Big Block Head Intake Bolt Thread Repair

[Richard M.]

Preamble: I had a very disappointing and discouraging day yesterday. A true showstopper. When restoring 50 year old cars, we sometimes forget that pitfalls are common. Salvaging old correctly cast numbered and dated objects can be rewarding, yet sometimes it gets nightmarish. I have one of these situations at hand. I'm sorry this is long, but it is complicated and need details explained.

I have a major task ahead and would invite input and advice. Unfortunately, after machining the intake manifold and trying a few bolts to fit the heads on this 427 engine build, it appeared something was wrong. When I took the engine back from the shop I thought I was ready for final assembly, paint, and installation. I only tried a few intake bolts at the engine shop and had a little trouble but figured I'd be okay. It was late so I took the engine home and I figured the rest would be easy after a good nights rest. Not so.

I acquired these dated heads as the mounted heads on the running engine were dated beyond the car build(by one day). Since we were rebuilding the powerplant to stock, it was decided that now is the time to get correctly dated heads. Even though the dates are covered up by the valve covers, authenticity is still always a factor. Knowing the "right stuff" is "undercover" is important to me.

I knew they would need rebuild, as evidenced by the discovery of needing new valve guides and a valve regrind, etc, and was told all intake threads were made up of threaded inserts. I bought the heads. I actually welcomed the threaded inserts as the threads on other heads I've seen had issues with stripped threads. However, little did I realize that there was a major problem lurking that everyone missed...... the seller(allegedly), the machinists, and of course....me. When I got them in I rushed them to the machine shop for rebuild. Lesson learned. AGAIN!


Here you can see with all bolts inserted that there is a problem. All of them are varying angles in ALL directions....front to back and side to side. Now it's quite obvious, that whoever did this had no clue as to what they were doing. It was clearly a rookie mistake and the party had no business attempting this task. The heads were used and ran at one point.
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I took some measurements on the original head with stock threads using a 3/8-16 straight long threaded rod and a small machinists protractor. It appears the angle of the stock threads with respect to the intake mating deck are approximately 75 degrees. If anyone can confirm the actual specification for me I would appreciate it.

Using the threaded rod I measured the replacement heads. I get approximately 85 degree angle. Some are more and some are less.

Here is a example using my rough measurement. The angle line tracing of the rod on the left is "stock". The angle line on the right is these heads with the previously(incorrectly) installed 3/8-16 thin-wall inserts.
P8140005.jpg

Stock apx 75* angle ------- Improper head(s), apx 85* angle. (ignore the pencil 70/80, it should be 75/85)
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Additionally, looking very closely on some locations I can see the original threads and bottom of the original hole. Clearly obvious when looking down deep. Some bolts bottom out on the original ledge left by the incorrectly angled inserts.
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Here is my plan and welcome your thoughts. The present inserts are 0.505 OD(measured). I did some research and they appear to be "Thin Wall" inserts.

I found "Heavy Wall" inserts here made by EZ-LOK. These are 9/16-12 external thread size and use a 31/64 tap drill.

http://www.grainger.com/product/E-Z-LOK-Thread-Insert-4ZB75?s_pp=false&picUrl=//static.grainger.com/rp/s/is/image/Grainger/4ZY16_AS01?$smthumb$

Also available are "Extra Heavy Wall" thickness here. These are 5/8-11 external thread size and use a 17/32 tap drill.

http://www.grainger.com/product/E-Z-LOK-Threaded-Insert-4ZE32?s_pp=false&picUrl=//static.grainger.com/rp/s/is/image/Grainger/4ZY16_AS01?$smthumb$

I am leaning to use the "Extra Heavy" part. However, I have concerns....

After removing the incorrectly installed inserts, re-drilling may be difficult to achieve the correct 75* angle without loosing some OD wall thickness.

Another concern is there are 4 of 8 (IIRC) blind holes in each head. 4 pass through to the pushrod/rocker area. The others are unknown. I am concerned about the risk or drilling into the water jackets at the sides of the insert area. Depth can be controlled easily, but the width is fixed by the insert drill size. Edit...I just checked and there appear to be NO blind holes near water jackets.

I spoke with my machinist late yesterday and he agreed this plan should work, albeit with concerns above. He is ready to set up his Bridgeport to help me repair these heads. Of course I have to disassemble them from the block and protect them with wrap for shavings intrusion.

Any thoughts or concerns about my plan for repair is welcomed. Please keep in mind I have now milled a 3x2 intake manifold to fit this assembled engine and am committed to use these heads at this time. These are not "throw away" items. I must succeed and ask for your advice.

Rich

[Michael M.]

Re: Big Block Head Intake Bolt Thread Repair

Rich, so sorry to hear about your issue. Nothing worse than shoddy machine work. If you have confidence in your machine shop, is it not an option to see what they can do?

I believe you're on the right track with thicker walled inserts. A competent and experienced machine shop should know whether installing thicker walled inserts will intrude on water jackets, etc. A high end machine shop with lots of big block Chevy experience should be able to fix the issue, it would seem it's just a matter of removing the existing inserts, and properly re-drilling at the correct angle, re-tapping for the new inserts, etc. It's all about the set up. I am fortunate that I have Garry Grimes (Grimes machine) near me (Atlanta, GA) and he does my engine machining work. He's top notch. Unfortunately, this may be a time where you have to ship the parts to someone outside of your area to finish the job.

[Richard M.]

Re: Big Block Head Intake Bolt Thread Repair

Yes my machinist and I spoke yesterday about my idea and he agreed we can do it. He has had much experience with big blocks which is why I went to him. For the record, he did not install these inserts. I acquired the heads this way.

I just verified there are no water jackets near the holes at the sidewalls. Depth can be controlled.

Thanks for your opinion on using the bigger OD inserts. I'm thinking that is my only option. Accuracy is critical to get the angle correct. I can get the bigger inserts Monday morning at Grainger 1/2 hour away north, then a hour back south to the machine shop with the heads and parts.

I am removing the heads today and contemplate removing the wrong inserts. I tried tapping with a drift but they must be locktite'd in.

If anyone has prints showing the hole angle please check that for me.

Thanks,

Rich

[Gene M.]

Re: Big Block Head Intake Bolt Thread Repair

I would think the machine shop would use an end mill and not a drill so the hole center is cut where you want it and not follow the old holes path. Good luck that you don't run too thin on casting.......!

[Michael M.]

Re: Big Block Head Intake Bolt Thread Repair

Rich, let us know how it works out. Good luck!

- Mike

[Richard M.]

Re: Big Block Head Intake Bolt Thread Repair

I would think the machine shop would use an end mill and not a drill so the hole center is cut where you want it and not follow the old holes path. Good luck that you don't run too thin on casting.......!

Thanks Gene, Your input is taken.

Yes a end mill is the way to go for sure after fixturing the head for the correct angle. Setup will take some time.

I just was in the shop looking closely at all positions and there is much material around the holes. I'll shoot some photos up after breakfast. I've been up since 4:30 working/thinking on this.

I just slowly loosened all rocker arm nuts in a circular fashion a little at a time at all 16 places to keep the nuts cool. I will then remove the heads and contemplate removing the old inserts manually. Each one is at a different angle so step drilling is in order to maintain the angle of the "incorrect" insert angle. IMO, it would be difficult to fixture it on a machine and get the correct angle. I envision if I get the insert thin enough I can carefully tap them out and get to the clean "wrong" threads and stop there. It'll be time consuming on each, x 16.

Grainger is open Monday morning at 7:30 AM so I'll be at the door waiting for them to open.

Rich

[Gene M.]

Re: Big Block Head Intake Bolt Thread Repair

Your good head with wrong date for your car should have good set up information so the new threaded hole in the bad head can duplicate. I think all reference dimensions are derived off the spring pin hole that align head to block. That established the "datum". Best would be if someone has a specification machining drawing GM used. I thought there was a drawing in the old Chevy engine HP book. Maybe someone on this site has that info for you. The exact angle of the threads would be neat to have setting this up on the mill.

Setting up an angle to establish the center line of a hole off is an imaginary point at the intersection of two planes when at an angle off the surface. Be patient setting this up. Measure a lot cut once....

[Richard M.]

Re: Big Block Head Intake Bolt Thread Repair

Your good head with wrong date for your car should have good set up information so the new threaded hole in the bad head can duplicate. I think all reference dimensions are derived off the spring pin hole that align head to block. That established the "datum". Best would be if someone has a specification machining drawing GM used. I thought there was a drawing in the old Chevy engine HP book. Maybe someone on this site has that info for you. The exact angle of the threads would be neat to have setting this up on the mill.

Setting up an angle to establish the center line of a hole off is an imaginary point at the intersection of two planes when at an angle off the surface. Be patient setting this up. Measure a lot cut once....

Thanks Gene, Good advice. I'll be with the machinist when this happens.

Yes I will use a good head for reference but it certainly would help to have the drawing details. I scoured the internet for information but came up empty. My machinist has access to that information as he told me he has access to the specs, so when I take them to him I'm pretty sure he can get it. I thought someone here may have it too.

I did find this drawing where one of the race boat guys wanted to add head bolts to his BB. Funny as just before I saw it I noticed the head bolt arrangement and there seemed to be "missing head bolts" in the design at the #1/#2 and #3/#6 bottom of chamber areas. There's the datum point as you mentioned.

Bore_Spacing_and_Head_Bolt_Locations.jpg

Here are the inserts I'm ready to remove by step drilling and prying out.

The photos depict the hole starting at cyl #1 and work to the 8th hole at #7. Note right to left sequence as it's the opposite end from the exhaust port side. Actually the head came from the 2,4,6,8 side but I'll use DS reference as it's easier to remember. I used a red straw to show the through holes and where it stops on the blind holes. I also show some of the side views along the path and the material thicknesses.


Holes
#1 feed thru
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#2 feed thru
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#3 Blind
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#4 Blind (in the crossover area)
P8150029.jpgP8150030.jpgP8150031.jpg

#5 feed thru
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#6 Blind
P8150034.jpgP8150035.jpgP8150036.jpgP8150037.jpg

#7 feed thru (pushrod area)
P8150038.jpgP8150039.jpgP8150040.jpgP8150041.jpg

#8 blind
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The head is on the bench and taped up to minimize debris intrusion into the ports. I'll set a old valve cover on it also.
P8150046.jpg

====

[Richard M.]

Re: Big Block Head Intake Bolt Thread Repair

16 inserts removed. I took my time as if I was practicing saving the threads. All saved.

This concludes today's exercise in mistake fixing.

Rich


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[Richard M.]

Re: Big Block Head Intake Bolt Thread Repair

Yesterday began the process of repair. This included a trip to Grainger to get the inserts.

Before heading out, I took more measurements after insert removal Saturday.
The hole size left from the previous inserts was apx 0.400"..... 7/16-20 thread size.
P8170008.jpg

Because of this I thought the Extra Heavy Wall(5/8-11 OD) insert may be too big and not needed. So I decided to get 20 of each, the Extra Heavy and the Heavy(9/16-12 OD). If any holes require the bigger insert, I have it available. Grainger rep said I can return any unopened bags. Inserts are expensive.

Another view of the wrong angle for the previous inserts. That ledge should not be visible.
P8170006.jpg

I came up dry trying to get GM threaded hole angle specs, so we're going to use a known good head to setup the correct milling angle for the heads to repair. I went to my engine shop late afternoon and we discussed a plan for the repairs. He will remove his pivot plate from the head decking machine and set it up on the Bridgeport. We are going to use a end mill appropriately sized to create the proper holes for the insert tap size chosen. Then if needed we can use a sized drill to follow the milled hole to get to the correct diameter for the tap.

This is the blind hole location on each head that is concerning me. There is not much material there. I think the safest plan here is to open the hole a bit more than spec for ease of tapping force, then ad extra Loctite to the insert.
P8170003.jpg

For future reference if anyone else ever has to do this, I also found "Time-Sert".
image003.gif

These appear to be a very good product also, however they have a very thin wall thickness, not useful for this task due to the angle of the cuts needing correction as I have about 10* to correct on all 16 holes. I need the larger wall type to grasp more sidewall iron. They also make a thicker wall but I believe now that is what "Mr-Dewalt-With-No-Clue" used the first time as they're what I removed by step drilling out.

Heading back to the shop this morning for a day on the Bridgeport. Hopefully I'll have good news later.

Rich

[Richard M.]

Re: Big Block Head Intake Bolt Thread Repair

Success! All 16 holes fixed.

I got a late start at the Engine Shop in the early afternoon right after lunch. We setup the Bridgeport with a know virgin BB head to get the correct angle for a tilt plate borrowed from the head milling machine. We then used a special reground end mill(0.490") and a 9/16-12 tap. We used the Bridgeport to mill, then right after milling we maintained the location, switched to the tap, lowered the speed then tapped the hole 1/2 to 3/4 depth. This procedure repeated 8 times on each head, 32 tool changes total. Took about 1 1/2 hours per head.

I have to finish tapping by hand, clean the threads then install the inserts with permanent thread locker. A little grind-off of the inserts where needed due to the angles, then clean the heads thoroughly.

Dog tired. Heading to bed. Pics tomorrow.

Rich

[Richard M.]

Re: Big Block Head Intake Bolt Thread Repair

Pictures.....

Known good head. We set it on the tilt table, adjusted using a reference stud level(forgot picture) with a straight ARP stud and locked the table.

P8180024.jpgP8180025.jpgP8180026.jpgP8180027.jpgP8180028.jpg

We then installed the head to repair. Since the angle was locked by the table that angle was preset. But we had to set the horizontal angle(insert/bolt) to 90* using the internal cam adjuster(lower right of the table) to make sure it was straight. We taped the previously machined deck surface with tape. I also left my tape on the intake surface to keep the ports and water jackets more protected from debris.
P8180022.jpg

Using R8 collets and the 0.490" diameter mill, we set the speed to 600 rpm, centered the mill by eye to the center of the original counterbore.
P8180023.jpgP8180029.jpgP8180030.jpgP8180031.jpgP8180032.jpg

As you can see after milling the hole to the proper 75* angle, the residual threads from the incorrect insert repairs are still visible down low on one side. The new 9/16" OD Heavy Wall inserts were chosen to overtake those threads. Thankfully they used thinwall inserts when they did it "wrong".
P8180036.jpg

Then a clean out with air, change speed to slow rpm(50 rpm IIRC) with a few levers on the transmission gearing, change to the collet with the tap, power tap with cutting fluid, reverse the tap out, clean.
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After machining we could now see the original deep hole left from the original threads being concentric with our new hole.
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A test fit was done on all inserts when I got back. 5 of the 16 needed a little more tap depth, but because we used the machine to start all of them, a by-hand tapping process would follow the properly machined threads.
P8190042.jpg

I scrupulously cleaned(sorry Jaguar Repair Manual talk) all of the threads with solvent and brushes to remove the debris and oil and let them dry. It was late when I finished up last night and had to call it a day.

Today I will install all inserts with permanent highest strength thread locker. After adequate cure time I'll reassemble the engine.

I would like to thank Lynn, the Machinist at American Cylinder in Vero Beach for his excellent help to salvage these rare heads. I'd also like to thank Harald(Norwegian spelling), the new Owner of the business for his support as well. BTW, Harald owns and drives a 1984 Corvette every time he comes to the shop. He's a Captain on a major airline on the European routes when not helping out at his new business. He loves Corvettes!

Rich

[Gene M.]

Re: Big Block Head Intake Bolt Thread Repair

Rich,
Sounds like a cylinder head build is to follow. Interesting coverage on fix up of some bozo's attempt to rethread by hand without a mill set up. Assembly should be a shoe in now.......!

[Richard M.]

Re: Big Block Head Intake Bolt Thread Repair

Thanks Gene for your guidance with my task. Your input is always anticipated and appreciated.

Michael, thanks for your support and confidence as well. It's a great feeling to get help with problems like this from fellow members.

Rich

[Leif A.]

Re: Big Block Head Intake Bolt Thread Repair

Attention to every detail, as always, Rich. Well done!!!