The BSA A7-A10 Forum

Technical => A7 & A10 Engine => Topic started by: duTch on 30.06. 2012 17:42

Title: Gudgeon Pins
Post by: duTch on 30.06. 2012 17:42
 I went on a hunt for a " gudgeon pin" thread for info or add to, but no-go but read some interesting stuff from the archives!!(Richards' 'Rocket 8')

  The other week when I set my timing I noticed that the pistons didn't quite make it to the top of barrels...hmm
First thought- I had some big-end holes re-sized at some time but not sure if it was these ones, that'd pull them down hey??
Next I measured each side of each piston to top of barrel- all different.
  From left to right-> top of barrel to top of piston->L .0175", 0.0135", R->0.015", 0.025"
                               Left -4 thou diff. across piston, Right-10 thou diff. across piston
  My summation and easy fix is that the little end bushes are reamed out of square, and possibly 'twisted' radially (fore-aft)comprende??
    The way I see it this may not pose much of a problem for a while, but could be potentially catastrophic??
           As I needed to do a total ~assemble to assess things like brake pedal fit(yeah right), Rocker box on/off, and other stuff ,general mock up, I'll be stripping it anyway to check bushes,C-rod length, and whatever, no big deal but if anyone has other ideas, I'm open to it??

 Saw KiwiGf post re- thunderrods?, sent them an email also, may be an even easier fix,+$$,££(option+'3' on macbook), which reminds me,
 bsaBill- I found ˚˚degrees˚˚,on the macbook >option+'k'<, Shift'5' is-%, could be different on PC, mine won't kickstart/bootup!!
Title: Re: Gudgeon Pins
Post by: chaterlea25 on 30.06. 2012 18:59
Hi Dutch,
Heres a method which checks conrod alignment (smallend, bigend, rod bend, or twist
I got this from a prewar Harley manual  *eek*

Sit two engineering parallels on the crankcase face either side of the piston, cut 4 strips of thin paper and fit these between the piston and parallel front and back on both sides !
turn the crank over gently till the piston traps the paper strips,
hold the centre of the piston down using one finger.
Then check which if any of the paper strips are free to move,
If the two on one or other side are loose,  then the alignment (rod or bush) is off in that direction
If the diaganol opposite strips are loose then theres twist in the alignment
Rotate the crank till the rod is going at the opposite angle, then check again, if the loose strips are opposite the first test, it confirms twist!!!

The Harley method of ""fixing these problems "" involved a long close fitting (to the smallend bush) steel bar
and some brute force  *eek* *eek* *eek* *eek* *eek*

I always check rod alignment on engines I build, its amazing how many are off kilter  ???? ????

Regardind new conrods, MAP cycle rods (USA) may work out a good bit cheaper than the UK rods, worth checking out!!!!!

John O R

Title: Re: Gudgeon Pins
Post by: fido on 30.06. 2012 19:01
I would have thought if the pistons were running that far out of square in the bores it would have seized before now.
Title: Re: Gudgeon Pins
Post by: trevinoz on 01.07. 2012 01:00
                 Machine a bar to a sliding fit through the bushes, long enough to go through both at the same time.
If everything is square, the bar will easily slide through.
Title: Re: Gudgeon Pins
Post by: duTch on 01.07. 2012 01:43
Thanks All,
              Johns' method sounds good, but no 'engineering parallels' on hand but should be easy to source, I had Trevs idea in mind, as I have 5 old rods in various conditions(origins unknown) slid snug onto a piece of stock stainless bar (19 mm/0.748"). 'Assuming' that the rods are same thickness top and bottom, I figure there should be no gaps between top/bottom/front/back, in all combinations of side by side? The results here are interesting to say the least.
 I think that bit of bar will do the job nicely, thanks Trev.
 Could be a few days before I investigate.
  Fido, that's kind of what I'd expect, and (reluctantly) needed to hear. I did about 2 and half thou miles(2 on the 1620 clock) on it before the maggy started playing up, but that was long ago, lucky the pistons aren't too bad but definitely be having a look.
  Thanks guys, cheers
Forgot to say ,John I had a look at MAP cycles- $180 US ea./ SRM's £156. will see what Thunder have to say, and outcome of strip down,cheers