403 Fuel Pump AC type UG stripped threads

[mikebro]

I've done a few hundred miles after rebuilding the 403's engine. I had already rebuilt the fuel pump with a repair kit a few years ago. I remember at the time thinking that several of the diaphragm flange bolts were a bit dodgy. Anyhow, now, there is a slight petrol weep at the flange, and three of the six bolts won't tighten. Has anyone on the forum made a repair using helicoils or V-coils? I'm keen to know if there is enough metal around the female threads to take a thread insert. Also, are the bolts 10-32 Unified or 2BA? I'm suspecting 10-32 as mine will only go about 3 turns into a 2BA nut.
Anyway, any first hand experience of all this would be very useful. And confirmation of the bolt thread type.

Cheers

Mike

[David C]

Threads on a spare AC pump I have are 10-32 so I assume yours will be too.

Here is what seems a fairly scientific test of different thread repair methods:

https://www.youtube.com/watch?v=jknMrFOGMOQ

Nominal diameter of original screws is 4.82mm. It may be worth considering M5 screws and thread inserts as these are far more readily available. I would normally avoid mixing thread types but since the screws have slotted heads it will make no practical difference. There is precedent on the engine - the Solex banjo fittings are a metric fine thread.

[innellan House]

mike,
the bolts are 10x32 UNF, and there are 2 simple solutions that do not need machining. The first is to get six longer (say 25mm) 10x32 UNF bolts and secure the flanges using nut and washers. The second method is to use M5 x 0.8mm bolts directly into the existing 10x32 threads in the bottom flange.
M5 and 10x32 UNF have different thread forms, but are very close in pitch (31.7 tpi versus 32 tpi) and outside diameter (5.0mm versus 4.83mm). The slightly larger metric bolt will be held very securely, but try this on a piece of scrap first to satisfy yourself. This method (trick) was used by a famous Australian manufacturer of model aircraft diesel engines to prevent the compression tommy bar from loosening (10x32 UNF tommy bar into BA 2 threaded alloy cylinder head). Lastly, if you still wish to use a 10x32 UNF helicoil repair kit, it looks like you have enough room (see pic) for the required 13/64 (5.16mm) clearance drill. But practise on a piece of scrap first but unless you can very accurately drill the six holes on the required pitch circle diameter (PCD) in the lower flange, the result will not satisfactory, nor easy to repair.

[mikebro]

Thanks for those who responded with various suggestions for the fuel pump repair.After weighing up the options, I decided to go for M5 thread repair inserts. To overcome the possibility of inaccurate drilling, I first bolted the two halves of the pump together without the diaphragm with just four of the original bolts. I then drilled the two remaining holes through with the repair kit's tap drill which is 5.2mm. I then tapped those two holes and fitted the thread inserts. I was then able to bolt up using those two holes with new M5 bolts and repeat the process on the other four holes.