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Exhaust manifold bolt misalignment – 3.9 V8


mickeyw
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Last night I went to refit the exhaust manifolds on a serpentine 3.9 that I am preparing to fit in my 110. I only removed the manifolds a couple of nights ago, as I could see the gaskets to the heads were falling to bits.

Anyway, on refitting of the left side manifold I found the upper bolt holes were a LONG way out of alignment, and I just couldn't get all the bolts in their respective holes. The end ones were out the furthest.

I have cleaned all the bolt holes, chased them down with a tap, but there are still three bolts that I just cannot get to start in the thread. Without the manifold there, all bolts enter the holes fine, so there is no thread damage at play here.

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I have unsuccessfully tried fitting bolts in different sequences to see if I can get them all started.

This seems a crazy situation, as the manifolds have only been off for a couple of days, and the bolts didn't seem that tight when I removed them!

This is not the first engine I have encountered the problem on, but not had it quite as bad as this before.

The weird thing is that where the lower holes are a smaller diameter, and are not a problem, it's just the larger diameter upper holes. Did Rover know they had problems with manifolds distorting???

Now before I start enlarging holes, has anyone got any crafty tricks to stretch/twist the manifold? Before anyone suggests it, I am not interested in tubular headers. IMO they have their own set of problems, despite not being necessary on a stock 3.9.

This is the style of manifold I am dealing with, but note this image is not the item from my engine. It's borrowed from the web, as I didn't have one to hand.

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Manifold has warped with the heating, cooling, heating, cooling over many years. I have done the same, not notice during removal. Try a large bar or pole to force it back, or get longer bolts/studded bar, line up all the bolts and holes with some distance between head and manifold, then use nuts to pull the manifold down the threads while keeping the holes/bolts lined up. Slow process, but it does work. You could just file the hole bigger. Or better still, dump the cast manifolds for 4 branch tubed.

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Yep, been there.

If you get the middle bolts started and clamp it down a little, watching the alignment, it should stretch it back out a bit, key thing is to only have the centre bolts tightened to allow the rest to move. Few gentle taps with a hammer loosens things up during the process.

Or get a big drill, job done.

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Think I'd go +1 with the big drill on this one. i've had this problem more than once on 3.5 and 3.9 V8s...and eventually gave up trying to get them to fit right and just drilled the pass-throughs out to fit.

Cast iron's funny stuff - it does build up stressed and move.

ajr

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Been there, done that, ended up enlarging the holes then finding the ruddy thing was cracked in the middle anyway... got bored, bought P38 manifolds, at least you can weld the cracks up on those.

John, do the P38 manifolds have the same interface with the down pipes? I am hoping to use these pipes that came from the same Discovery.

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Yes I know they have cats, and I don't need them due to the truck being 1986, but if they'll fit, I'll use 'em ;)

Edit: I shall be changing the gearbox x-member to accommodate the 3.9 pipes.

Do cats create much obstruction to flow?

Manifold has warped with the heating, cooling, heating, cooling over many years.

This was rather the conclusion I had come to.

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John, do the P38 manifolds have the same interface with the down pipes? I am hoping to use these pipes that came from the same Discovery.

OK, answered my own question here. Ports are not in the same place, or same mating pattern.

P38

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Disco/RRC

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I'd like to use as many off the shelf parts here as possible. I am hoping my down pipes will mate to a Td5 centre/rear set of pipes for a 110. I managed similar on my V8 90 - RRC 3.9 front section to Td5 90 rear.

The tail of the V8 cat pipes measure Ø2.5".

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I'd like to use as many off the shelf parts here as possible. I am hoping my down pipes will mate to a Td5 centre/rear set of pipes for a 110. I managed similar on my V8 90 - RRC 3.9 front section to Td5 90 rear.

The tail of the V8 cat pipes measure Ø2.5".

With regard to manifolds, there are loads of cast items floating around, Rover P5, P6, SD1, Morgan, TVR, Sherpa van, e.t.c. You may find one in your local scrap yard that may fit.

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My plan with the P38 ones is the get a stainless Y-pipe made that then joins the standard exhaust system at the usual place (this is for the Ambulance), same plan on the 109 but minus the word "standard" although the stainless system was built around a standard start. A Y-pipe shouldn't tax the exhaust guys / my wallet too much and stainless won't rot so no worries on finding a replacement. Just so fed up with the older style ones, the 109 has the 4-2 cast manifolds and they're just hateful and never seal at the join.

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  • 2 weeks later...

To close this story off, I can report that persistence paid off in the end, without having to enlarge the holes.

More fiddling each bolt in different sequences, getting each one so it was 'just started', and with the manifold as far from the mating face as possible, I did eventually get them all in. But what a PITA!!! The other side fortunately doesn't seem to have the same problem.

A stainless system would be nice Fridge, but that will have to wait for funds to recover.

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