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R686ST project in UK


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I'm based in the South East of England, halfway between London and the south coast. I've wanted one of these for 25 years and I've finally taken delivery of a project 1975 R686ST.

I bought this in Holland and had it delivered here a few weeks ago. It is a partially completed project which has been mocked-up but needs to be dismantled to be completed. We are very busy with my WW2 truck and armour restoration business so I have not had much time to look at it yet. I have a few questions now and I am sure there will be a lot more to follow. These are very rare trucks in Europe and there is only a limited amount of information and expertise available locally. I hope people on here will be patient if I ask what may appear to be stupid questions, but I had never even seen one of these trucks in the flesh until this arrived! This one was delivered new to a haulage company in Switzerland who had a fleet of them. This truck was sent to Antwerp docks in Belgium to be exported to Zaire but was bought from the dockside by a collector. As far as I can tell it has not been on the road since 1992 although I am still piecing together its history.

Annoyingly it has a broken halfshaft in the rear axle. There is a spare halfshaft that came with it but I noticed that the diff covers have been loosened and I do not know why. Are these axles known for snapping half shafts, and if so, why?

The other immediate question I have is how does the tip turbine work? Am I correct in thinking it uses turbo pressure to spin the turbine to pump air through the heat exchanger?

Also, what is the best way to re-route the air intake so I can get rid of the scoop on top of the bonnet?

The truck has twin gear levers, how can I tell which type of gearbox it is?

Finally for now, what diameter pipe would be appropriate for a pair of stock-looking exhaust stacks?

Thank you in advance, Mike



Edited by Tootallmike
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Welcome!  Fine looking lorry!

Axle shafts don't break often.  Might have to investigate further.

Bleed pressure from turbo does spin the turbine.

Hood scoop is KooL!  A dual hose air cleaner would provide the secondary air source or second air cleaner assembly on the left side of the truck have been utilized.

The trans info will be stamped on the left side of the trans such as X107.

4" exhaust pipe was the stock size. 



It doesn't cost anything to pay attention.

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You have quite a ambitious project ahead of you but by pics it looks solid. Axle shafts may have been removed if it was towed instead of pulling driveshaft. Good luck and ask as many questions as you need. Don't feel dumb asking it's great your interested in old Mack's just like the rest of us here.

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You do have your work cut out for you but R model Mack’s are as tough as they come.  I would agree that the axles where probably pulled for transport.   Are they actually broken?   If your truck was make with a good scoop you should keep it.  They are cool.  I see you just have hope in the hood though.   Good luck.  Mike. 

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Thank you for your responses and answers to the first of many questions.

The halfshaft is definitely snapped and the broken end is still in the diff. The first job is to find out what has happened there.

I'll post up photos of the engine and gearbox later on.

I have contacted the Mack museum to get the build sheet and hopefully a set of manuals, but in the mean time does anyone have a schematic for the air system on this truck? Ideally this would be for the European export model as there are differences in the brake plumbing and controls.

Thanks again, Mike

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