Engine Rebuild (Part 13) - Fitting the Exhaust Manifold

This is a bit of a saga…..

When removed, the manifold parts were very rusted and brown. 

Originally I intended to de-rust them and then paint them with VHT high temperature paint. I told a couple of people this and they just laughed – telling me the VHT would either simply fall straight off, or would gradually fall off – leaving less and less area painted. I wanted to soldier on and convinced myself that a good clean surface for the paint to key-to would make all the difference to paint adhesion.…..

While I decided on my next step, I set about removing the manifold studs so that they could be replaced with new ones. I tried double-nutting a stud but that had no effect – the two nuts yielded and turned before the stud moved. I stripped several nuts in this way!

Busy Stripping Threads......

There was no easy way to do this. In the end I used my biggest, strongest mole grips to grips the end of a stud. With a can of WD40 on standby, I applied a gradual, too and fro, wiggling action. The grips slipped off many times and the stud ended up very gnarled, but eventually I started to sense almost microscopic signs of movement.

I applied a squirt of WD40 and kept wiggling (the tightening was as important as the loosening) and the sense of movement turned into a reality – the stud began to loosen it’s grip and, with more wiggling and WD40, was eventually persuaded out. I did that eight times……until all the downpipe and heat shield studs were out. I was VERY lucky that none of the studs broke off - as I didn't fancy having to drill them out.

Gnarled and New........

So if I was going to get good paint adhesion, now I needed to de-rust and de-grease the manifold…..so took them to my local machine shop for sandblasting. Unfortunately, after sand blasting, my local engineer – thinking he was doing me a favour - sprayed the bits over with WD40 before handing them back! He did it to stop flash-rusting. No chance of paint sticking now! 

A Generous Coating of WD40

Apologising he suggested that a good clean with painters ‘panel wipe’ would remove any grease. However with the parts rough and pitted, I wasn’t at all confident that I would get them clean again.

So now I needed to de-grease them (again)……With the parts now thoroughly covered in WD40, choosing a day when Gayle was out of the house, my Plan B was to give the manifolds a wash in the dishwasher! I added a little carburettor cleaner (meant for my ultrasonic cleaner). Now I cannot advise you follow suit: I had suds pouring onto the floor! And, as expected, the parts began to show surface rust as soon as the rinse water had dried off.

Sparkly Clean!

So now I needed to remove rust (again)……I thoroughly coated the manifolds in strong (85%?) phosphoric acid to kill the rust. Coming back to them a few days later, this had worked very well: the surfaces were now blackened, but a hard crystalline deposit  had formed in patches on the surface.

So now I needed to remove the acid!!! Wearing gloves and goggles I applied some dilute phosphoric acid and used a wire brush to scrub the manifolds clean once more – removing the crystallised layer and getting down to bare metal. They didn’t look so bad!

I decided to abandon plans to repaint the manifolds and resolved to going with the bare metal finish: I used a thin, diluted wash of phosphoric acid to stop any rusting and, when that was dry, gave them a squirt over with WD40….. Which is what my engineer had done in the first place!!!

With the manifolds looking cleaner, I fitted the new studs. To my surprise, a couple of studs to join the manifolds to the down pipes didn’t fit all the way to the unthreaded ‘waist’ in the middle of the stud. This meant that when the downpipe was fitted and the nut tightened, the nut would stop at the ‘waist’ and would not ‘bite down’ onto downpipe to provide a gas-tight seal. I tried tightening the studs again but didn’t want to risk stripping the threads in the manifold. I used a bottoming tap to make sure that the thread was clear and clean, but could not risk tapping deeper for fear the tap drill would damage the existing thread and weaken it. For the two studs that would still not go ‘all the way’, in the end, I resorted to using a thread die to cut a little extra thread on the nut side of the ‘waist’. That did the trick.

Tapping Threads for the New Studs

With the studs in place, I fitted the manifolds to the car. I had deliberately built my engine trolley so that it allowed sufficient space on the exhaust side for the fitting of the downpipe.

For the two-piece DS21 manifold the technique is covered in Operation DX.180-1 in Section 1 of Volume 2 of Manual 814.

Check that the joining surfaces are as clean and flat as possible.

Smear a little exhaust assembly paste on both sides of the exhaust gaskets and hang them over the studs on the cylinder head. Slide both manifold halves over the studs together: a little wiggling is necessary to get them to align and ‘nest’ one in the other.

Fit flat washers and nuts (copper nuts recommended) and tighten them finger tight only

Smear a little exhaust assembly paste on both sides of the gaskets that join the two manifold halves to the down pipe. Press these gently into place on the studs on the underside of the manifold. (They should stay there while you prepare for the next step).

Introduce the downpipe to the manifolds. Fit nuts (no washers used) finger tight.

At this point I re-checked the nuts at the cylinder head end for any slack that had developed, but otherwise still left them only finger tight. I wanted the manifold to be hanging parallel to the side of the cylinder head, but still be loose enough to have a little ‘give’ in them because of the next step.

Returning to the manifold/ down pipe join, I tightened these nuts fully. Only when this was done did I return to the nuts on the cylinder head and tighten them fully.

Exhaust manifolds and downpipes fitted

My reward for this was that I was then able to fit the outer heat shield over the manifold. For some reason this felt like a significant step. I had also given this heat shield a polish. In this case, it's the inner surface that does all the work - keeping exhaust heat away from other components through radiation - so I worked hard to give that hidden surface a good shine.

I haven’t tightened the two shield bolts on top yet. I will save those until I come to fit the air cleaner and its base bracket.

Heat Shield Fitted

It’s starting to look like a proper DS engine now!