casting flaws in crankcase

[steve davidson]

Hi all,

As per the photo my crankcase has numerous small voids in the surface that will mate with the sump, some very close to the threads. 

My intention is the lap both the base of the crankcase and the new semi-deep sump to make that connection as water-tight as possible. My theory is that the better I make that surface the less work the gasket/gasket paste needs to do and thus the chances of oil leaks are reduced.

My thought was to clean out those little voids with thinners and then fill them with some sort of quick metal before I lap the surface.

Is this course of action ok and if so did anyone have any suggestions on types of fillers that might suit this application given that the crankcase is an old alloy?

["Slack Alice" Simon]

I would try with a silicon gasket first, without filling the voids.

Make good all the threads in the crankcase so you know you can tighten them all properly.

Leave it on the bench with oil covering the joint, and see what happens.

Plastic metal works well if you really want to fill the voids.

The gasket does the work, though.   It is fairly easy to get a half decent finish on the 'case - though I wouldn't spend very long on it.

The design of the standard sump, though, means that they will never be truly flat, once you tighten them down.

Try it as it is, with a decent gasket.

[Chris KC]

I have used aluminium-filled epoxy in the past to patch up unsightly porosity in crankcases, it blends fairly well visually and hasn't yet come unstuck.

[steve davidson]

thanks chris, I'll give it a shot

[Steve Bryant]

I had a lot of small air holes in my crank case. The trick is to get the holes clean enough to accept a filler which in my case was J B Weld for Aluminum. I cleaned and opened the holes up slightly with a suitable bit in a Dremel tool and blew the debris  out with a blow nozzle. Once ready to fill the holes, warm both tubes A and B in hot water to thin the viscosity of the resin as it makes a better mix and easier application. If you want better penetration of the resin warm the area with a hair dryer or heat gun. Allow to cool and harden for 24 hours then smooth surface out with files, and abrasive paper. I would recommend priming the area and painting the crank case to finish.

AIR HOLES PREPPED FOR FILLING:
   

HOLE FILLED
   

HOLE FILED AND SANDED READY FOR PAINT
   

Hope this helps,
Stephen