SDS - Throttle Body Install / Spacer Plate

When we were building the engine, i decided a simple quick job was installing the SDS throttle body. Mating it up to the sump however, revealed interference between the MAP pickup nipples and the sump casting - this is where i need to install the silicone hose to run to the MAP sensors. 

You can see the only boss for mounting a throttle cable bracket is on the right hand side of the sump.

No way i can get a hose onto these barbs - the sump casting is in the way. 

I posted a query on the VAF forums about this, and i figured i had three options. 

Option 1 - Rotate the Throttle Body

This would be a simple fix - the MAP barbs would be facing upward, and there is plenty of space to get the hoses on. However, this meant that the throttle arm would be on the bottom of the engine. To get good geometry on the throttle arm, i think the throttle arm would need to be come from the left hand side of the sump. My particular sump only has a boss (with mounting holes) on the right side of the sump casting. This is not unachievable, but would be more of a challenge to get the geometry to work and find a place to mount the throttle cable. 

With the throttle full open, the geometry becomes challenging if the throttle cable was mounted on the right hand side of the sump, in the only location where there are threaded holes.

I might have been able to use the above boss on the left hand side of the sump, but would need to fabircate a custom steel bracket for this, and the throttle cable would be exiting the engine at a 45 degree angle. 

Option 2 - 90 degree barbs / Move the barbs

Option 2 would be to obtain some 90 degree barbs, to allow the rubber hoses to exit the throttle body; one up and one down. The issue here is that the threads are 10-32 straight threads (the barbs have an o-ring). So I would need a banjo fitting to allow this to work. 

Banjo fittings like these were previously supplied by SDS

Ross at SDS says:

Alternatively, Ross said i can drill and tap some new holes in the throttle body (on the lower or upper side), and then make a plug for the original holes. That's a bit scary.

Option 3 - Spacer

Option 3 would be to just space teh throttle body away from the sump by a small distance, to give some more room for the rubber hoses to fit between the sump and the throttle position sensor. I calcualted i could use an approx 4.5mm spacer (0.180"), and not need to change the length of the studs in the sump. I really wanted to avoid this - if we broke a stud or damaged the threads in the sump, it would be a disaster. 

The gap here is approx 4.5mm using standard 5/16 nuts and one plain and one split washer, allowing for a thread out of the end of the nut. When measuring this, i forgot about the thickness of the gasket! 

Just enough room for the hose to fit.

Making the Spacer

To make the spacer, i headed to the aviation section of my Dad's shed, underneath his (unserviceable) milling machine, and found some 5mm aluminium plate. 

I started out with a dodgy, out of round 54mm hole saw (i think i used this to drill holes in my doors for locks) and hacked a hole.

I then centered the throttle body on this hole, and marked out the mounting holes. These were drilled, then i cut the plate to size

It central hole was carefully filed to match the hole in the throttle body and the sump, to make sure there was no 'step' in the air flow. 

Once filed, it was punched at the bottom to make sure it went on at the correct orientation.

I even lapped both sides using 600 / 800 grit paper on the surface plate in the back of the engine shop.

It was then alodined. 

It was installed on the engine using the original gasket against the sump, and the o-ring seals the throttle body against the spacer plate. Grease was used on the gasket and o-ring. 

The slightly thicker plate (5mm versus 4.5mm), combined with the thickness of the forgotten gasket, meant i didn't have any threads showing out the nuts. 

This worked out well.

The lack of threads out of the end of the nuts was solved by replacing the thick washer and split ring, wiht a thin washer and an internal tooth star washer.

Much better with the thin washer and internal tooth washer.

The next step will be to mount the bracket on the sump, and work out the linkage to the throttle arm. I will do this once the engine is on the airframe. For running on the truck we will jsut jury-rig something up.