My engine rebuild / crank balancing considerations

[MG]

Hello all!

After some delay (delivery of the SRM rods and machining the t/side bush) I finally got around to start rebuilding the A10 engine after the the small end bush failure I had (reported here: http://www.audioworld.net/BSA/forum/index.php/topic,2893.0.html).

I started yesterday by dynamically balancing the new LJ crank (kindly provided by one of the forum members  ), which looks like a Swiss cheese now, owing to the slightly heavier con-rods, pistons and different balance factor. Next time I'm going to machine material off the sides of the flywheel, that would definitely look better. It had been reground when I bought it already and I had it nitrided, hence the greyish colour.




I decided to use a balance factor of 58%. Let me explain what the considerations for this very number were:
Assuming the weight of the original con-rods and pistons the crank must have been balanced to exactly 54%, which is the original factor used by BSA (as stated in Eddie Dow's tuning sheet).
For an engine with 42mm crank throw radius and a rod lenght of 165mm, using 54% results in the lowest average value for the resulting force over the crank angle (Fmittel/Fmax/% in the spread sheet, sorry it is in German). The polar diagram bottom right shows the force progression in vertical and horizontal direction, the red graph representing 54% and the green one 58%.
The average resulting force is slightly better with 54%, but the peak values are smaller at 58% (graph not visible on the screenshot, sorry), furthermore the amplitude is smaller in vertical direction, while the bigger amplitude in horizontal direction isn't felt that badly than vertical vibration.
I found 58% to be the best compromise between peak and mean amplitude, experience will show how well it is going to perform in the BSA frame. I will let you know once I've covered a few miles.




FYI: 70% as recommended for racing compared to the original 54%. The average and peak forces are worse, resulting in higher bearing load, but the deflection in vertical direction is rather small.




The phosphor bronze t/side bush (supplied by SRM), nicely line-bored with a play of 1.5thou.




The crank inside the cases fitted with the SRM rods. Great stuff, but one word of warning if anyone here intends to fit them:
Check the oil hole in the lhs rod, mine was blocked 7-8mm deep with a mixture of grit and polishing compound. I had a real tough time prising the stuff out with a piece of 1mm dia spring steel wire.




Furthermore the recesses in the cylinder spigots have to be enlarged. The SRM rods are wider than the original ones and the edges will hit the spigots. Hopefully I will have time tomorrow to machine the barrels to fit.




As I have everything apart anyway, I'm going to incorporate the cam follower oiling mod. Cutting threads into the rear return oil holes is real fun, but it worked with the setup shown in the pic, this being an M6 tap and a 6BA spanner  .




I wanted to use hex bolts and lock wire to blank the holes off, but there's not enough clearance to the crankcases to acommodate the heads. So I used Allen grub screws, fitted them with Loctite and peened them into grooves in the base material. There's no way these will ever come loose (and there probably is no way of getting them out again without drilling I'm afraid  ).




Machining the slots in the followers is on the to-do list for tomorrow, will keep you updated.


Cheers, Markus

[A10Boy]

Blimey Markus, it looks good, I wouldn't have recognised it.  Keep us posted on progress.

[MG]

Some more work done today.

Cut-outs, enlarged to clear the new SRM rods:




Grooves cut in the followers. It's incredible what one can do with an angle grinder and a steady hand  






The Wisecos fitted




Engine back in frame.




That's it for today, unfortunately I'll have to wait until next weekend to finish it, too bad I can't retire already  .

[mike667]

Markus
 i am already jealous - can't wait to see more as you progress!
m

[muskrat]

  Great work Marcus. I just use the seat of my pants to test my work.
Cheers

[MG]

Thanks chaps, I shall keep you updated as work progresses.

I just use the seat of my pants to test my work.

That's about what it's going to boil down to.  
At least I know now that this crank must have been balanced to 54% originally (there was only one hole drilled in the flywheel, so it obviously hadn't been messed with, well, up 'til now that is  ), hopefully some test rides this weekend will show how it will perform using 58% on our twisty mountain roads here.

I found it interesting that the BSA engineers obviously have chosen the balance factor that results in the smallest average resulting force for a given stroke and con-rod length.
The calculations I have done are based on basic geometric correlations of the crank gear known since mankind has built the first steam engines (probably), so I guess the guys from BSA had well considered what they were doing.

Eddie Dow stated a balance factor of 54% as standard and recommended 65% (and above) for racing, and from what I've measured now I'd say this data seems to be correct, unlike some other balancing information circulating on the internet.

Cheers, Markus

[muskrat]

Yes it's all give and take. You can make it smooth in one part of the rev range but it will be worse at another. My race A7 was at about 75% and was smooth at top revs (7500) but would rattle your bones at 3000.
I just got a LJ crank that has been stroked by about 2mm (4mm on stroke) and the flywheel was cut away greatly adjacent to the crankpins. I will do a lot of checking before I use it in the next motor.
Cheers

[MG]

IT'S DONE!



She started first kick, I just did a short test ride down the street and back, everything seems fine so far. Tomorrow I'll go for a longer ride to see whether my balancing efforts were successful. I'll report back.


The dynamo belt drive kit, nice piece of kit, but no one told me I wouldn't get the dynamo back in with the pulley on  . The flange is 1.5mm wider in diameter than the original sprocket and I couldn't get the darn thing through the opening in the crankcase. But it runs very very quietly.




I had the dynamo body sandblasted and chromed over the rough surface, giving a nice dull finish that matches the alloy cases pretty well. Looks good in there, just what I wanted. And it ensures a good earthing connection and a firm grip of the mounting strap on the rough surface. The strap is home-plated with the Caswell CopyCad kit btw. (as are all the nuts, bolts, studs, etc.)




This will go on for next year's riding season, a late Super Rocket head (67-1549). It will need some attention though.




Cheers, Markus

P.S. I'm sure the engine will be okay now, told her she'd end up on ebay if something like that ever happened again.  

[a10gf]

Congrats. Great topic, great results.

[muskrat]

She looks fantastic MG. I'll have to remember that dynamo chroming method for my next time. I have been thinking of a home cad kit for quite some time. Looking at your bike has moved it up the list.
What? A drip tray under her already. Yea of little faith. LOL.
Cheers

[MG]

Thanks for the kind comments gents.

What? A drip tray under her already. Yea of little faith. LOL.

Just being realistic  


The CopyCad kit works a treat, I only bought the liquid from Caswell UK though. I'm using an adjustable laboratory power supply I had anyway and free sheet zinc oddments from the roofer as anode. That's much cheaper than buying the whole stuff.

[MG]

I've just returned from a 60 mls trip through the countryside.  

What should I say, the weather was perfect and the engine is sweeter than ever. In 4th there's almost zero vibration up to 60-65mph, then at 70 it starts to get noticeable in the footrests, it only gets a little bit worse up to 80. I haven't ridden any faster today, as I tried to keep the revs in reasonable limits while running the engine in.
For highway speeds you might want to go a tad higher with the balance factor probably, about 60-65% I'd say, but for the alpine roads here it is ideal, almost vibration-free between 50 and 70 in 4th gear and 35-55 in 3rd, where it is used most.
The irritating high-frequent handlebar vibrations are almost completely gone, which makes it so much more comfortable to ride.

That's what I call a successful operation.  

Cheers, a happy A10 owner.

[Stu55Flash]

MG

Very nice job well done enjoyed reading the topic.

Stu

[trevinoz]

Markus,
               Did you dynamically or statically balance the engine?
Trev.

[muskrat]

And with or without oil in the galleries ?
Cheers

[MG]

Stu,
thanks for the nice comment.

Trev,
it was balanced dynamically. Usually this is nothing to pay out of petty cash, but my engine reconditioner allowed me to use his machinery during an afternoon when he didn't need it, so all it did actually cost was some hours of work and a crate o' beer for the staff. 

muskrat,
I didn't have any oil in the galleries and the sludge trap, but I calculated the weight of the oil in there (volume of the sludge trap hole and galleries less the sludge trap tube and plugs, multiplied by the specific gravity of the engine oil at 80°C) and added it to the rotary weight of the bob weights (more calculating, less oily mess )
This makes quite a difference, I seem to remember it was about 25 to 30grams.

See, you won't catch me on that one! 

Cheers, Markus

[muskrat]

Smartypants 

[Hubie]

I'm now very curious to see how mine will turn out.  My crank is going in for balancing tomorrow so I am very keen to get it back in and see how it goes.  Balance factor is going to be 60-62%.

Great job mate, you'ev only got me more eager to complete my rebuild!

Cheers,

Hubie.

[MG]

Hubie,

I'm sure it'll turn out good with 60-62%.
But don't forget fitting the sludge trap tube, plugs and flywheel bolts prior to having it balanced.
Plus, like muskrat mentioned, the sludge trap should be filled with oil (the oil holes covered with adhesive film to keep it in there). Otherwise you won't have the rotating masses right and the final balance factor will be lower than the 60% you were aiming for. The other way is adding the weight of the oil to the bob weights, like I did.

Let us know how the engine runs once you got it all back together!

Cheers, Markus

[A10Boy]

Blimey Markus she looks great, well done. Now for the CSR...........