Bolt Material advice needed

C

chriswesterman

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Hi,

Details:

Engine Bukh DV20, manifold and Thermostat housing made of Cast Iron

The thread in the manifold which holds the Thermostat hosing bolt has stripped / enlarged and will not hold the bolt tight. The Bolt only has about 5mm of thread length into the manifold. The Original BUKH bolts are M6 70mm long ignoring the HEX bolt head

Now there is plenty of thread in the Manifold so I could get a longer bolt or I could helicoil the damaged thread.

Question:

Either way what material should either the bolt or helicoil be made out of ?

Regards

Chris
 
I believe you'll find that Helicoils are always stainless steel. Should you chose to use them, then whether you use a mild steel or stainless fastener, it would be a good idea to use some Copperslip to prevent the thread seizing or (in the case of stainless on stainless) galling. Of course, if you have some longer screws, it would be worth trying to pick up the remaining thread with those, but I'd still use some Copperslip. The mix of metals shouldn't be a problem as there shouldn't be any saltwater getting into the thread.

Rob.
 
AFAIR there is nothing at all special about the bolts on the Bukh thermostat - socket headed I think. A common or garden automotive steel bolt would be perfectly OK. There is a reason that car manufacturers use studs and nuts - it is for exactly what you have discovered. A stud in a regularly used fastening mostly remains in place and the nut is removed. If the nut strips/wears out it is easily replaced. If the stud strips/wears out it will be the part at the top that goes, so the stud can be replaced.
 
The thread on the stud that fits into cast iron housings should be a coarse thread where the thread on the stud that the nut fits on is normally a fine thread.

That's how BMC A & B series engines in my MGB and mini are made anyway.
 
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