so how come? (big end chat!)

yes my crank spreads, so does every crank Ive ever seen, welded pin or not, you can't stop it

to get a (my) welded trued 628 crank to spread as far as it does in use I have to flex it 3-4mm in the press. so I say the flex in my engine (35mm pin) at the flywheels is around 3-4mm

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GWR over trues and welds the pin so that when the crank spreads its closer to being right ie he sets it up "unspread" but not trued.

what Im getting at is the husky may "get away" with a smaller pin because it has a tighter fit and because it doesn't have the silly CB forcing the mains further out and increasing crank deflection.

just a tad im going for 0.03mm toe out opposite the pin

well if we knew the interference fit of a new husky pin we'd have something to say.

cranks do move on the pin. not just spreading but also twisting, thats why we weld the pin because it does stop twist

there is little point welding this 250 2t pin cause its so tight I can't true it lucky its about right

well i don't know how we can predict how much the crank can flex when in use? pretty much impossible i think.can't quite follow you on that one bushy?
when we pressed the crank pin out of my crank, we also pressed the crank pin out of my mates wr450,no comparison mate, the interference fit was far greater in the wr's crank.
tell me,if you take the crank out of the motor after you done a couple of hundred hours of use & the crank is spread opp the crank pin(like it usually is) then the crank webs have to move on the pin taffy,how could it not.
with a welded crank pin at least the crank spread can't move,although it may flex.at least it will always come back to where it was welded.a smaller pin has less surface area to grab on to,so if it does not have a very tight interference fit,crank spread & trueness/allignmentcan change.on the motors that you are pulling down to rebuild taffy, are you taking note of crank spread trueness& allignment when you disassemble the engine? i know i will be.
..weed..

not saying this is it but a crank can bend just once to a new position and then stay there.

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yes thats it and the 32mm pin berg cranks move further than the 35mm pin before they stop moving.

the welded pin cranks still move on the pin because each flywheel is only welded on one side. it elimanates twisting but not flexing or spreading.

last build I had 0.02mm runout after welding and peening, its now got 0.06mm runout and I had to flex it 3-4mm in the press to move it back to where it started.

so the flywheels move on the pin till they are skewed enough to stop moving, more skewed means more resistance to further movement. more interference fit or bigger diameter means less skewing needed for a given resistance

theres not enough info to say why the husky has only a 30mm pin but without the Hberg placed CB its 30mm pin might take on less spread than my 35mm berg pin crank even with the same interference fit.

why does that 0.04 - 0.08mm matter if the crank flexes a full 3-4mm during use?

Im guessing it only flexes that much very occasionally so the rest of the time the crank needs to be as true as possible.

I don't really know, I do know that a high HP road bike engines with a one peice cranks and plain main bearings go upwards of 100 000km of being absolutely thrashed and abused.

there is a 100hp+ hberg hillclimb bike in the US, it has a billet crank with the pin being part of one side. normal pressed cranks twist with high tourque outputs and loads.

its 0.06mm not 6 thou mate keep up at the back!

I just pressed it true again in the press squashing the flywheels together opposite the pin

bushmechanic said:

[quote
the welded pin cranks still move on the pin because each flywheel is only welded on one side. it elimanates twisting but not flexing or spreading.
pressed cranks twist with high tourque outputs and loads.

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yeah... bit of a bummer.i thought welding the crank pin would eliminate spreading ,but yeah..i got the picture now.only makes sense that it wouldn't.
not much we can do about it lads, just keep doing the bandaid fix.
she'll be right mate hey...
..weed..

Hi y'all

I don't know if this is relevant but if you look at race engines you will see far more chev 350's than ford 351's.
The main reason is because the 351, being a truck motor, has a massive big end resulting in much higher big end bearing surface speeds. So they don't like sustained high speeds without a little fettling.
A white metal bearing with a good oil supply can spread the load over the whole bearing surface. High spots get worn off and everyone is happy.
A roller bearing big end can run with just oil mist so doesn't need an oil pump as such. See original berg motor.
A roller big end assembled with just one roller 2.5 beesdicks bigger than all the others will fail because that one roller takes all of the big end load on a very small area instead of being spread through the whole bearing.
The hardest part with crankshafts is that the crank pin comes from one mob, the roller bearings and conrod from two others so assembly needs to be so meticulous. Who mics every roller to guarantee they are exactly the same size?
A roller bearing needs to get the lubrication to the middle of the roller. the wider the roller or the higher the speed, the harder it is to evenly lubricate and the more chance of skating as the rollers try to climb over the layer of oil.
Roller bearings are incredibly strong- in one direction only. Any roller bearing run out of alignment will fail. If you guys are talking about 4mm crank deflection at full power (I hope I'm moving in the right direction) then the wide roller bearing on the drive side must have a fairly out of centre shaft twisting around in its centre as it rotates. There are self centring roller bearings out there but it depends on what fails.

Anyway, I'm an electrician who fixes ovens and i haven't yet blown a berg big end. I'm happy to say i'm on my third. Hey, has anyone thought about hooking a variable resistor to the map switch instead of fixed resistances.

There is a finite element analysis of a crank flexing on the JBS
web site,
http://www.jbsracingpage.co.uk/Shop%20C ... g_Shop.htm
scroll down a bit.
How many tons does it take to press the big end pin in/out?,
maybe not enough interference would be my "guess".

Cheers spanner

Do the old 600's rev as hard as the newer generation motors, revs are always killers of reliability..... and as for pin size, husky's are only shopping baskets and maybe berg have gone a step closer to the limits, hence narrower rod/smaller pin. Comparing with kawasaki backs up my argument, ie the berg will be ready for a rebuild when the kwaka gets a service at the same miles. But then my KLR sits unused whilst the bergs been thrashed to death. sez it all bro!