The BSA A7-A10 Forum

Technical => A7 & A10 Engine => Topic started by: KiwiGF on 06.09. 2012 07:55

Title: Crank Balancing DIY
Post by: KiwiGF on 06.09. 2012 07:55
I thought I'd share a bit of information I've gathered done prior to putting my small journal A10 engine back together.

I'd heard that JP pistons and billet rods were heavier than the original BSA components, and consequently I might end up with an engine with abnormally bad vibration, due to the non OEM components altering the crankshaft balance factor significantly away from standard.

So I thought I'd have a crack at checking the balance factor "DIY style".
To start the job I needed some scales. I used cheap 0-5000 gram scales which I checked for accuracy using several different sizes of tins filled with water. I found the balance was consistently within 2% of the calculated weights of water (1 liter=1000 grams) which is good enough for what wanted.

Service sheet 712X gives useful information on cranks balancing, a copy is attached below.

The service sheet for my engine gives the standard counter balance weight as 2 x  18 oz 10 drams, which I reckon is 1056 grams. 712X also says there is no need to rebalance the crank unless the new big end assembly is different in weight by more than 1 to 1 3/4 oz (approx. 30 to 50 grams).

As 712X shows, without special equipment it's not easy to measure the counter weight (CW) of a crank, on the other hand I have no reason to think the crank I have is not counter weighted as standard e.g. been messed with.

For a rough check of CW I measured the weight required to make the cranks counter weights stay at horizantal using the method shown in the attached photo. The weight of the bottle of water and rod came out at 1050 grams, which is near enough to the BSA figure of 1056 grams for me to safely assume the crank is standard. I also  turned the scale upside down and used a pencil to measure the upward force from the crank pin, again the "counter weight" came out near enough 1056 grams.

The next job was to weigh the piston components.I measured a few pistons to gauge whether the JP pistons I had bought were likely to be significantly heavier than the OEM.

The results of measuring are below.

Desc                                   Weight (grams)     Comment
========                             =========      ===========
Bare Piston JP brand 7.25:1          256             Bare means no pin, rings, clips
JP Pin                                         78               Not tapered ID, 0.475' ID
JP set 3 rings                               25
JP clips x 2                                  1.5                      
Bare BSA piston flat +060 bore      261             Its "partner" weighed 258 grams
Bare piston 8.5:1 std bore              241             Unknown brand
Bare hepolite 9:1 std bore              251
Pin, tapered ID                              78
Pin  unknown brand                       89              Not tapered in ID, 0.43 ID, HEAVY!  

Next up was to weigh the con rods, the weight of each con rod end needs to be measured separately, the method I used is shown in the photos below. I weighed the rods with both ends resting on nails placed under the middle of the small and big ends. Not having the nails exactly on centre did not affect the measurement too much so I reckon this gave an accurate enough measurement for my purposes. It would not be that hard to make some weighing fixtures to measure with the rods on edge and not horizantal, but I did not find that to be necessary. As another check, I also weighted the whole rod and this came out near enough the total of the 2 ends weighed separately (only 2g difference).

The results of measuring are below.

Desc                           Weight (grams)             Comment
======                         =========              ==========
OEM BSA SJ rod Big end         296                  Rod has nuts/bolts/SE bush/-040 shells
Ditto small end                         110                  Ditto
Complete BSA rod                    404                  2g different to measuring ends separately
Thunder SJ rod Big End            298                  Rod has nuts/bolts/SE bush/-040 shells
Ditto small end                         122                  12gr more than OEM rod
Complete thunder rod                422                  2g different to measuring ends separately
-040 shells                               41                    2weight given is for half shells, for one rod

Now this is where it gets a bit tricky. I could have stopped here as i found the weights of the non OEM components I'm using are not much different in weight to the OEM parts....but I thought I'd look up how to calculate the balance factor to be sure a small increase in weight does not make too much difference.

The calculation  of the reciprocating weight (RCW) using non OEM parts is as follows:

Desc                          Weight (grams)
=======                    ==========
2x JP Piston              512
2 sets JP rings            50
2 pairs JP clips             3
2 x JP Pin                 156
2x Thunder Conrod     244  (measured at the small end)
Total (RCW)             965g

Rotating Weight (ROW) = 2 x Thunder Conrod/nuts/bolts big end = 596g
Counterweight (CW) = 1056g (BSA standard)

BF = 100 x (CW - ROW)/RCW    (this seems to be an accepted calculation for BF)

BF = 100 x ((1056 - 596)/965) = 47.7%

Using weights for OEM parts, BSA rods and pistons (at 261g ea)

BF = 100 x ((1056 - 592)/951) = 48.8%

I've not included for the weight of oil in the big end gallery in the above calculations. I reckon it would not make a big difference to a SJ crank, which holds very little oil. It would maybe add 1 or 2% to the BF.

The OEM BSA balance factor (small journal engine) seems to have been in the region of 50% using the above calculation method, and the actual balance factor using my non OEM components is not much different to that. need for me to drill extra holes in the crank :-)

If I've made errors in my calculations or methods of weighing - please let me know! (also if I've got it right would be good to know).
Title: Re: Crank Balancing DIY
Post by: duTch on 06.09. 2012 11:18
Kiwi, you need a medal for all that !
  Believe it or not I did read it all, Fairly much correlates to mine, except for LJ. I calculated about ~25 grm for the oil.
I used strips of lead(old flashing at the metal merchant $2 /kg x 4) as Musky suggested, but it was clumsy, so melted it down with a sleeve in a green bean tin-yuk & 1 pea tin(~72mm dia x ~24mm deep/wide), and halved it & strapped around journal with tape.
  My pistons are 360grm ea  x 2  =       720
  Thunder Rods   440    (310/130) x 2 = 880 (620/260)
  BSA rods (LJ) ~382   (280/102)

  To cut it short, it 'balanced' with ~1166grm, calculated what I had as ~54% but had a new difference of about 90grm (1290-1199), which would've given me about 47%, so drilled a couple of holes, and I think I've ended up with about 58%.

  These are figures of top of head, if need be can dig out the writings(after deciphering hieroglyphics) .
 I've been endeavouring to take pictures, of bits of 'aids', too, hopefully that'll happen.

 Gotta go now, more when I remember what I did, duTch


Title: Re: Crank Balancing DIY
Post by: MG on 06.09. 2012 20:52
Hi Kiwi!

Your calcs are spot on! I had posted my balancing efforts a while ago, and back then also found an original balance factor of 54% with the weight of original components (incl. the weight of BSA pistons at STD size) and a previously undrilled crank.
I have however altered the BF to 58% during my small-end-failure-conditioned rebuild, and I'm quite happy with the result. Next time I mgiht go a tad higher though, maybe 60-62% to move the sweet spot a bit further up the rev range. She's a bit rough above 70mph in fourth, but smooths out again above 85.

Title: Re: Crank Balancing DIY
Post by: KiwiGF on 07.09. 2012 11:58
Thanks Dutch and MG. I was not 100% sure about the arithmetic part so it's good to get confirmation and other evidence that's how it's done.

It sure was a long post! Probably TOO long for most!

Title: Re: Crank Balancing DIY
Post by: Topdad on 07.09. 2012 13:20
found it very interesting but totally outside my capabilities I'm afraid , I'll just have to splash the dough when I need anything like this done  best of luck to you techno's BobH.
Title: Re: Crank Balancing DIY
Post by: duTch on 07.09. 2012 13:40
 Thanks, but just because we had similar maths, doesn't mean it's confirmation of correctness!!! 'specially if I'm involved, I was just going on what I read in the other end of the story, and made the rest up *smile*-as I went along-kind of?
     Mind you MG sounds like he knows what's the g-o.though.
 I made a doo-dad, with 4 bearings on it like one of those scissor wheels in the service sheet, and it was fun to play with, but found better results with the couple of bits of 25mm angle 'bout a foot long that I 'sharpened one edge of each to simulate 'knife edges', had to get them straight tho' or else the crank ran out of steam on the hill and ran away downhill....!! (if you get what I mean).

 I also found it necessary to make a sleeve the same OD as the T/S main journal, to fit where the D/S roller fits, otherwise crank wanted to run in a curve.
  I also realised today, that my crank has had a 'chamfer' (only about 5x8mm x130mm around the arc)on the outside edges of the fat side of the outrigger bob weights, probably doesn't affect my calculations, would've saved a bit of work if they weren't ground(as the other old crusty crank is)?
 Another significant thing that Kel (my mate)stressed, is to check the 'Balance of Symmetry', where you sit the crank 'naked' on the knives (yes I did make them nice and level), and check that the line through the Big-end and shaft centre is vertical, B/e NOT leaning one way or t' other.

    Have ta leave it there, for now -tell me if I got anything wrong??
 Cheers, duTch

Ya shanghai'd me Bob, rounded me up
Title: Re: Crank Balancing DIY
Post by: Rocket Racer on 19.03. 2014 00:20
I guess I'd summarise the post as highlighting that balance factors are as standard fairly low and that oversize pistons which are typically heavier reduce them further.
Whereas modern roads and their associated higher speeds are more suited to increased balance factors.
So if you're replacing pistons with heavier ones or fitting heavier rods its worth getting your crank balanced.
Title: Re: Crank Balancing DIY
Post by: muskrat on 19.03. 2014 08:56
72% works well at 8000 RPM  *eek*
Title: Re: Crank Balancing DIY
Post by: KiwiGF on 19.03. 2014 10:22
well its been a while since I posted this up and nice to see it gets read at least a bit!

I've now done over 3000 miles on the engine (JP pistons and billet rods and std crank and extra tooth on engine sprocket) and can report on the vibes albeit I've not ridden nother A10 as a comparison.

To me the engine is smoother than I expected at low rpm, in fact not much worse than my Suzuki 1400 v twin, at 60mph a tingle starts, at 70mph it's still ok is but the tingle is more of a buzz and is beginning to get uncomfortable for extended periods, and I cannot see the local NZ plod in the mirror.

I don't rev it much more than its going at 70 mph even through the gears (so muscrat I can't comment on what its like at 8000 rpm)  *smiley4* but obviously the vibes would get worse at higher rpm.

If I spent more time at above 60mph it would be worth changing the balance factor to reduce the vibes I guess, but for me its fine as it is.

If I have to tear the engine down for any reason I might get the drill out though  *smile*