15. Drive shafts and tail pipes

October 2023

A quiet-ish time at work between catalogues and a spark of enthusiasm ignited by our acquisition of a rather special, part-built Ultima GTR has got me going again on our final P4 - and it's time to sort out the drive shafts and CV joints.

The rear hub stub axles accept good-old, Ford Scorpio 100mm dia. CV joints - circa 1986 which are still available from motor factors. The output flanges on the Porsche transmission accept larger, 108mm diameter joints. I found a box of new/old CV joints gathering dust and surface rust in long-hidden boxes at the back of some workshop shelving. Fortunately, they're easy to strip and clean. I'll probably buy new ones but these'll do for now.

Driveshaft length is obviously non standard, as is the mix of 25 tooth spline on the Ford end and the 28 tooth spline on the Porsche end, so the driveshafts will have to be custom made and I'll have to make some accurate measurements to get them right first time.

Optimum driveshaft length is measured when the car is sitting at normal ride height. Our little hydraulic lift is supporting the P4 chassis with all four suspension corners on full droop, so I borrowed a rear wheel from the Ultima (same PCD) and set up a jack under the tyre to raise the suspension on one side to an equivalent ride height of about 5 inches. It's important at this stage to make sure both flange faces are parallel by tweeking the rear suspension camber adjustment.

Over a decade ago I designed and made the engine and gearbox mounts to align the Porsche output flanges perfectly with the rear wheel hubs - across the car but, because the transmission is inverted, they sit slightly higher than the hubs vertically - no problem for CV joints - that's what they're designed for, but it necessitates accurate measuring of the distances between the two flanges.

I took three dimensions as accurately as I could using a tape measure -

• From the top of the hub flange to the bottom of the gearbox flange: 512mm

• From the bottom of the hub flange to the top of the gearbox flange: 532mm

• From the centre of the hub flange to the centre of the gearbox flange: 511mm

Of course all the drive flanges are dished making it difficult to measure the centre to centre dimension so I cut some accurate aluminium rectangle templates of the side profile dimensions of the drive flanges - 100mm and 108mm and marked the half way (centre height) on each. I laid them on a flat board, accurately squared and positioned to the dimensions measured on the car and measured the centre dimension from that.

Historically, GKN manufactured all of our driveshafts but this time, 20 years on, they failed to respond to my three attempts at communication so Matt searched around and found an engineering company called 'Brand Mechanical Engineering' in Birmingham. I received a call back from Brian within five minutes of sending my email enquiry. He asked me to send a sketch of the dimensions and one each of the CV joints - apparently there are now many versions of the 'same' joints. I made two stiff cardboard discs for the driveshaft flanges with the centre clearly marked. Two stiff aluminium welding rods held together between my fingers enabled me to set the exact distance between the centre of the two discs and transfer the two rods to a metre rule to read-off the dimension.

Here's my fag-packet sketch of all the information he needed.

Brian quoted me £325 each + vat and shipping for the driveshafts - compared to GKN's price of £66 each in 2003. That's a lot of money so I thought I'd check and measure the dimensions again using a revised method. Here's the revised drawing.

Here's the nearside driveshaft with CV joints trial fitted on full suspension droop. Perfect. I've ordered high tensile stainless bolts so next it's paint the shafts, CV's and gaiter shrouds.

I made some gaskets for the CV to flange joints using the CV case as a pattern/jig. the metal is hard enough to cut the holes accurately with a 'doming punch'. I added bolts through each hole as I punched them. I then used a little brass hammer to tap around the inside and outside edges of the flange to mark the gasket paper with an accurate cutting line for scissors.

I powder-coated the shafts, CV housings and gaiter cups before assembling it all.

At some point in the (hopefully near) future I'll have to load the car on a trailer and take it to have the exhaust fabrication completed, so, in the absence of the proper wheels, Matt found four space-saver wheels on ebay. Not the right rolling diameter but good enough for now.

I jacked-up the rear of the car, clear of the lift, on both rear wheels, adjusted the ride height to 5 inches and the camber to 0 degrees on the rear suspension ........

...... ensuring the chassis was level with my digital protractor.

You can see here just how wide the quad-cam engine is and how little room there is for posh four branch manifolds........

....... so I opted to keep the original cast manifolds (see their preparation in an earlier post). This is the left side of the engine bay. You can just see the manifold outlet flange on the right with it's Lambda sensor thread.

The exhaust tailpipes are a significant feature of the rear view of a P4. The original cars had two, staggered 50mm-ish pipes either side of the lower cut-out in the tail panel. IVA now dictates that all tailpipes have beaded ends so we had a batch made at 75mm diameter. Fortunately there were two left on the shelf. Positioning them and spacing them correctly in the cut-out is essential or it'll be one of those 'first thing you see' moments every time I look at the car.

The tail section has been aligned and pre-fitted for years so I know it's in the right place. My challenge here is to mount the tailpipes alone - effectively in mid-air and secure them firmly in their correct, finished position before the car goes to an exhaust specialist to manufacture the components between the tailpipes and the manifolds - that is, Catalytic converters, silencers and the pipework in between. The tailpipes must be neatly and evenly spaced to look right but must allow some clearance when the tail section is open. I don't want to start thinking about final mounting options for the exhaust yet so I just need to find a way to hold the tailpipes firmly in position.

The tail section is hinged at the rear and supported by a subframe section at the rear of the chassis. The subframe has an angled steel cross-piece 40mm x 20mm, twisted at an angle of 29 degrees from horizontal. There's a sample of the tube in the picture below. I measured and cut two pieces of 25mm ply and glued them together to make a 50mm thick wooden beam. My plan is to cut a 25mm deep channel in the ply at the same angle as the chassis cross-piece so that I can clamp the beam to the steel crosspiece leaving it vertical roughly in the area where the tail pipes will have to be.

Here it is in situ. You can just see the black steel cross beam through the small opening. The tail hinge is also mounted on the cross beam. The wooden beam is pushed over this from the engine side.

I taped two short pieces of 25mm dowel at 90 degrees on top and on the side of a tailpipe to space it evenly from the panel. I simply held the tailpipe in position and drew around the end on the wooden beam with a sharp pencil.

And here they are - perfectly positioned for welding onto the exhaust. I'll probably cut the wood right cross the middle of the holes so I can screw the two pieces together to clamp the tailpipes.

The view from inside the tail. The camera angle has distorted the view. The wooden jig/panel is actually vertical and the tailpipes are horizontal and in the right place. So, the manifolds are on and the tail pipes are in position. Just the middle bit to do.

I've mentioned in a previous post that I have a complete, replacement Engine Wiring Loom that retains all the legal UK emissions features of the engine but does away with the systems I don't need - like ignition switch and door lock security integration and exhaust gas recirculation.

Here it is, and the reflashed ECU, perched roughly in the position they'll eventually stay - in the side pod. Big decision time..... To fit the centre tub or take advantage of easy access all round and keep working on other stuff.

Find the answer in Blog post 16.

Bye for now.