Dommie barrel spigots cracked

Previously wrote:

Ooops, got bit by a dog yesterday and hit the control key by mistake with my sore fingers!

I was cleaning up the set of Dynamo 99 Barrels I was planning to use in my project, they are a STD bore set I've had a long time, so I got a shock when I noticed 3 hairline cracks in the spigots radiating from the clearance notches on the back side, doesn't look like blow up damage, more like fatigue cracking, anyone seen this before? I'm wondering now if I need to line them or otherwise do something to stop this progressing. It seems to have been running like this.

Cheers,

Niall

yes you can get the spigots machined off. the other option is to have them resleeved yours anna j

These will be the spigots that project down into the crankcases I presume. It depends (of course). You don't want bits of a barrel dropping into the crankcases. If cutting away the cracked area of the spigots, effectively extending the cutaways, does not take away too much (question - what is too much?), just carefully cut away. I take it the cracks do not extend above the cut-outs. If the cracking is extensive and/or heading upwards, re-sleeving may be the way to go. Many a 99 has run happily for years with chunks of the lower barrel missing following a con rod breakage.

Thanks for the description of the cracking. Sounds like a good plan you have there. I did have a set of A65 barrels welded up when part of the base flange broke off. The weld worked - the barrels were heated to dull red then welded and then allowed to cool slowly. However, it did introduce a distortion in the bores which a boring bar couldn't sort. One part of the bore had hardened. So beware of welding. I flogged the bike...

The problem with sleeving is that the barrel walls are somewhat thin. However, it does work (I have the barrels to prove it) and you end up with a rather thin sleeve showing where any lower spigot is missing. And of course the lower spigot is now rather thinner than it was. But it works and it's out of sight so no-one else knows.

Previously gordon_johnston wrote:

Thanks for the description of the cracking. Sounds like a good plan you have there. I did have a set of A65 barrels welded up when part of the base flange broke off. The weld worked - the barrels were heated to dull red then welded and then allowed to cool slowly. However, it did introduce a distortion in the bores which a boring bar couldn't sort. One part of the bore had hardened. So beware of welding. I flogged the bike...

The problem with sleeving is that the barrel walls are somewhat thin. However, it does work (I have the barrels to prove it) and you end up with a rather thin sleeve showing where any lower spigot is missing. And of course the lower spigot is now rather thinner than it was. But it works and it's out of sight so no-one else knows.

Just an idea - If you can clearly find the ends of the cracks and all are still well below the flange you may be able to carefully drill a nice clean say 3/16" hole right through at / to include the end of each crack. Lightly clean up each hole with a warding file or the like to remove any sharpness or discontinuity on it's edges (so as not to allow a new crack to propagate from there). This could halt the propagation of each existing crack.

Yeeessss but. If the crack has propogated that far there is a fair chance it will ignore the stress-relieving hole and just continue happily on. In the days when lots of commuter motorcycles had the old handlebar fairings, you would see where people had drilled a succession of crack-stopper holes in the perspex on the screen to no avail as the crack just carried gaily on despite them.

Previously gordon_johnston wrote:

Yeeessss but. If the crack has propogated that far there is a fair chance it will ignore the stress-relieving hole and just continue happily on. In the days when lots of commuter motorcycles had the old handlebar fairings, you would see where people had drilled a succession of crack-stopper holes in the perspex on the screen to no avail as the crack just carried gaily on despite them.

It can be very difficult to see the true tip of a crack so maybe the holes weren't in the right place.

But the stress tending to openy the sleeve cracks will presumably become smaller the closer it gets to the main barrel so either the crack will then stop growing or the crack stopper hole will be more effective than on, say, a screen where the stresses would get worse and worse.

Crack propogation tends to speed up as the crack progresses. For a good read, The New Science of Strong Materials is highly recommended. It is very rare for a crack to stop propogating once it has started - it is more likely just to change direction. My worry is that the crack will extend to the cylinder base flange and then head off round it until a big chunk of spigot lets go and falls into the mill.