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Bungled exhaust stud threads


ccnoel123
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I bought a 93 Pathfinder a few months ago. I noticed that the exhaust manifold leaked so I took it back to the place where I bought it and he had his mechanic "fix" it. Needless to say his repair didn't last long. I decided to do so myself. So I'm mid installing a set of Pace-Setter headers that I got out of the junk yard for $50.00. :rolleyes: Today I took the stock manifolds off and low and behold, there was good news and bad news. The good news is that 8 of the 12 studs are all brand new! The bad news is that 4 of the holes are completely trashed. :( It looks like he may have tried to drill out the broken studs but that's all he did. On the drivers side the first two studs towards the front of the engine are missing and there is no threads left. The same on the passenger side but these are the two towards the back of the engine. At least the holes are in an easy to reach position. My question is, can I drill through these holes or is there something important like a water jacket behind them or can I install Helicoils in a "blind" hole here?

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I don't think that he drilled in to far as there are no leaks. I am thinking of using size 10 studs and buying the appropriate Helicoil set for that. How deep should I drill to properly set the Helicoil?

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If your going with a larger stud you may just be able to tap new threads into the current hole if it is not too buggered. If you do end up needing to use a helicoil grab some scrap aluminum and install a couple until you get the hang of it they can be a bit tricky but all in all not to bad. And the helicoils come in different lengths so you can just get one of the proper depth for the hole.

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I don't think that he drilled in to far as there are no leaks. I am thinking of using size 10 studs and buying the appropriate Helicoil set for that. How deep should I drill to properly set the Helicoil?

Two approaches I'd do in this order (hopefully you have a set of calipers).

Measure the diameter of the holes in the header and the blind holes in the head, both at the mouth and as deep as you can go to see if they are bell mouthed, Measure the depth and write all this info down for each hole.

If the holes in the head are not larger than 8.3-8.4mm (.326-331", other than perhaps right at the mouth) you can drill out to that size and tap/insert helicoil as long as you have sufficient depth VS insert length.

If the holes in the head are larger than that, but smaller than 8.8mm (.346"), you can drill them out to that size and directly tap M10x1.25.

If the holes in the headers are larger than 10mm (.394") you could go with straight M10 studs; if not you need to drill the holes larger or use stepped studs (M8 to M10).

 

It all comes down to what dimmensions you have now to determine what is the best/easiest way to approach this. Post that and I'll give you my professional opinion.

For the record, I am a machinist who has honestly tapped countless holes and helicoiled thousands of them.

 

Here is a little light reading...

http://www.helicoil.in/pdf/HeliCoil%20Catalogue.pdf

 

B

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So I guess I have one question left which is this: What is the maximum depth to which I can safely drill these holes? Is it 1/2 inch, 5/8th or what? I can then get or improvise a depth gage/stop so I then don't drill too deep.

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The safest thing is to go no deeper than a) you have to or B) no deeper than they already are. Put graduation lines on a bamboo skewer with a marker and just do a down and dirty check what the current depths are first, then get yourself the proper sized drill stop...

http://www.harborfreight.com/7-piece-drill-stop-set-38336.html

 

B

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Ok. So I went and relooked at my heads. This is what I found. On the drivers side on the top hole, it looks like someone tried to drill out the droken stud and drilled a hole off-center. 31a91971270d12e2e35fd71db9aee159.jpg The hole on the bottom still has the broken off stud in it 230e05dfa1153d2ea8fd937471e87136.jpg On the passenger side, it looks like the studs were drilled out and the holes are about an 1/2 inch deep and I can insert an 8mm stud in them but it's loose and wobbles. 845119114018d90bd6e9e1cd5f733d38.jpg6cb50c41d9a69845a44dbe45c9829047.jpg So I know enough that I wont be able to fix the off center drilled holenwith the head still in the truck and I quess my options are:

1. Remove the broken stud and replace it on the drivers side and call it good. I'll still have 1 stud missing and may suffer an exhaust leak due to this.

2. Take the head to a machine shop and have it fixed. Lord knows I can't really afford to do this.

3. Replace the heads. The Pathdinder I got the headers from.last weekend is still sitting in the junk.yard with food studs all the way around. I already bought a head gasket kit and all the tools needed. What would you do?

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If you already have the stuff to rebuild your heads (most top end gasket kits) and the tools to take proper measurements to make sure the junk yard heads are good; I would swap the heads and have the top end all nice and refreshed. While your in there might as well change the water pump and the timing belt if you change the heads. :my2cents:

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The only one that is a problem is the off center one. WTH is wrong with people? :doh: You can probably get it out and put an insert in the right place, but it will take a little finesse.

The rest are quite salvageable, but at only 1/2 inch deep, are there still studs in there? (test with something magnetic)

If the motor runs fine, I'd be more inclined to address those 4 holes rather than replace both heads.

 

B

 

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Ok. I was going to go with replacing the heads. My question is what tools does one need to ensure that the replacement heads are ok?

 

The proper answer is... take a set to a machine shop and have them trued and the valves grounded/seated. I forget how much it typically is but when I looked into it, it did not seem outrageous. With a nice set of rebuilt heads, it's fairly easy to do the swap.

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http://www.ebay.ca/itm/87-94-Nissan-3-0L-V6-SOHC-FI-Left-Side-Aluminum-rebuilt-cylinder-head-/280801347118?pt=Motors_Car_Truck_Parts_Accessories&hash=item416110862e&vxp=mtr&_uhb=1

 

You could most likely still fix that off center stud as precise1 is saying.. i would repair the stud first, if the motor runs well..

 

If you want to replace the heads then you can get quite reasonable fully rebuilt replacement heads on ebay. They really don't cost very much for what you get and a fully rebuilt set of heads.

Edited by Nefarious
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Well I just found out that my friend who's been helping me on this project got OK from his dad to use his machine shop to repair the heads. So thanks for all the sdvice and I'll let everyone know how this all works out.

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First off I'd like to thank everyone for all of the advice that they have given me. I found out today from my friend who has been helping me that his dad has offered to let us use his machine shop and some of his time to repair these heads. I all ready purchased the gasket kit which includes the valve seats so it looks like a complete rebuild and repair of the heads. The water pump was leaking so that and the timing belt are being replaced along with the spark plugs. I also plan to change ALL the fluids, rear end, transfer case etc, flush and refill the tranny and then do a Sea Foam clean on the engine and did I mention that I also need to put in new radiator and while I'm at it I plan to install a tranny cooler as well. Then off to the Russian guy to get a $39,99 front end alignment. Whew. One hopes that the truck will last quite some time after this! Thanks to everyone and I'll post pics of the finished product!

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You are welcome, and good luck. Just stay organized and don't rush or guess. Sounds like you have the right idea, so you should do just fine.

Have you downloaded a copy of the 1994 FSM from the Garage section yet? That will have most of the information you could need...

 

B

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  • 3 weeks later...

Ok. So here is an update of what went down so far on this project. We took the heads to my friend's dad's machine shop. We had to make jig so that we could mount the heads on the milling machine. fc6eba7428e57713f9e74845b97d351b.jpg85eef51e23217d280d622203044ca80a.jpgd451bbd6e44300cfd1251dc1e7b645be.jpg As you can see, we first drilled out each hole then tapped themnto accept helicoil inserts. The one where someone had tried to drill out the stud and the drill walked into the head we had to drill and insert a 10 mm helicoil into and I found a 10mm - 8mm stud at the local Coastal Farm and Ranch store. As the hole wasn't quite on center, I had to grind out the hole on the headers so everything fit. Now it's all back together with a new water pump, timing belt, head gaskets, fuel injector cushions (purchased at my local Nissian dealer as I couldn't find them anywhere else and they only cost $14.00 for 6 of them), a K and N air filter element and I installed a transmission cooler. I had a little bit of trouble getting the timing right as the picture in the 1994 Service manual didn't match up but with a bit of trial and error me and a friend were able to get it running smoothly. I am veey happy that a now have a set of Pacesetter headers on my Pathy! 5bd196eb251500d5f22f0eb3f683b759.jpg and I forgot to mention I also put in a new fuel filter while I was at it. Next up is to change the tranny fluid out and install a new filter and new pada on the back brakes and I should be good to go fir another 50,000 until it's time fornanother timing belt and water cooler. I loved how easy it was to work on this car. Maybe all automotive engineers should be schooled in Japan!

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