Repairs in Dar es Salaam
Personally I wouldn't recommend using Tuktuk for repairs but rather only for getting pre-orders like Rich suggests. Hussein is a good guy but his mechanics work in a filthy workshop on a sandy dusty road, not good for an open engine. Try Hussein's TukTuk shop for your quick spares, it's down town and anyone will be able to help you find it if you say Tuktuk.
For repairs I would recommend a german mechanic named Frank Carbort (sp?). His number is (255) 22 - 0756443940. He has many mechanics working for him in a huge workshop about 10 km west of downtown dar, on the road to morogoro (and zambia).
I hope this helps.