Due a crank rebuild - what would you do...?

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bbenxe
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Due a crank rebuild - what would you do...?

Post by bbenxe »

Well the day after this photo I had a bottom end blowout 200 metres into the morning's riding and leaving a long black tyre mark in the road...
20180707_084229.jpg
I haven't investigated any deeper than taking primary cover off yet as first I've had to repatriate the bike from darkest Cornwall, but that showed up a rotor out of plane with the stator and that rotating the engine sprocket through the 20 or so degrees through which it was free to move elicited no movement of the piston so who knows what's happening down there... Oil change the fortnight before was pretty clean, but there was a lot of miniscule swarf in the oil when I drained it after the bang.

I had planned a rebuild this winter with MX cam, but it looks now I'm going to need at least a con-rod and maybe a crank too. And that opens up some interesting options...

Most economical would appear to be an NEB crank, conrod and piston - although not sure if they would balance the crank to match a piston that isn't one of theirs (i.e. not intended for their nikasil bores - I may be due a rebore and therefore an oversize piston).

I would very much like to use a longer con-rod, as recommended by many, and since I'm forced to rebuild anyway it would seem to make sense to put the best in there and be done with it. This would imply a PES crank and PES/Carillo con-rod? NEB only do a 6" rod.

If I was really pushing the boat out I'd probably go PES crank and 6.5" conrod balanced for NEB piston in 600cc nikasil barrel. There's an interesting idea.

I use the bike for blasting around town, some off-road riding in Wales at the weekends, little bit of track riding (e.g. Prescott Hill Climb), and maybe some proper hill climbing too (would like a crack at Red Marley next year).

This is a little hypothetical until I take it to bits and see what really has gone bang... But I'd like to know how much I should start putting aside for when it comes time to splash the cash in a couple of months, I've got a house move and some other bikes that need clearing out to deal with first, both of which should make more cash available.

So the question is if you were forced to rebuild, what bits would you put inside? What do I not want to in a year's time think "Ooooh, I wish I'd put that in there while I had it open"
'71 BSA B50 street scrambler, '75 Bultaco 250 flat tracker racing as #404 in the UK DTRA National and European series
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AJ CCM
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Re: Due a crank rebuild - what would you do...?

Post by AJ CCM »

You might wreck your Bike if you do Red Marley and finish last all the time if using Petrol.
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Ian Hingley
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Re: Due a crank rebuild - what would you do...?

Post by Ian Hingley »

Sorry to hear this Ben. <218

Feel free to come round and borrow garage space, tools etc or just chat and drink tea.

I was down in Devon three weeks ago and had a massive primary drive failure on the Victor which took the gearbox with it. Maybe we could make one good bike our of our two!

As for upgrades it just depends what you're after and how deep your pockets are. I think I'd go just aim at maximum rideability and reliability and only mild tuning.

Red Marley? Brilliant event! I'd love to have a go, but... Yes, you'd finish last, but what a blast! I'm sure you've YouTubed it by now.

Gonna need my clutch tools back sooner rather later too...

Cheers

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66 BSA Victor Enduro, 72 BSA Lightning, 62 BSA B40 trials, 2015 Truimph Street Twin, 89 H*nda Bros 650, 2000 Aprilia Moto 6.5 Starck
mlb50
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Re: Due a crank rebuild - what would you do...?

Post by mlb50 »

Plenty of thought for what do you want to use the bike for most.....as high performance tuning may not suit every option.Stuff that suits road race can be less than satisfactory for off road. I'd go for gearing and broad torque at lower revs for dirt.

Just a comment on the long rod and what it can entail - barrel raised up 1/2" or so, with special longer pushrods - rocker box to frame clearance, exhaust pipe positioning changes, carburettor to frame clearance etc'. Big bore motor needs improved primary, possibly stronger gearbox.
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Re: Due a crank rebuild - what would you do...?

Post by minetymenace »

bbenxe wrote:I may be due a rebore
:laugh
bbenxe wrote:a lot of miniscule swarf in the oil when I drained it after the bang
You don't say! Just be grateful that the cases are still holding in the big bits.

I think you under estimate the damage. If you are going to give the engine to someone, let them take it apart, it saves on the heart ache. Expect bent valves, knackered guides, damaged seats, bent push rods, bu@@ered piston & bore, hopefully crank is OK, check the cases for cracks etc etc. It will be costly whatever you do to it, but the investment will return a sound engine of known condition. You could start a poll on the ultimate cost :???:

Reliability and rideability seem inversely proportional to performance, but there is a happy medium (maybe he can predict the cost :ban ).

Sensible mods:
1. Strong Rod carrillo, PES etc
2. What mmb50 said about the long rod, but if you are flush, now is the time!
3. Machine the bore to match the piston.

Performance mods
1. MX cam
2. improved breathing (carb, valves and head)
3. Piston (I put this here because you seem set on a particular make), most balancers won't mind who's piston you use, it will be the ring pack that they will quibble over.

Try to find out why it went bang, my guess is oil supply or over revving, you may never know, but some sort of smoking gun give yo peace of mind that the thousands spent will prevent a similar failure in the future.
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bbenxe
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Re: Due a crank rebuild - what would you do...?

Post by bbenxe »

Thanks all for your input!

Expecting heartache ahead then as I'm young and really enjoying learning and have every intention of doing the rebuild myself...

Three points that made me glad I started the topic:

1) 1/2" longer rod: I understood the impacts on rocker box clearance and pushrods, but hadn't considered exhaust position and carb clearance; but I've also been measuring up a flatslide mikuni I want to put on. Clearance for that is tight already, and I think raising it 1/2" would scupper that plan...

2) Finding the smoking gun: important indeed if I'm to be confident in the £££ ahead... I suspect not over-revving as I was just barely accelerating as it went bang while the engine was still warming up and I'm generally pretty conservative in terms of max revs.

3) Piston: I know very little about the piston market. If I was sticking to standard barrel then what would be some reasonable recommendations for a modern piston with good availability in standard and oversizes and similarly for the ring pack?


I'm being brought round to the reliability rather than performance angle. I want to enjoy the bike for the next 50 years and have no intention of racing (Red Marley and other hill climbs are just for the hell of it, who wants to win!?)

A little tuning then but only to bring it up to MX spec and fuel nicely (no bogging etc...) with the pumper flatslide. Low torque rather than max top end, so I didn't go crazy with the Mikuni, it's only a 33mm.

I currently use the bike for road and off-road and seem to be changing the engine sprocket almost monthly to suit what I've got planned for the month ahead... Slightly longer term plan is to keep it aimed at town and off-road and get a twin for fast road riding. Working on that...
'71 BSA B50 street scrambler, '75 Bultaco 250 flat tracker racing as #404 in the UK DTRA National and European series
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