Author Topic: Crank Measurements  (Read 4719 times)

Offline Drew Back

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Crank Measurements
« on: 04.06. 2015 04:13 »
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?

Online KiwiGF

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Re: Crank Measurements
« Reply #1 on: 04.06. 2015 12:34 »
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.

New Zealand

1956 A10 Golden Flash  (1st finished project)
1949 B31 rigid “400cc”  (2nd finished project)
1968 B44 Victor Special (3rd finished project)
2001 GL1800 Goldwing, well, the wife likes it
2009 KTM 990 Adventure, cos it’s 100% nuts

Offline chaterlea25

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Re: Crank Measurements
« Reply #2 on: 04.06. 2015 23:14 »
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   *eek*
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

 *conf*
John

1961 Super Rocket
1963 RGS (ongoing)

Offline Drew Back

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Re: Crank Measurements
« Reply #3 on: 05.06. 2015 02:35 »
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..

Online KiwiGF

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Re: Crank Measurements
« Reply #4 on: 05.06. 2015 03:14 »
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.
New Zealand

1956 A10 Golden Flash  (1st finished project)
1949 B31 rigid “400cc”  (2nd finished project)
1968 B44 Victor Special (3rd finished project)
2001 GL1800 Goldwing, well, the wife likes it
2009 KTM 990 Adventure, cos it’s 100% nuts

Offline Drew Back

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Re: Crank Measurements
« Reply #5 on: 05.06. 2015 13:52 »
here are some pics of what I have..How do you even identify it as a long stroke crank??

Online KiwiGF

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Re: Crank Measurements
« Reply #6 on: 06.06. 2015 05:09 »
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.
New Zealand

1956 A10 Golden Flash  (1st finished project)
1949 B31 rigid “400cc”  (2nd finished project)
1968 B44 Victor Special (3rd finished project)
2001 GL1800 Goldwing, well, the wife likes it
2009 KTM 990 Adventure, cos it’s 100% nuts

Offline Drew Back

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Re: Crank Measurements
« Reply #7 on: 06.06. 2015 15:30 »
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..

Online bsa-bill

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Re: Crank Measurements
« Reply #8 on: 06.06. 2015 16:30 »
never done this so here's a guess (gurus feel free to fire from the hip) *smile*

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
All the best - Bill
1961 Flash - stock, reliable, steady, fantastic for shopping
1959 Rocket Gold Flash - blinged and tarted up  would have seizure if taken to  Tesco

Offline duTch

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Re: Crank Measurements
« Reply #9 on: 06.06. 2015 18:26 »

 T'is a bit scientific but that's what I'd do too....  *smile*
Started building in about 1977/8 a on average '52 A10 -built from bits 'n pieces never resto intended -maybe 'personalised'
Have a '74 850T Moto Guzzi since '92-best thing I ever bought doesn't need a kickstart 'cos it bump starts sooooooooo(mostly) easy
Australia