Weirdness with new pistons

[Brandis]

So, I had the bottom of the long stroke A7 all together.  Crank spun very smoothly. Conrods spun like velvet.  New standard 62mm pistons. I had a hard time getting the gudgeon pins in.  But with some gentle heat, all was well.  The pistons were difficult to rock back and forth on the con rods but not much more than on a 500 single, which is my main area of experience. (I've never done a full twin rebuild). The cylinder barrels slid down with the a little more effort than I expected, but I told myself it's 6 new rings in freshly cleaned cylinders.  I had used a 'dingleberry' hone. 
But now the crank is very difficult to turn.  My first thought was that it's not because of the new rings.  Because at top and bottom dead center, when the rings hardly move, it's just as difficult.  So the issue, I thought, has to be some resistance in the the big end/journal interface.  Or it it the when spinning at TDC or BDC, you are at the maximum movement at the gudgeon pin?   So the resistance at top and bottom might come from the gudgeon pins and then the resistance of 6 new rings takes over for the rest of the arc.  Should I pull the barrel off and measure the piston and cylinder spacing

And yes.  I know you are all thinking of how hopeless I am.  If I get past this, I think I have the rest under control

[chaterlea25]

Hi Brandis,
The piston pins MUST be  free to slide easily in the small end bushes, You will have to go back and hone or ream them out..
Place a pair of parallels front to back either side of the conrods with the pistons fitted, rotate the crank until the pistons sit on the parallels, place 4 strips of shim or paper at each contact between the pistons and parallels
use a finger of one hand to hold the piston onto the parallels, tug on each strip in turn, the resistance should be equal if the piston is square to the crankcase / cylinder. rotate the crank to the opposite rod angle and repeat the test..
If the strips are loose on one side when the rod is at both angles, then the rod / small end needs to be re aligned.
If the looseness moves from one side to the other when the rod angle is changed then the small end /rod is twisted !!!

John

[Brandis]

Makes sense.  The pistons seemed to be perfectly spaced when they slid in the barrels.  They were probably both offset equally which, when the barrel is bolted down, caused the tightness.
That's quite a procedure but I guess I can only put off learning proper mechanics so long before it bites me in the ass


I knew one of you would bail me out on this. 

[Swarfcut]

 Pistons usually need a bit of heat to fit and remove the pins in the backyard workshop, the pins being a tight fit in the piston bosses when cold. The initial movement comes from the freedom of the pins in the small end bush, so revisit this aspect as suggested by C J . Another trick is to assemble and check at every stage, eg Check ring gaps in the bore, try pistons without rings, that way you can find where the tightness is coming from.

 A newly assembled unit will be tight to turn, but not excessively so. Stiff pistons on the rods is your most likely cause, always assuming the pistons are made to spec  for the bore, with the correct running clearance. I don't know if the rods have had the small end bushes replaced, new ones usually need to be reamed to match the pins.

 Swarfy.

[RDfella]

Not convinced the problem is with the small ends. On several engines the pin is a press fit in the conrod (have to heat the steel rod SE to get them in - some even had a pinch bolt) and the piston needs warming too. I'd be more concerned about the honing. Not the way I'd do it - what is the surface finish?
What I'd do next is pull the barrel & pistons off and put a 3/4" bar (silver steel is ground to size) through both small ends concurrently. Should fit nicely and engine rotate smoothly. Next check fit of pistons (less rings) in the bores. What is the clearance at bottom of skirt, 90* to gudgeon pin? Lastly, try each ring in its bore for gap.
If all that checks out, I'd start worrying whether the bores are perpendicular to the base flange....

[Brandis]

It's all apart again.
The crankcase was empty when I got it.  I got the used steel rods from a reliable supplier.  Small end appears to be an old bushing.  Colouring suggests it saw some use but it's tight on the pin.  This bike is a time capsule, put in storage after 12K miles when the engine failed. Whoever put it up used some long lasting waxy grease in the bore.  Standard size so we assume it's still the factory bore from '47. Measures correct at 62mm.

I will do a gentle hone on the rod until I can force it through by hand and leave the bore in the pistons as is.  I don't have a steel shaft piece that fits to do the test suggested.  If it goes back together and fells good on the spin, I'll assume the problem is solved.  If it's still to tight, I'll pull it apart and follow all your 'worst case' suggestions.

Thanks again.  I wish you were all here, looking over my shoulder, drinking beer, and whispering behind my back about how someone should rescue this bike from all my ham-fisted best intentions.

[RDfella]

I note you have new pistons. Pistons are (should be) manufacture to give correct clearance when fitted to specified bore size. Unfortunately, these days one cannot rely on that. Whilst you've got it apart, check those piston / bore clearances as I described. May save you yet another strip-down.

[Swarfcut]

 BSA Service Sheet 704 details clearances between the cylinder wall and the bottom of the piston. Convention is that this is measured at right angles to the pin.

For the A7 solid skirt it's .002 to .004". Split skirt type is less, .0011 to .0031"

 Swarfy.

 

[RDfella]

For the A7 solid skirt it's .002 to .004". Split skirt type is less, .0011 to .0031"

Of course, that assumes the coeff of expansion of the material the makers of Brandi's pistons used matches that used by BSA. Which is a matter for conjecture...

[BSA_54A10]

Apat from very expensive hand made race pistons all cast pistons are made with 1 of 2 alloys depending upon weather they ara gravity cast or pressure cast
Same for forged pistons again only 2 alloys .
Now every piston maker has their own special named alloy but they are all te same stuff with different names.
We found during WWII aluminium alloys with ecpansion co-efficients almost the same as grey cast iron.
BAck then the got names like Lo-Spand / No-spand / Spandex / S-Alloy / etc etc etc
They are all lientical as far as the major alloying elements ( Al-Si-Cu-Zn ) and major impurities ( Cu-Zn-Fe-Mn-Mg-Ni ) but vary enough in minor impurities ( total less than 1% ) to be given patients and commercial names.
Back in my foundry days we made hundreds of secondary foundry ingots ( remelted scrap ingots ) and from the same melt would often cast a dozen or more "special piston alloys " particularly as in those days Australia was in the top 10 foundry nations with lots of small foundries that would be lucky to cast a ton of piston blanks a year .

[Triton Thrasher]

For the A7 solid skirt it's .002 to .004". Split skirt type is less, .0011 to .0031"

Of course, that assumes the coeff of expansion of the material the makers of Brandi's pistons used matches that used by BSA. Which is a matter for conjecture...

If Brandis’ pistons are modern manufacture, then I think they are unlikely to be low-expansion alloy.

If they are not low-expansion, then 0.004” looks like a reasonable minimum clearance near the bottom of the skirt.  I think that’s 0.0017” per inch of bore.

[chaterlea25]

Hi Again,

I will do a gentle hone on the rod until I can force it through by hand

 

The pins MUST be a floating fit in the rods, They should fall out under their own weight

John

[Brandis]

I honed the con rods with a small rotary flap wheel and coarse valve grinding paste.   Pistons are back on.  Tomorrow I'll slide the barrels down and check rotation before the nuts get fitted.

An off topic question.  I've drilled the 2 1/16" holes on the con rods.  I would like to do a temporary oil pressure check once the engine is running and hot.  Is there an easy test point to attach to?

[Brandis]

It's all good. Perfect movement. Thanks to you all.

[fido]

One of my pistons has some friction between the gudgeon pin and the little end bush, although the engine was running fine. I only stripped it to check the sludge trap.