Fuel Injected intake manifold: studs or bolts

[bdinardo]

I'm working on my 1979 spider 2000 that is Fuel Injected (might be a california car). I've got the head out of the car on the bench to refresh it and upon removing the intake manifold from the head found it was using 4 bolts and 1 stud/nut to attach it to the head. To boot, the bolts were various lengths.

1. Upon looking at the exploded view diagram it appears that factory used studs (I'm assuming 8mm x 1.25). Does anyone know what length is correct? Vicks and AutoRicambi seem to sell exhaust manifold stud bolts but not intake manifold stud bolts.
2. Can I just use bolts instead of studs? What has been people's experiences?

Much appreciate the input!

[18Fiatsandcounting]

What has been people's experiences?

Early engines used studs for the intake and exhaust manifolds, while later ones used mostly bolts. I'm not sure of the year when the changeover occurred, but I'm thinking it was mid 1970s. There were some years where there was a mixture of two studs and four bolts on the intake, but I don't recall those years.

Either can be used, but studs do have some advantages:
1. Studs "hold" the manifolds in position while you put the nuts on.
2. Since studs don't get removed very often, there is less wear on the threads in the cylinder head side if you are the kind of person who swaps manifolds frequently or does other head work. Numerous bolt removals can eventually wear out those threads.
3. You can put thread sealer on studs to prevent the oil seepage past the threads on the #2 and #5 intake manifold holes. You can do this with bolts too, but you have to redo it every time you remove the bolts.

Studs on the intake side do have one disadvantage, and that is clearance issues with the manifold and master cylinder/brake booster as you pull the manifold away from the head to clear the studs. Not an issue if you pull the cylinder head out with the manifolds still attached, obviously.

My personal preference is studs, but I will generally just use what the engine came with. By the way, I think the bolts/studs are around 45 to 50 mm long, but you would need to check your intake manifold to see if some of these should be shorter. As long as you have about 3/4" or more of thread gripping, you should be good.

-Bryan

[bdinardo]

What has been people's experiences?

Early engines used studs for the intake and exhaust manifolds, while later ones used mostly bolts. I'm not sure of the year when the changeover occurred, but I'm thinking it was mid 1970s. There were some years where there was a mixture of two studs and four bolts on the intake, but I don't recall those years.

Either can be used, but studs do have some advantages:
1. Studs "hold" the manifolds in position while you put the nuts on.
2. Since studs don't get removed very often, there is less wear on the threads in the cylinder head side if you are the kind of person who swaps manifolds frequently or does other head work. Numerous bolt removals can eventually wear out those threads.
3. You can put thread sealer on studs to prevent the oil seepage past the threads on the #2 and #5 intake manifold holes. You can do this with bolts too, but you have to redo it every time you remove the bolts.

Studs on the intake side do have one disadvantage, and that is clearance issues with the manifold and master cylinder/brake booster as you pull the manifold away from the head to clear the studs. Not an issue if you pull the cylinder head out with the manifolds still attached, obviously.

My personal preference is studs, but I will generally just use what the engine came with. By the way, I think the bolts/studs are around 45 to 50 mm long, but you would need to check your intake manifold to see if some of these should be shorter. As long as you have about 3/4" or more of thread gripping, you should be good.

-Bryan

Thanks very much for the thorough analysis Bryan!

[Spider951]

I have an '81 FI Spider that has a completely stock engine. I also have an '82 "spare" engine. Both engines have studs for both the intake and exhaust manifolds. My $0.02 worth is that studs are easier to work with when removing or installing the manifolds.

[bdinardo]

I have an '81 FI Spider that has a completely stock engine. I also have an '82 "spare" engine. Both engines have studs for both the intake and exhaust manifolds. My $0.02 worth is that studs are easier to work with when removing or installing the manifolds.

Thank you Spider951. Any idea on length of the stud?

[Spider951]

This is probably too late to be helpful, but in digging around in my "well organized" (read NOT) stash of Fiat parts I found two baggies labelled intake and exhaust manifold (probably from work done about 3+ years ago on '82 engine). Measurements (all approximate with a cheapo caliper) are as follows:

Intake: studs (2), 2 1/8" (54 mm) fitted to upper part of cylinder head
bolts (4), 3 of them 1 5/8" (41.3 mm); 1 is 1 7/8" (47.6 mm) [the longer length doesn't appear necessary - probably just what was available to PO or mechanic as a replacement]; bolts all fitted to bottom row (nearer to engine block)

Exhaust: studs (5); 3 longer, 2" (50.8 mm) fitted to bottom row (nearer to engine block)
2 shorter, 1 5/8" (41.3 mm) fitted to top row

All the studs (intake, exhaust, short or long) have a a longer threaded end and a shorter threaded end, with a short unthreaded portion separating the threaded ends (typical studs). IIRC the shorter ends thread into the cylinder head with longer threaded parts outward to accept manifold. I was wrong, then, about being all studs. But because all of these (studs w/ nuts or bolts) are relatively low torque fasteners, probably just getting them long enough and right diameter & thread (M8, 1.25) are most important. I would go with what Bryan said.

[18Fiatsandcounting]

I think you have it right. Early spiders used all studs, while later ones used a mixture of studs and bolts. Yes, all are M8x1.25 and the length varies depending on where it is and what type of manifold you have. On the studs, as long as you have enough threads showing for the washer and nut to be tightened all the way, you're good. And yes, the shorter threaded part of the stud is the end that goes into the head.

Usually the top ones are the studs and the lower are bolts. Note that the top two studs on each side go into an oil galley, so you want thread sealer on those. I just put thread sealer on all of them just so I don't have to think too hard.

Don't hold me to this, but 18 ft lbs is what I recall for the torque on the manifold nuts/bolts. On the exhaust side, check the torque after the engine has been through a few cycles of warming/cooling.

-Bryan

[Spider951]

Yes, 18 ft-lbs is what is called for in the manual.

[bdinardo]

You all rock, thank you very much for the insights.