400 sbc block with one smaller headbolt

I have just finished assembling my short block (400 chev small block) and went to put the heads on and couldn't get one of the central bolts to start. Upon further examination, i find that someone had helicoiled the hole and now the thread size is the next size down (sorry i don't know the exact sizes although i'm pretty sure they are both unc). My question is, what should I do? Should I just get a strong bolt the right thread and length and use it at the same touque ratings as normal or invest in an expensive helicoil kit and attempt to fix the thread/hole properly. Could there be some reason why someone has done it the wrong size in the first place? There is only the one on the whole block that is like this. It is only a mild street engine (270 degree cam, dished pistons with 350 rods, gear drive and fuelie heads (ported moderately) with victor juniour and 750 holley vac sec.) so I'm hoping that I can just use a high grade bolt. I do notice that the proper head bolts have a raised shank near the head which I assume may be important but where would I get a single bolt like that? What say you engine gurus?


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Different size bolts require different torques to achieve a certain stretch. I would repair the thread to the correct size and use the correct bolt to ensure proper clamping force across all of the head bolts.

Although I've never used a helicoil on a head bolt before but I don't know why it would be an issue? When installed properly helicoils are quite strong.

You may be able to find and use a ARP head stud with the correct diameter, thread pitch and length.

Okay, two different schools of thought. The wallet wants me to just use the smaller size bolt and dont mess with the installed helicoil and then get the car going again quicker but its silly if i end up blowing head gaskets and/or up cracking a head or block. I guess I'll see what everyone else says and in the meantime look for a bolt of maybe pay someone to install a single helicoil for me to save buying the whole kit etc.
thx for the response guys.

The repair is probably not a HeliCoil. Their kits are designed to return a damaged thread to a condition where it accepts the original fastener. To that end, damaged female threads are drilled out, a custom tap run thru to make new threads, the Heli-Coil installed having the original thread size and pitch for the original type fastener.

As was previously stated, the fasteners use a torque value to relate to an amount of stretch. This varies by fastener size, alloy, and hardness. A fastener from a hardware store may or may not be capable of withstanding the same torque value and provide the same clamping force. In the end what the engineer is trying to get at is a clamping force between parts. The problem with using a different clamping force is that the parts are now distorted between that fastener and the others. This can lead to loss of clamping force allowing the head gasket to leak compression and or coolant. It can also lead to casting cracking from the differential forces. Castings especially iron castings are both stiff and brittle, they don't like uneven forces with in them and have little resistance to forming cracks as a way to relieve themselves of these forces.

It's possible to get away with a less than proper repair, but the potential price of a down the road failure will be a lot higher than the proper fix now.

Bogie

The repair job worked fine before the motor was torn down didnt it? End of story.. Just get the right bolt and put your motor together.. Dont forget the thread sealer.

A picture is worth a thousand words, post one if you can and let us look at it, I don't think you can put a smaller size heli-coil in a hole that is already tapped for a 7/16 thread. Is this the bolt that is adjacent to the steam hole?

That is odd for it to be smaller. Sounds to me like it was either something other than a helicoil or maybe somebody used a metric helicoil. Figure out what size bolt fits the repaired bolt hole.

Are you sure that it doesnt just need a good thread cleaning job? That should have been part of the process anyway.

Whatever type of insert is in there now probably had to have a larger hole drilled for it. By the time it was tapped for the insert, the head bolt hole got a lot closer to the cylinder(s).

If it hasn't yet, it's going to form a crack between the head bolt hole and cylinder(s).

tom

I had decided yesterday to helicoil it to the right size and ordered a single size kit that cost 50% of my weekly income
As you can see, it is definately a smaller size than the original, It appears to be the very next size down from whatever the original chevy bolt was.
You may be able to see from the pic that I do or don't have enough meat to try to helicoil it back to right size in which I will just find the best bolt or stud i can find and try it. Like someone said up above, it must have been working before...although that could be why I ended up with a block and not a complete engine! I do not know the history of the block although all else looks to be in good order. I will now attempt to upload the pics.
Please let me know your thoughts. Keep in mind, I am on an extreme budget and parts dont come as easy here in nz as in the states nor even australia. It wont be a daily driven car but just a weekend cruiser with a bit of poke to keep the jap boys at bay lol

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I know there are one or two cracks from steam holes through to the bolt holes but I am not concerned about those. I have seen enough evidence that these don't really matter on a lower performance engine like this. Some may disagree though.

I would run it as is, any attempts at a remedy will make an already dicey situation worse. It's already been overly modified beyond an adequate fix. Any attempt will just make it worse. Send the heli-coil back and try to come up with a bolt or stud that you think can do the job, cross your fingers and a nod to the combustion gods that it holds long enough to make you happy that you gave it a whirl. Another block is the only real adequate fix.
Seeing where you are on the globe, run it till it fails if you can't get another block.

Somebody please correct me if I'm wrong but to me that looks like somebody stripped the original hole, drilled it out and installed a larger bolt, machined it flat with the deck then drilled and tapped that for a replacement bolt....

^^^^ Exactly

And to avoid ruining that bigger "bolt insert", he decided to make the new threaded hole slightly smaller. Or maybe that was just the closest he came with his equipment?

I would say use it as it is if you can get a suitable bolt that is strong enough.

I might be able to help you with a grade 8 metric bolt if that's a problem.
Just find out the thread size & type on it.

That is clearly an insert that has been installed. Not a helicoil. If done right it should actualy be superior to a helicoil. If you try to open it back up to the original size you run the risk of ending up with an insert that is too thin, which may be the reason that it is smaller in the first place. I would take the helicoil back and get back your cash. Just make sure you have a good strong bolt that is up to the task, clean the threads good and treat it like any other head bolt. It should be fine.