Author Topic: Crankpin oil hole not there!  (Read 2065 times)

Offline Rusty nuts

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Crankpin oil hole not there!
« on: 10.09. 2009 15:58 »
Well I was going to wait till winter for the next one but have started fiddling with the left over bits from the plunger.
There is pretty much a complete longstroke engine in boxes, including rebored barrels, reground crank new hepolite pistons.
Only problem is this was done in 1971 & with the exception of the pistons is all rather rusty!

Anyway, the timing side crankpin has only one oil hole drilled.
Have removed sludge trap plugs & there is no evidence that ever was a hole in one half of the crankpin.
My original thought was a shell must have seized into it but looking from inside of sludgetrap it is completely undrilled!
Have spoken to SRM, the guy I spoke to has seen some early cranks like this, he recommends drilling it out.
So that's the engineering perspective.

My lazy perspective: I'm thinking well it's been like this since nineteen fourty something & is only on second regrind both crankpins same size so maybe one hole is enough.

 Theory 1: BSA special experiment, one hole supplies enough oil to crankpin/shells while increasing residual oil pressure to troublesome T/S bush which needs it more.
Theory 2: This is a "Friday afternoon" crank.

Anyone had similar experience, run a crank like this, have any other theories, want to shoot my brilliant theories down in flames?

Cheers,
 Rusty
1949 A7 Plunger
1947 A7 Rigid Star Twin
1969 Triumph T120R
1972 Triumph T120V

Offline LJ.

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Re: Crankpin oil hole not there!
« Reply #1 on: 10.09. 2009 21:20 »
Right... well... I was sitting comfy at my computer untill I read your post!  *smile* Curiosity got the better of me and just *Had* to go out to the shed and look at my early A7 crank. You'll be pleased to know that mine is the same as yours.... See  photos.  ;)

As far as your theories go.... I think I'd leave the crank as it is unless mine was made on the following Friday!
Ride Safely Lads! LJ.
**********************
1940 BSA M20 500cc Girder/Rigid- (SOLD)
1947 BSA M21 600cc Girder/Rigid-Green
1949 BSA A7   500cc Girder/Plunger Star Twin-(SOLD)
1953 BSA B33  500cc Teles/Plunger-Maroon
1961 BSA A10  650cc Golden Flash-Blue
1961 BSA A10  650cc Golden Flash-Red

Online RichardL

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Re: Crankpin oil hole not there!
« Reply #2 on: 10.09. 2009 23:59 »
Seems it must be that a single hole on right increases olil flow to two holes on left, which are fed through the small oriface between the crank pins. I don't think it has to do with improving flow at the T/S bush, as that is fed first.

Richard L.

Offline Rusty nuts

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Re: Crankpin oil hole not there!
« Reply #3 on: 11.09. 2009 07:55 »
LJ
 Thanks, that's very reassuring, will leave as it is.
Manosound, I think you're right.
Sitting in the garden enjoying the autumnal sushine & a couple of beers, crank & cases spread over garden table, as you do!
Sip beer, peer down sludge trap, sip beer study cases, enlightenment!
Sip more beer, aah, contentment *beer*

Off to Netley Marsh now to amass more bits!
Cheers
Rusty
1949 A7 Plunger
1947 A7 Rigid Star Twin
1969 Triumph T120R
1972 Triumph T120V