Sal­vaging crankcases with smashed off mount­ing bosses. A sim­ple, ef­fec­tive fix for a com­mon prob­lem

Practical Sportsbikes (UK) - - Contents - Words and pic­tures: Alan See­ley

You know how it goes – tight, tight, snap, crack, slack. And there goes a cast boss. I have two sets of Honda CRM250 crankcases, both have snapped­off bosses where the lower screw of the gear­box sprocket cover threads in. Whether this was caused by a snapped chain or the care­less­ness of pre­vi­ous own­ers is a moot point, it needs sort­ing.

As I still had the bro­ken off piece for this set of cases, I thought I’d go for a solid fix with­out re­course to weld­ing. You wouldn’t want to do it this way on a boss with a crit­i­cal struc­tural ap­pli­ca­tion, such as an en­gine mount, but in the con­text of a non-stressed area the com­bi­na­tion of se­ri­ous epoxy ad­he­sive and a thread insert as set out here are more than up to it.

01 The prob­lem

There it is. A nice clean break, although it ob­vi­ously went a while back go­ing by the level of ox­i­da­tion.

02 Brush up

Us­ing a stain­less steel wire brush to clean the worst of the dirt and cor­ro­sion from the prob­lem area.

03 Deeper clean

Give the af­flicted spots a deeper clean with some cel­lu­lose thin­ners or brake cleaner.

04 Hole in one

A pil­lar drill as­sists in get­ting the pilot hole for the thread tap dead square. Don’t go too deep and risk break­ing through the cases.

05 Hole in the other

Drill a cor­re­spond­ing hole through the bro­ken-off piece. A drill press vice al­lows it to be held dead square.

06 Tap it

Cut­ting a thread all the way through the boss with the tap from the thread insert re­pair kit.

07 Ta­per trick

The tap for the insert is ta­pered. Wind it into the boss just enough that it doesn’t pre­vent the boss be­ing placed flush on the case.

08 De­burr

Drilling the crank­case left a burr around the top of the hole. This can be re­moved with a larger drill bit.

09 Tap it on through

By wind­ing the tap through the now threaded boss un­til the thread starts to cut into the crankcases, we can be sure the insert will thread in cor­rectly. Re­move the tap and the bro­ken boss and cut the thread in the case prop­erly. Blow the swarf out.

10 Mix it

JB Weld is our favourite work­shop epoxy. The full-house stuff does take 24 hours to go off how­ever.

11 Spread it

Us­ing a rivet to put a thin bead of JB Weld on the sur­face of the break. Don’t use too much – it will only squeeze out.

12 And spread it some more

Put some JB Weld on the newly cut threads too so the insert will be glued in by it as well.

13 The insert

Set the thread insert on the tool with the tang lo­cated in the tool’s slot.

14 Wind it in

Twist the insert into the bro­ken-off bit and into the crank­case, tak­ing care that the former doesn’t lift from the lat­ter.

15 Grease your bolt

Lightly grease an M6 bolt and thread it in while the ad­he­sive is go­ing off. This pre­vents the new thread be­ing gummed up with ad­he­sive

16 Let it set and you’re sorted

Now re­move the bolt and re­mem­ber to knock the in­sert­ing tang off the insert with the tool sup­plied in the kit. Your work here is done.

Alan about to boss the job. Look how se­ri­ous he is. Blimey

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