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November 7th, 2008, 01:55 PM   #1
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crank pin fitment and crank twist especially on the 650's!


a few months ago i had a problem when an engine i had buildt went wrong, i have mentioned it before and those of you that know me know that i'm relentless in my search to try and cure the factory's mistakes.

essentially, this '02 650 crank twisted just tootling along running it in. the '02 had the 32mm crank pin and as the mathmaticians among you will know, the resistance to the twist is relative to the amount of contact area between pin and each flywheel web.

all things being equal then, the 35mm pins can afford to have exactly the same interference fit and yet be far stronger. the clue is that it would take more tons of pressure to press it in. harder to fit, harder to twist would be the general rule.

the one that wrecks that equation is the one i have hinted at before. there is a point at which the interference fit is so great that the metal spreads and the metal deforms so the resistance to twist goes down.

anyway, i put the question to the new lineaweaver: thomas!

after using a hidden camera and breaking into the factory at night he wa hushed out the next morning dressed as a swedish blond secretary with knee high red boots, luckily he was dressed inconspicuously and his disguise was completed with a dutch beard and two prosthetic legs.

police reckon that when they catch him: he won't have a leg to stand on!

anyway, the word is from our man 'inside' is that the old 82mm cranks with the 32mm big end could be too loose - and anything less than 0.09mm (.004" known as 'four thou') difference between the pin and the hole and they could twist easily so the factory changed the specs.

Now they are between - 0.10 to 0.12mm (also known by all previous members of our huge empire as .004" to .0045" or four to four-and-a-half thou) on the 450cc and - 0.11 to 0.13mm (that'll be .00425" - .005" and again four and a quarter to five thou) on the 628cc with the 80mm stroke and 35mm big end.

thomas' advice is that it should be no less than - 0.10mm (.004") on any crank.

if a crank does twist after a rebuild it will have broken the mains which must be replaced but also to just straight it and weld it, it will work for another 100-200hrs with out any problem.

to press the old pin out may take between 10 and 15 tonnes of pressure

to install a new pin will be between 5 - 8 tonnes and often more!

"We know that nearly all 650cc cranks, twist by time.
The 644cc more than the 628cc"

Ben Ballard was telling me recently that he has pins .0015" oversize on the left and right of centre (the big end remains the same) which i think are for the 35mm pins. i think he has pretty much given up racing the 32mm pin engines.

today i collected two cranks from the engine reconditioner and they had measured them. a 470 crank was .0035" and tthe 501 was .0045". i have to make a decision about the 470. it is going to be hill climbed down in the west country.

this will go into the doc as you know....


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November 7th, 2008, 03:26 PM   #2
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RE: crank pin fitment and crank twist especially on the 650

Preparation can play a part too. I have assembled lots of machinery over the years and deburring or chamfering the related parts goes a long way on heavy fits. Initial alignment is critical. Any "off-square" and there can some oscillation deforming the contact area. I usually oil everything well to help it get moving smoothly.
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November 7th, 2008, 03:41 PM   #3
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All good info above.

I think Neil hit the nail on the head re. pin to web alignment.

Getting that pin in and out of the web bores perfectly square is a bit of a challenge with the average hyd. press- after watching a fussy guy with a purpose built crank press makes you realise just how "agricultural" the typical press is, and the need for extra care supporting/shimming webs true and flat on the press suporrting areas
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650, crank, fitment, pin, twist

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