion, as I galled the first set The solution was to used dry ice to freeze the cam -- the hot gear slipped on no problem. I learned this from one of your old posts, Mal - so thanks. It was a breakthrough moment.

Piston to wall clearance needs to increase as the piston/bore gets larger ... more piston = more expansion!

I don't know why those numbers don't add up. Good question. Could it be that the pistons are different alloys/coefficient of expansion and thus have different recommended wall tolerances?

The piston set I bought (for the R71 project) came with specified tolerance requirements.

-Chris

Good points Chris.
Were the specified tolerance requirements stamped on the piston or on a piece of paper?

Mal

[caker]

Mal,

Pistons from RRC Tuning...

77.92

Translated from German (Google Translate app is amazing): 0.08 - 0.10mm clearance, measured at the widest part on these pistons.

Speaking of translations, I'm pretty sure "Sp" is short for Spiel, or "play".

-Chris

[malmac]

I consulted someone who I have great respect for. Here is what he said.

So it all depends on the manufacturer. Thank you Chris for sharing your information. Obviously some pistons do require .08mm clearance.

Hi Mal,

It is the manufacturer of the piston that determines what the minimum clearance should be.
I assume that the type of alloy has a lot to do with the expansion coefficient of the piston and based on that KS set the minimum at 0.05 mm Sp = Spiel = clearance
In the ‘70’s and 80’s the Italian made Meteor pistons had serious seizure problems under normal operating conditions, but I’ve not heard about KS seizure problems.
Of course, the clearance is valid for properly tuned engines running under normal operating conditions.
If you foresee exceptional conditions, it won't hurt to increase the initial clearance a bit.
And follow the correct run-in process for the piston rings, but I’m sure you know that.

Kind regards,

[vechorik1373]

In all the years I built bikes, and engines for customers, and supplied parts, I have a few observations concerning pistons.

Never mix brands of pistons in an engine. Not a good idea. The weight of each piston is VERY important. If you are using a pair of used pistons, be sure that they weigh the same! And be sure there is not excessive wear in the ring lands that allow the rings to cock back and forth every time the piston changes direction.

To measure the diameter of a piston, measure the skirt at right angles to the wrist pin. Pistons are not perfectly round, they are cam ground for lack of a better term.

After being in the parts supply business, early on, (I learned the hard way) that aftermarket pistons were problematic junk.
Very thick skirt walls, (which cause excessive expansion) and improper alloys that cause excess expansion resulting in seizures and galling. In some instances, using aftermarket pistons, the bore had to be so large, to prevent seizure and galling, that the engine had piston SLAP dead cold, which would gradually go way as it got up to operating temperatures. The bore was so loose that a brand new cylinder bore was considered 3/4 of the way to being completely worn out according to factory specifications in order to avoid seizure. After these hard and costly lessons, the only pistons, that I sold, and used, and recommend, are the KS brand. They are by FAR the best quality.
OEM pistons, such as Nural and KS originals, had and stolid steel ring, cast IN the aluminum, above the top ring land, to control expansion.
Today, even the new KS pistons are not made that way. But the alloys are much better. In inches, (since in the USA everyone bores to inch dimensions) the piston wall to cylinder recommended clearance is .003 of an inch.
In my opinion, you are wasting your money, buying any other brand.

[malmac]

Thank You Vech for passing on your valuable experience.

I actually have three sets of barrels and each has new pistons. Each set has a matched set but different brands.
KS has the smallest clearence. - .05mm
Nural - .08mm
Kloben - .08mm

In my running engine that I am currently breaking in the new Nural pistons - which I did weigh (and tweak even when brand new to get the weight as close as possible to the same) - so far so good.

I did notice that, no matter how I measured the new pistons - I could not make the math work without paying attention to the numbers stamped on the top of the piston. When it came to secondhand pistons - I imagine it would be very to measure old pistons and if you re-sleeved barrels - to allow just .08mm clearence it would not work.

I experimented with matching pistons and barrels - which had .08mm when cold and then I heated the barrels in my oven to running temps and they were stuck. So had I run that combination it would have failed very quickly.

As you point out experience can be an expensive teacher.
I am sure others will also benefit from your comments.

Thanks.

Mal