more on tapping aluminum 1

[carpenterson]

I read a recent thread on tapping aluminum with interest, but have some questions on the subject regarding a current project. I have an aluminum intake manifold from a Dodge Stratus which bolts to an aluminum lower intake manifold. One of the 6 bolts was broken off in the hole, and the owner's attempt to drill out the bolt and retap the hole were not successful. In addition, one of the other holes is stripped, and will not hold a bolt. I am considering drilling out both holes for a larger diameter bolt. The existing holes were originally treaded for a #8 x 1.25 metric bolt, but the hole that is stripped now allows the bolt to slide through without even threading; the attempt to drill and retap the other hole left it a bit wallowed out and slightly out of round. In order to avoid purchasing a metric tap, I am considering using a 3/8" SAE tap, but would like to know if, in the opinion of those with more experience tapping aluminum, that size hole would be sufficiently large to provide enough threading given the existing #8 metric hole size. If so, I would also appreciate recomendations as to which would work better in aluminum; a coarse (20tpi) thread or a finer (28tpi) thread? Finally, what size hole would you recommend I drill for tapping a 3/8"x 20 or 28 tpi hole in aluminium. The hole I will be drilling will be approximately 7/16" deep; one hole being a through hole, and the other a bottomed hole. The threads will need to be strong enough to stand up to the manifold being torqued to the recommended 160 inch pounds.

In reading the previous post on tapping aluminum, several people recommended using helicoils or keyserts. I am not sure if these would be a better option in this case, but when I looked up the instructions for installing keyserts on line, I was not clear how they would be an improvement, since they still require tapping a new hole in the aluminum.

Thanks in advance for your suggestions
carpenterson

 

[KENAT]

I'd probably suggest the helicoils. Spreads the load a little and reduces corrosion concerns if you're using SS fasteners etc. Also the helicoil is only inserted once into the aluminium. From then on any threading or unthreading is the bold in the helicoil.

KENAT, probably the least qualified checker you'll ever meet...

 

[JDana]

3/8 coarse is 16tpi -recommended drill dia. is .312"
3/8 fine is 24tpi -recommended drill dia. is .332"

You're probably already too big for coarse, fine thread would be better anyway.

i don't know off hand about the torque values, but if the 8mm was good for 160 in/lbs you should be fine.

 

[ssn596]

FIRST No matter what you do the first question is will you have enough metal around the hole when all is said and done ie "enuff meat"??

SECOND do you have to tap the hole - can you use a bolt and nut?

THIRD If it were mine I would keep all fasteners the same ie metric. I do enough of that metric versus SAE at work (carry full set of tools for each) and get tired of that fiasco.

FOURTH Get a good tap and pilot drill chart.

FIFTH Heli coil and other type inserts are a LAST option. They are a good option when NOTHING else can be done. They are not foolproof and like anything else break or wear out. When that happens you often end up scrapping unless you are able to build up by welding.

Dan Bentler

 

[mullins]

If you dig around on the net, you can calculate the thread engagement area which you need for an aluminum body and a steel bolt. This will show you that for a softer female body, you will need less depth with a course thread.

So if you are going to use a UN thread, I would suggest a 3/8"-16 UNC.

Taking the science out of rocket science...One step at a time.

 

[Ataloss]

I'd recommend a helicoil too if you want to repair this manifold instead of oversizing the bolt.

Another option would be to just buy a used lower manifold. You didn't mention what year or engine size your car was but I looked on

http://www.car-part.com

and they have used lower manifolds for Dodge Stratuses starting at $30. Throw in another $20-30 for gaskets and you're done, no worrying about having enough meat around the new threads, cracking the boss on the upper intake manifold when you tighten the bolt down because drilling out the clearance hole to accomodate the larger bolt weakened the boss too much, or opening up the hole in the gasket to accomodate the larger bolt decreased the sealing surface enought to create a leak path (doubtful, but you never know).

Bob

 

[derekkarp]

I disagree ssn596. I think Helicoil is the proper fix. In fact your helicoiled threads will be stronger than the original. We machine jet engine cases, and every threaded hole is helicoiled. I don't think they would do something that is second-best option. Also speaking from my own experience, I recently rebuilt a H-D motor, that has aluminum cases, and I drilled out every tapped hole, and helicoiled them. No problems, plus if the helicoil gets screwed up, just put a new one in.
-Derek

 

[KENAT]

Yeah, back when I worked in aerospace/defense we normally used helicoils, in fact I can't think of one instance where we had thereaded holes directly in aluminium.

KENAT, probably the least qualified checker you'll ever meet...

 

[Tmoose]

For low torque applications like intake manifolds there are epoxy like products that sometimes can make an in-place repair that squeaks by.

http://store.summitracing.com/partd...CT-37390&N=700+401068+317608+115&autoview=sku

There are thick walled thread repair inserts that are useful when attempts to remove seized bolts go bad, or when you make something for Northropp Grumman or the US Navy, who pretty much demand fasteners not be threaded directly into aluminum.

http://www.scandiloc.dk/billeder/GEVINDINDSATSE/Keenserts.JPG

I'd try to repair it back to metric.

Those used manifolds might be the best solution.