HOW TO FIX BUSTED BOSSES
Salvaging crankcases with smashed off mounting bosses. A simple, effective fix for a common problem
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.
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