Re-threading & TAP kits * metric/standard questions [Archive] - Chevelle Tech

: Re-threading & TAP kits * metric/standard questions

Nov 12th, 03, 4:09 AM
I still can't get my oil pan bolts to seal, the bigger ones in the front, I think they are 7/16 bolts. They basically slide right in. The bolts are low thread but I looked up in the holes and tried different bolts and it was a no go. So I think it's time for a tap and dye kit? I saw one at Kragen for like $7 which is a small kit comes with metric I think? 10mm 11mm etc. Im still confused the difference between the metric and standard threads. Also If I redo these front holes what size should I do and what would be the best way of going at it?
I even tried slopping sealer around this area for a temporary seal. I don't think it's a go on that. Oh well. graemlins/sad.gif :confused:

Nov 12th, 03, 4:19 AM
The holes you are refering to are 1/4" (I believe). If they are srripped, you have two options: tap them to the next size larger, or repair the exsisting size with a "Heli-coil". Either way will involve some drilling.

Now, as far as standard or metric. Standard bolts are measured in inches and metric is in millimeters.

Nov 12th, 03, 4:24 AM
Ok, the bolts along the sides are 1/4-20 threads,
the front and back two bolts are 5/16-18 threads.
These are all standard threads. There are NC or
National Coarse threads and NF National fine threads
for the different standard thread types. The ones
you are dealig with are all NC threads.
Metric threads are a completely different thing.
I imagine your bolts threads are messed up, get
some new bolts of the same lenght or 1/4" longer
if your holes are deep enough.
If that does not work you can put in a Heli-Coli
or just drill and tap the hole out to 3/8-16 threads
and use a socket head bolt instead of the hex head
bolt to help clear the lip on the pan.
Hope this makes sense, if not just ask and I or
someone else will try to help you.

Nov 12th, 03, 4:50 AM
One of the very first things I ever did when I got my car was replaced the manifold, it had an old torker on there and the bolt holes were stripped on that on the left side of the water-neck. I tried heli-coiling that section and used a little bit of JB weld around the sides. Looked perfect, drove around the block and lets just say it really wasnt a good thing graemlins/waving.gif But since then Ive always thought of them as a bad fix, or better put... I suck at using those! But it does sound like a good way of doing this. Either way with drilling maybe tapping a new hole and thread size would be best?
I'll definately buy new oil pan gaskets, sealer and a new bolt set for this fix. Thanks for the quick answers, very helpful! With tapping them to the next size, what all is done? I'd imagine using the kits, drilling out the last of the old threads with the right size bit, then manually twisting in the tool, I may be way off thats what I think these kits are.

Nov 12th, 03, 5:06 AM
Now remember, when using either a tap or die, use a lot of oil, only turn the tool about a 1/4 turn, then turn it back, and repeat. Keep doing this as it breaks and clears the chips that is made by the tool. It will also prevent the tool from breaking. Once the new threads are cut, run the tool in and out a couple times to "clean the hole". graemlins/thumbsup.gif

Nov 12th, 03, 5:08 AM
PS - your new tap and die set should come (hopefully) with a chart of some sort which will tell you exactly what size to drill the hole for each respective tap.

Nov 12th, 03, 7:02 AM
Originally posted by TJW:
PS - your new tap and die set should come (hopefully) with a chart of some sort which will tell you exactly what size to drill the hole for each respective tap. You can also buy taps or dies seperately at
your local hardware or auto parts stores. Most
all taps have the recomended drill size right on their shanks.
To go to a larger size you will drill out the hole
with the recomened bit. Then being sure to keep
the tap square start to thread it in, only go a
turn or so then back it out a half turn. Cast is
easy to tap but still use a good cutting fluid
or WD-40 at the least. when you reach the bottom
of the hole back the taper tap out. If you want/need
to have threads all the way to the bottom of the
hole you will need a plug tap to follow up with.
It will cut the last 3 or 4 threads at the bottom
of the hole.
Without looking I imagine 3/8-16 NC is what you will
need to go to. Like I mentioned before you can use
a socket head (allen wrench) screw or if you have
some small head header bolts they should work also.

Nov 12th, 03, 7:05 AM

If I had to choose between heli-coil or going oversize, I would heli-coil.

A heli-coil repair will cost you a little more, but it's an excellant repair, and will allow you to use the original size fasteners.

I'm guessing you are fairly new at this auto repair business. One thing you are going to learn is that you get what you pay for when buying tools. Invest in a quality tap and die set - it will last you a lifetime and you will probably use it a lot around home and while working on the car. I would avoid the $7 Kragen set.

For a late sixties/early seventies Chevy get the "standard" set. Chevy didn't start using metric fasteners till 1977 or so.


Nov 12th, 03, 10:11 AM
Mike took the words right out of my mouth. Always buy quality tools. This is especially important with regard to taps/dies, drill bits, hacksaw blades, etc. The job will go much easier with good quality tools and the good stuff will last almost forever.

Pat Kelley
Nov 12th, 03, 12:04 PM
And, as said, you can get just the size of heli-coil or tap you need to avoid spending the money for a quality set. Use only quality taps. I got a Harbor Freight T&D set as a gift. The taps will not even go into a nut, the dies will not go on a bolt. Pure junk.

Be sure to protect the engine from chips. Cover up everything around where you are working. The correct bolt size if 5/16" NC. On a SBC, the hole may go all the way through. Be sure to hold an oil soaked rag on the back side of the hole to catch chips. A Heli-coil is the proper fix. Permatex make a thread repair kit that uses an epoxy type material for the repair. I've never used it but it might work since this is a low stress fastener.

Nov 12th, 03, 8:40 PM
Buy a heli-coil kit and save a lot of frustration.If the holes are 5/16-18,get the repair kit for 5/16-18. It will contain the correct drill bit size and 5/16-18 Heli-coil tap as well as Heli-coils, and the most important part--the heli-coil installation tool.The heli-coil tap will have 5/16-18 STI marked on it,which stands for heli-coil tap.Take a piece of aluminum or steel and do a practice install. Drill it,tap it, and install a heli-coil.You will be glad you did. It will make a good repair and you can still use the original size hardware.

Nov 12th, 03, 10:51 PM
First, I am not an expert (not even close) with oil pan bolts. But it sounds like from above posts that there are two different sizes of bolts there - make sure you tried the right size bolt in the right hole. It can seem close enough sometimes, but not be the right one. Make sure you have a bolt with good quality threads too. As mentioned, try a bolt that will use most of the threads in the hole (almost as deep as the hole is) if it is the top few hole threads that are worn. Just trying to think of things that would save drilling. Good luck!

Nov 12th, 03, 10:57 PM
Yeah I tried the same size bolt but a little longer in length and it didnt seem to catch. I'll buy a new bolt set which is good no matter what I do. But if those dont work I'll look into re-thread tap kits. Also with the heli-coils it might be extra difficult other than me having badluck with them this is an upsidown install so they would be tricky to get them to stay as well. Lots of good ideas so far though.