Might have left your run a bit late maybe, 3 days to get in shape including 1 day drivingOriginally Posted by F5 Dave
Forum whore
Might have left your run a bit late maybe, 3 days to get in shape including 1 day driving
My neighbours diary says I have boundary issues
Long Haired Yobbo
Beer fixes that.
Heinz Varieties
No8 Wire Extraordinaire
Had a good nights sleep. That was worth it. I was a write off last year. This time of year for some reason
Don't you look at my accountant.
He's the only one I've got.
No8 Wire Extraordinaire
We'll with the BOB been and gone there were mixed results. The bike was clearly the fastest thing out there although I never lined up against Scott. But it was still too peaky esp with the tall gearing. The temp stayed constant at 60* +\- 1* which was good considering the tiny pump. Cooler would be better but much better than air cooled.
the bike seized twice into the hairpin and was shite off closed throttle. Must check for air leaks. I put in a drilled pilot I found and had no more issues. There were two minor seizes on the rear skirt.
but the big issue is the destroke offset pin broke 2nd yr running. Fortunately the top end has survived. Hopefully the bottom will have too. The pin is broken on the flywheel side but a dinner party has stopped further autopsy.
So what to do? I need to run the 52mm piston but the pin either has to be remade better (expensive and still not certain) or the crank offset bored to fit the bigger pin. I avoided this in the first place as it will undercut into the shaft on the other side of the flywheel. Greg however says this is common practice on some old beaters so maybe it's worth a try.
the std pin was offset ground with generous radius. But clearly it's not good enough and peak power is a lowly 12,000 so further revving planed wil be more stress.
and I clearly need another pipe to get some midrange. So I'll modify the transfers as it's easier.
Don't you look at my accountant.
He's the only one I've got.
Forum whore
The offset pins radius in the corner of the step maybe the issue with blending the radius at the transition of the offset. It may be possible to see where the origin of the crack is from the fracture lines.
Without a decent radius a crank will break sure as eggs.
On this one it looks like the cracks origin is the top corner where the pin meets the crank web.
Low personal B.M.E.P.
Dave has previously said he's happy with the radius at the transition. What i'm not happy with is the amount of welding on the cases...I'd really like to see someone check just how accurately the mains are lined up...The MB cases don't have a lot of inherent strength/stiffness.
Any motor with a pressed up crank and mainshaft/crankpin overlap will have a reduction in mainshaft section area at the point where the pin shows past the web. If you're seriously concerned, chamfer the end of the pin - drive side only - and once assembled and aligned, weld the bastard...
As an aside, Dave, did the guy assembling the crank say anything about the pressure required to assemble it ? I assume it is a solid pin ?
Bucket Racer/Crasher
Pretty sure Dave's pin is dimensionally the same as mine though he got there by grinding the ends down to 20mm to fit the MB crank, starting with a 22mm pin. I had mine made from scratch by Heat Treatments. It had lots of "0"s in the cost. Possibly a need for heat or surface treatment after machining.
No8 Wire Extraordinaire
If I understand heat treatment will affect the hardness of the surface but not the strength.
Don't you look at my accountant.
He's the only one I've got.
Low personal B.M.E.P.
Depends on the material....If you're starting from a bought pin - bought as part of a rod kit - I wouldn't do any heat treatment post grinding the end down.
If you're grinding back from 22 to 20mm you're going through the case - or should be. If it's hardened deeper than that it's probably too brittle.
If you're making a pin from scratch - as i've done several times - the right material, EN36 is my preference, rough turn leaving a grinding allowance. Get it case hardened then finish grind. In the sizes we're talking about stress relieving shouldn't be necessary - and nor is hardening it right through...
Dave, when you press the broken pin out, have someone test the reduced portion for hardness. If it shows as being case hardened that deep, I'd say the pins have been too brittle - I'm assuming it's a bought pin ground down.
No8 Wire Extraordinaire
yeah bought pin.
Who would test that?
Don't you look at my accountant.
He's the only one I've got.
Bucket Racer/Crasher
At least one Griffith has a hardness tester, I think.
Low personal B.M.E.P.
[QUOTE=F5 Dave;yeah bought pin.
Who would test that?[/QUOTE]
In ChCh, Wallace has one so I'd ring small precision engineers who advertise they can do grinding...
(no reflection on Wallace's height intended....)
No8 Wire Extraordinaire
Yeah actually Dave Griffiths was saying he had one at the BOB now I think about it. Thanks for the ideas.
Don't you look at my accountant.
He's the only one I've got.
Long Haired Yobbo
I remember you asking about interest to get a run of these done.
I'd be keen, depending on the final pricing, Dave, maybe Mike C?
would it make it that much cheaper do you think?
Or do we do go for a rule change lobby?
Heinz Varieties
No8 Wire Extraordinaire
Well here are some pics. Break between 1.1mm from radius and 4mm so not that near stress raiser. sort of torn.
Looking at it I'm keen to build a jig and offset bore to 22, there is actually a bit of meat in the webs. Just cant go 'through' as seal runs there, has to be done from inside the webs.
Don't you look at my accountant.
He's the only one I've got.
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