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.a7a10.net/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
