InjectedSS

my fluidampener caused the crank bolt to back out i guess.. either way it fu*ked up the crank threads so i retapped them and they didnt turn out too bad but their just not as strong anymore! so i wanna drill it out and retap it the next size bigger! my ? is the stock size is 7/16" FINE THREAD well i believe the next size is 1/2" so how do i tap for that size? do i drill the hole out with a 7/16" bit and then tap it with the 1/2" bit??

also what kind of tap would i need? is there even a 1/2" FINE and a 1/2" COARSE??

thanks

shoebox

Crank bolt threads are 7/16-20. Get a Helicoil or Timesert kit to repair it. You will be drilling it out to a larger size and using a supplied tap in the kit to make new threads. Then you will insert a new thread device to restore it to the original thread size.

97FormulaWS-6

exactly what shoebox said.. Helicoil it... Mine is.. I ended up stripping mine out swapping out the WP/Opti... and trying to pull the dampner back on with the bolt... I will NEVER do that again...

Have fun drilling it out though... I did it with an air drill and it still took forever.. make sure to use some oil when you drill.. it will make it a little easier.

FireAm94

I agree....I think I will be using allthread rod next time I do mine!

Joe

InjectedSS

REALLY thats ok to do then huh?? will that hold up though? i mean will it seat in there good enough to have torque on it?? where do u find a kit like this at??

thanks

95 Z/28 LT1

The tap should say right on it what size drill to use. If not, there are charts that come with the taps that say.

Personally I would drill and tap it instead of helicoil there, but that's just me.

And use LOTS of oil when you drill too.

InjectedSS

i was thinking the same thing! shoebox said differently though and he knows his sh*t pretty good so thats the only reason i was considering doing the helicoil thing

95 Z/28 LT1

Call your local machine-shop and ask them what they normally do.

magius231

in many cases the helicoil repair is stronger than the original thread, mainly due to the material used. I've heard of helicoiled head bolts with no problems.

shoebox

This is normally true. Helicols have been around for a long time.

NAPA or a well stocked parts place should have what you need. You will need the Helicoil (maybe a few in case you have some difficulty), corresponding tap (exact size will be dictated by the Helicoil) and the Helicoil insertion tool.

It would not hurt to get an opinion from a machine shop. I would first make sure that a 1/2" bolt would fit inside the hub bolt hole, before going with a larger bolt.

InjectedSS

well i talked to my buddy down at the machine shop and he said he would just drill it out and go to 1/2" he said helicoils are good but for the crank theres just too much vibration and he wouldnt reccommend it *shrug*

the parts i have to drill the new thread are

TAP - 1/2"-20
DRILL BIT - 29/64" (which is what the box on the tap says to use)

so hopefully this turns out ok i think ill practice on my old crank first before i go trying to drill on my eagle forged crank!

do you guys think it will be a PITA trying to drill on a forged crank??

rskrause

I would think you should use a Helicoil. When drilling a hard material like 4340 steel there are a couple of tricks. As mentioned, use a lot of cutting oil. Also, the harder the material, the slower you should spin the drill. Strong steels should be drilled so that the cutting edge of the tool is at ~50 feet per minute. The formula is rpm =12xV/PixD where rpm is the drill motor speed in RPM, and V is the cutting speed of the material in fpm. So, for a 7/16" hole you should drill at not more than (50*12/3.14*.4375) ~450rpm. Slower is ok too.

I don't remember too much about this stuff, but I hope that helps.

Rich Krause

Mad Machinist

Don't see too many people call a surface speed calculation for hand drilling. As long as you don't start quoting feedrates in manufacturer specified IPR we'll be ok. No offense, just made me chuckle a little bit.

If you can control the rpm at all, yeah, stay under 400 and use a good cutting oil. Less rpm would be better.
Your 29/64 drill is correct considering 75% thread with a 1/2-20 tap. Take your time and use a good quality drill bit. For the stuff you can get at the local hardware stores, cobalt is your best bet. A cheap high speed drill bit won't cut it (pun intended).

Mike

rskrause

Yeah, I think I was mixing my metaphors a bit, as with most hand drills it souldn't be much of an issue. Like I said, my memory of this stuff is kinda hazy. Agree about the cobalt.

Rich Krause

InjectedSS

well i guess the crank threads are stronger than i thought i guess what happened is that the bolt got all torn up and left chunks of thread INSIDE the crank snout threads and was causing the NEW bolt that i was trying to put in NOT to go in without getting caught up on the broken threads... so on i hunch i thought id try and see if i couldnt rethread the crank threads with the 7/16"-20 TAP and sure enuf it cleaned them right out thank god cause i didnt really feel like drilling and chewing at my $800 crank