Inlet manifold bolt holes

[mole42]

I am Helicoiling the bolt holes for the inlet manifold, most are stripped, some have been epoxied, the manifold was held on with silicone.

How deep should I drill the holes before the Helicoil tap goes in? Measurements on one head suggests 20mm but is that safe for all the holes?

Also, is the inlet manifold face truly parallel to the exhaust manifold face? That would make it much easier to set up in the drill press.

TIA, Richard

[stag8manod]

Hi Richard, Q1, That is more than deep enough. Measure the Heli-coil length plus 1/4" and drill to that depth. Q2 I don't know. Could be measured with a large pair of outside callipers or digital calliper.
Cheers Ian A

[Paul Roberts]

No idea about question number 1.
Question 2; I realise that you are referencing original machining and the need to establish a datum.
Both faces would, originally, have been machined from the datum of the cylinder head mating face.
What you (or anyone else) cannot possibly know is: has the head been machined, in any way, since the day of manufacture?
A rough and ready check, as has just been suggested, would be to measure with calipers.
The important thing, during reassembly, is to ensure that you align the heads and the inlet manifold before everything gets too tight.
The heads are able to move (a little) longitudinally. The inlet manifold is of fixed dimension and the heads must be aligned to it.
Make absolutely sure the heads align with the manifold (or vice verse) in all planes.
Respect the fact that the heads move inwards (both independently) in direct proportion to their (independent) downward movement.
The exhaust will bolt up whatever the alignment.

[KOY 23]

Richard.
Helicoils come in various lengths varying from 1D to 3D, where D is bolt diameter, the kits I have seen usually have the shorter ones. So in addition to Ian's statement. I would check the bolt length protruding into head and add 1/4". Don't drill too deep as there are hidden waterways.

[mole42]

I am using 2D Helicoils which are 0.625" (15.9mm) long. Alowing a bit for the half-turn below the surface and some room for the tang to break off I think 20mm should be sufficient.

I am asking the question about depth in case anyone knows that a particular hole should be drilled to less than 20mm! As I say, I measured all the holes on one head and they are all 20mm ±1mm so that seems to be a reasonable limit.

Paul - I'm asking for a datum for drilling for Helicoils, not for the manifold/head assembly. I know about that part!!

Thanks!

[milothedog]

You can also, with a bit of care, put a short one in until near the bottom of the hole and then shorten another to the required length with the tips of good sharp side cutters and put it in on top of the first one giving you thread for the full length.

Ian.

[Motorsport Micky]

^ +1 with Ian. The "stacking" of the shorter helicoils works very well to the deeper depths required.

Micky

[milothedog]

Cutting them down is also useful if you have a shallow threaded hole and the helicoil inserts you have are too long.

[nib]

I am Helicoiling the bolt holes for the inlet manifold, most are stripped, some have been epoxied, the manifold was held on with silicone.

How deep should I drill the holes before the Helicoil tap goes in? Measurements on one head suggests 20mm but is that safe for all the holes?

Also, is the inlet manifold face truly parallel to the exhaust manifold face? That would make it much easier to set up in the drill press.

TIA, Richard

All I say Richard is go easy with that drill, some of them waterways are a few mm behind them I/M bolt holes. I've got away without drilling before just tapping a thread in the ally to take the new helicoil.
Mark

[richardthestag]

On mine,

one of the long bolt threads had to go at the very bottom of the hole in the head due to much damage in the outer end of said hole. This caused huge issues with the fixed length "long" inlet manifold bolt fully reaching the thread. Stacking helicoils was no good because the outer part of the head was knackered. they simply pulled out or mangled themselves.

Ended up in a moment of unusual sobriety and creativity switching the inlet "long" bolt for the longest elbow to carb bolt (which is IIRC 5mm longer) and all is 20 ft lb again and water tight too with just a smear of Stag wellseal.

Oh and why would you want to drill any deeper? surely that will only end in disaster

[marshman]

Hi Richard,
When I did mine......... see here for pics....

http://socforum.com/forum/showthread.php?26031-Helicoiling-Heads&highlight=helicoil

I just used the helicoil tapping drill which takes off very little material and drilled down to the bottom of the existing hole. I did ust a pillar drill and fed the drill in gently, you feel it as soon as it meets the bottom of the hole. Just go gently and it will be OK. As I recall the exhaust and inlet faces were parallel and I didn't take any precautions to check. I would have thought that even if the faces had been machined they would still be very close to parallel to make no difference in terms of the "squareness" of the thread.

Roger

[tractorjames]

If all goes badly wrong you could go up another bolt size and get a local engineering place to make you up some steel inserts threaded internally and externally or use wurth time serts.

James

[mole42]

Oh and why would you want to drill any deeper? surely that will only end in disaster

No, I don't want to drill any deeper. The holes on mine are 20mm deep at the moment, complete with stripped threads, and I just want to make sure that I can put the Helicoil tap right down the holes without fear of breaking through. I was just checking that no-one had found any nasties while doing theirs!

I'll be setting up in the bench drill press with a positive stop at the 20mm depth to make sure there are no accidents!

Richard

[mole42]

Hi Richard,
When I did mine......... see here for pics....

http://socforum.com/forum/showthread.php?26031-Helicoiling-Heads&highlight=helicoil

Roger

Thanks Roger. Just the sort of reassurance I need! It's funny how different one model of a Triumph can be from others, I used to know about the straight-six 2.5PI, but this Stag has many surprises.

Richard