Main Bearing Thread Size

[Rod]

Hi All,

I have been slowly (very slowly) cleaning up my motor in parts get ready for a basic rebuild.

I have come across a broken Main Bearing Bolt (Rear Journal). I am correct that the thread of these bolts are 7/16 UNC?

The broken piece is about 10mm below the surface. For a couple of weeks I have soaked this bolt. Today I was successful in drilling a centre hole through the bolt to use ezy outs. After using some heat and the ezy outs I wasn't successful. I feel if I apply much more pressure the ezy out is going to break. This is something I don't want to do.

As the centre hole has gone right through (not through the casting of the block ie: bottomed out), I am now able to fill this void up more with WD 40 etc... to hopefully have some further pentration from both ends of the broken bolt. I will try again tomorrow with an ezy out.

If I am unsuccessful this is what I am thinking. Please tell me if this is a stupid idea. I plan on drilling the hole out a little further without compromising the thread, then to use a 7/16 UNC tapered tap to hopefully start "cleaning the thread out. Onc this is completed I would then use the same size tap but this time a bottoming tap. The idea would be that due to the taper of the first tap it would go through the hole to start claeaning the thread. Does this sound like a plan of attack? There maybe another approaach I havent considered.

Cheers

Rod

[my8thholden]

G'day Rod .Do you know what caused the bolt to break off in the first place ?

[Rod]

Good Morning Vern,

That is a very good good question. This is something that I am not sure of. I don't recall any "binding" of this or for that matter any other bolts when I dismantled the motor 6 weeks ago. I wonder if there was a issue when it was assembled in a previous rebuild. Are you able to confirm the thread size.

Have  great weekend or what is left of it.

[ardiesse]

Rod,

7/16" UNC is correct.

At this point, I would suggest taking the block to an engine shop.  They will be able to set it up in a milling machine, locate the hole centre to micron accuracy, drill the bolt to the root diameter of the thread, and then chase the thread out with a tap, as you describe.  At the very worst, the hole will need heli-coiling.

You can use the main bearing cap as a drilling jig, but you would need to make a sleeve to suit the "root diameter" drill.  Easy if you have access to a lathe . . .

Rob

[ardiesse]

The minor diameter of a 7/16" thread is 0.3525"; the tapping drill is a letter "U" (and we all know what diameter that is . . . not).  0.3525" is 9.0 mm.

Wikipedia is your best friend.  The letter "U" drill is 0.368" (=47/128", so it's half-way between fractional sizes), 9.347 mm.

Rob

[Rod]

Hi Vern and Rob,

Once again thanks for your feedback. Both of you are real rippers!

Vern thank you for asking the question what caused the broken bolt. Your question made me investigate further and I think I have found the issue and to be honest your comments about end float etc in another thread may very well be part of the "knock" I have had (in addition to the clutch plate).

I went out this morning and had another go at removing the broken piece. Gave it another good heat. Made a few creaks which I thought was movement but to no avail - it was the ezy out making the noise. Stopped again fearing a breakage. At that point I investigated further in regards to the bolts. What I found was the following.

1. There are two sets of different bolts, one with a star pattern and one with a circle on top of the head.
2. The circle pattern bolts were on the front and rear main bolts while the star pattern ones were on the intermediate mains.
3. These bolts are of different length. The cirlce ones being longer. Then I had to investigate why. It is due to the heights of the main cap.
4. Front and Rear main caps are lower in height than the intermediates.

With this in mind I came to the conclusion that the bolts were around the wrong way. I then installed the front cap (couldn't test it with the rear due the broken bolt) with one bolt of each length. The short one went in fine and could have been torqued up correctly, while the long one "bottomed out" and torquing up would have been very difficult. The main was a little loose. I then looked at the full length long bolt which was taken of the rear main (unbroken one) and had a close look at it. The end of the thread was slightly damaged from bottoming out on the end ot the hole. This explained why it binded a bit when testing it in other holes. I can only assume that the front and rear mains would not have been torqued up correctly and would have been loose. It might also explain why the rear main leaked like a seive).

I looked at the Master Parts Catalogue and sure enough from EK onwards there are two sets of bolts listed for the main caps, while there is only one set for 48 to FC (all different numbers). While I was aware of the change to the Front main from L313838, at some point there must have been a modification to the heights of either the Front and Mains or Intermediates hence the different bolts from EK onwards. (My motor is a replacement of EK / EJ vintage) Vern some of this might confirm your thoughts about end float particularly if the Front and Rear was torqued properly.

Rob thank you for the confirmation of the bolt size and suggestions of moving forward. The broken piece is in real tight. I am sure this is because it has bottomed out. A while ago I saw on Youtube of a bloke who made a guide. He took a bolt of the correct thread and then using a lathe as you suggest to drill a hole through it. Once screwed in the guide allowed for true centering of the broken bolt. I have already drilled a hole which is close to centre but not exact (for ezy out). I hope this doesn't compromise your suggestions.

Have you heard of the different bolt lengths. I have searched here but can't find any reference.

Once again thanks fellas. I hope my issues may be of help for those rebuilding in the future.

Cheers

Rod

[ardiesse]

Rod,

You'll only find reference to different length main bearing cap bolts in the EJ workshop manual, which makes me think you have an EJ motor.

About the sleeve: I'm suggesting you make the outside diameter the same as the hole in the main bearing cap, and the inner diameter 9.4 mm.  Then install the rear main bearing cap with the remaining bolt, slip the sleeve into the other hole in the main bearing cap, and drill the broken bolt out.

Still a job for an engine shop.  And since the motor's been run with (effectively) only one bolt securing the rear main bearing cap, it would be a good idea to get the cap checked to see that it's not distorted.  You can do this yourself with alfoil, once you have removed the remains of the broken bolt.  Put a strip of alfoil on the mating face of the block. Put the main bearing cap in position, put a bolt in the opposite side to the alfoil and torque to spec.  The cap should hold the alfoil tight.  Repeat the procedure with the alfoil and bolt on the other side.  If the cap doesn't hold the alfoil tight, then it's distorted, and you will definitely need the services of an engine shop.

And the motor probably would have had a worse-than-normal-for-Holdens oil leak at the rear main bearing seal too . . .

[my8thholden]

Rod ..Rob has covered everything you need to do ,sadly , it seems some one has done a really rough job on that engine ,also you need a huge amount of torque to break off a main bearing cap bolt ,and they don't break in service ,not correctly installed ones anyway ,most people would stop when your off scale on the tension wrench ,he probably had a inch drive socket and length of pipe..I imagine it would "snap" .And then you would not continue with assembly and installation...So .thinking that .I would investigate that engine ever so thoroughly ,what does the bore measure ? what do the crank journals measure ?
Then how does it measure up ,IE amount of taper ,ovality etc..condition of cam ,has top of block been machined ? ,it may measure STD in the bore ,has it been re sleeved back to standard? That did happen , and it was done in the past  ,just grind one big end journal ,or sleeve one cylinder ...also are the con rods correct ,a lot to check ,but now is the time ,I feel .someone possibly tried to build one engine out of two ,and mixed several things up, because they both were not the same series engines ...cheers Vern ..

[Rod]

Success!!!!

I had the inside of a 7/16 bolt bored out to serve as the guide. Used that to guide the drill to centre out the broken bolt. I used the tappered tap followed by the bottom tap. It took some time as I was very patient. I had to clean the tread regularly to get rid of the old bolt as I went. I was paranoid of a broken tap. Anyway eventually all was cleared.

I then used your suggestion Rob with the alfoil. Once torqued up there was no distortion indicated in the main cap as the there was no way the foil was moving without ripping.

Thanks once again for your help and suggestions.

Cheers

Rod

[my8thholden]

Rod ..Did you try a easy out after you drilled the broken bolt before re tapping the thread to clear remains of old bolt ?  Vern

[Rod]

Hi Vern,

Yes I used the ezy outs multiple times but resisted the temptation to apply more torque in the fear of breaking the ezy out. I tried everything - soaked the top and bottom (once hole was drilled) with every known substance know to man. I applied heat multiple times, cold (Loctite Freeze), and a combination of both heat and cold but the broken bolt would not budge. I even used different sized ezy outs but I just couldn't bring myself to go harder. I thought it would much easier in drilling out a broken bolt than a broken ezy out.

Thank again.

Rod

[my8thholden]

Rod..Ok ..You persevered and went cautiously and got the right result ..good on you ..Vern ..

[Harv]

I thought it would much easier in drilling out a broken bolt than a broken ezy out.

A broken ezy-out is the hardest substance known to man. Makes diamond look like butter. Don't ask me how I know  .

Cheers,
Harv

[vmx-mxr]

For anyone who hasn't used an Ezy-Out beware!

Even fitters, toolmakers and other handy tradies can easily snap off one having the best skills / machines at their disposal. For that reason they generally avoid using them and prefer to accurately centre the offending stub and drill them out and pick out the remaining thread. Ezy-Out's were designed for extraction of only the easiest jobs and no-one knows whether the stub will co-operate or not.

There are essentially two types. Corkscrew form, or square tang with gripping flutes (my preferred choice if using one). The tang of a file will often work too. You drive it by turning with a shifter as close as possible and because the tang isn't hardened it wont' snap off like an E/O can.

Other methods -

Use a good penetrating spray - INOX make a great product and every workshop should have a can and it's great for un-siezing virtually anything.

Often if a small amount of the bolt is protruding grind a slot in it with a dremel or 1mm cut off disc and remove with a screwdriver. a few good hits with a hammer on top of the stub beforehand can shock the thread and release rust bonding or stretched threads. Before doing the cut leave the job for a while (overnight it better) soaking with penetrating spray. 

If part of the stud or bolt is still sitting proud building the top up with weld to a point where it can be grabbed with vice grips. With the heat involved  in welding, more often than not will see the stub easily screwed out by gripping the weld material.

Centre and drill out with a left handed drill preferably with a drilling block to ensure correct alignment (a drilling block is a section of steel pre-drilled to the drilling diameter in a pedestal drill) . I've had plenty come out doing this. as soon as the drilling starts you often see the stub come turning out Try gmdengineering.com.au for appropriate drills - these guys really know their stuff and will advise on the most suitable drill.

If it's a broken stud in an exhaust flange heat the whole area with oxy to a dull red which will usually release any rust bonds the usually culprit here.

Harv is right. When an E/O snaps off which is all too common you now have a major problem and it is more than likely machine shop time. Or the $$$ services of a spark erosion company.

Good luck

Dave Mac

[Errol62]

Good tip about the left hand drill bit Dave. I hadn’t heard that one.

Those who have snapped an ezy out off once are very cautious not to do it again.


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