Crank Measurements

[Drew Back]

Looking at my crank tonight I noticed it was a little different on each side primary was 1.396 and timing.  was 1.425 and the shells were .30 and .40 does this make any difference??There is no play up and down on each..I was also confused about measurement of timing side  which is 1.366 what would a standard bearing be or what size under should I get to have it line bored??Can you go any more than .40 on the big end shells?

[KiwiGF]

I have no experience of long strokes...but does this help?

http://www.bsawiki.com/chaps/9-engine/15-chapter07

Generally when one big end needs re grinding so does the other so usually they will be both on the same undersized shells, if the the crank balance is not affected I guess them being different is ok.....I can't imagine the balance would be affected much if at all?

Note aftermarket -040 shells are readily available for short stroke engines but bsa did not envisage going that much undersized in "period" service sheets etc.

The generally accepted approach to the timing side main (short stroke) is to grind the minimum off the journal to make it round then make a one off bush to fit, often a one off is needed anyway as the case is worn so a standard bush cannot be used anyway, so a one off bush with a bigger o.d. Is needed to fit the case.

[chaterlea25]

Hi Drew,
The measurements you have given show a difference of 0.029in 
The bearings you mention are different by 0.010 
Somethings NOT RIGHT   
Are the long strokes different to the small journal ?

Below are small journal sizes from SRM's website, http://www.srm-engineering.com/technical/bsa-a7-a10-big-end-journal-sizes

 "STD"..................... 1".460 - 1".4595                                                                 "STD".................1".374 - 1".3735   

       010"...................... 1".450 - 1".4495                                                           -010"................ 1".364 - 1".3635

       020"...................... 1".440 - 1".4395                                                            -020"................ 1".354 - 1".3535

       030"...................... 1".430 - 1".4295                                                            -030"................ 1".344 - 1".3435

       040"...................... 1".420 - 1".4195                                                                         -040"................ 1".334 - 1".3335

 
John

[Drew Back]

That is what I measured and those were the shells with the rods that came off the crank..Seems funny to me also but you never know what you are getting when digging through a junk pile..From your chart am I correct to assume that the timing side is -10??I think I will have the crank resurfaced to the standard size anyway..I will check my machine shop to see if they can do it..

[KiwiGF]

Hello Drew, it might be worth double checking the short stroke journal sizes are the same as small journal long stroke? But if they are the same yes the timing side journal is standard at 1.375 so yours is about 010 less than that. Both mains and big ends wear oval of course and it's really the amount of ovality which determines the need to re grind or not.

If you are lucky you can buy say a -020 main bearing bush and line bore/hone it to suit your journal, but as per my prior post the cases are often worn by a spinning bush so that often does not work out.....

Make sure you get new shells before grinding the crank! I have a feeling the long stroke shells are different to short stroke (wider?) but hopefully someone will chime in soon who can confirm / deny this.

[Drew Back]

here are some pics of what I have..How do you even identify it as a long stroke crank??

[KiwiGF]

I don't know if there is an obvious visual difference between crank types, but you could measure the stroke?

If it's a short stroke crank then it's probably no use, as it's gone past the accepted wear limit. That would possibly explain one pair of the shells being marked oddly as they maybe are not made for an a7 crank but for something else.

[Drew Back]

This may sound like a dumb question but how do you measure the stroke on just the crank this is all new stuff to me and im not a machinist in any terms I need it in po boys terms..

[bsa-bill]

never done this so here's a guess (gurus feel free to fire from the hip)

measure from centre of crank to centre of crank pin if that's correct term (the bit the rod rotates on) and then double it

[duTch]

 T'is a bit scientific but that's what I'd do too....