Head bolt block thread failure 2

[Deltona]

Hello all,

I recently got a car with a four cylinder turbo Lampredi Twincam in the workshop with a recent rebuild from some 'expert' engine builder. The engine started to knock whilst in my care so I stripped it to find it failed on worn out crank bearings (mains and big end), scuffed bores and pistons due to what looked like oil starvation. A large dent in the pressed steel sump pan was blocking the oil pick up, it must have been done when dropping the engine out or putting it in by whoever built it and caused oil starvation.

When taking the head off the bolts were caked in copper grease.

So fast forward to re-build time and I'm doing it by the book, but when I come to torque up the head bolts one pulls the thread out of the cast iron block. Take it apart, repair, try again. Another one fails.

I'm oiling the bolts and letting them drain for 30 mins as per factory instructions. The head gasket must be fitted dry and clean so it bonds to the block/head, yet when I take it apart oil has been squeezed into the mating surfaces.

So I'm thinking maybe the bolts (stretch type which don't need re-torqueing) are past their best and I need new ones.

The first question: When they are worn out or stretched too far is this what happens?

And how do I oil the bolt threads without it getting under the gasket? Presumably with less or thinner oil?! I'm following the factory manual instructions, but it seems the excess is squeezed out of the threads and under the gasket.
How is it getting under here? Cappilary action, but why doesn't it get passed through the gap and up into the bolt holes?

Hope you guys can help here,
Thanks.

 

[desertfox]

Hi Deltona

Thanks for the info on the length I'll run some numbers and let you know for sure about why I think those threads failed.

As to the torque plate I wouldn't recommend using anything but the right length bolts, whilst its sems possible to achieve the correct preload, it could affect the stresses in the block.
I'll explain more later, let me look into it a bit further before I give it the complete thumbs down.

Happy new year

desertfox

 

[desertfox]

Hi Deltona

I calculated the extra load generated on the bolts for the two quarter turns you give them during assembly and working back to a torque figure it comes to about 80Nm which is at capacity for the bolt and the cast iron threads would stand no chance and would definitely strip.
Now the reason they don't always strip is that I am assuming a worse case situation, which won't happen in practice, firstly I am assuming with the 90 degree turns on the bolt that all the strain is taken by the bolt, this means that I am not allowing for any compression of the gasket or the head and block materials which is unrealistic and any compression of the clamped components would reduce bolt load.
The only sensible conclusion I can come to is that the bolts are tightened to achieve about 90% of the yield stress,but because of the error in torquing bolts its possible that some bolts are loaded beyond yield, this additional load coupled with only 20mm thread engagement is to much for the cast iron and so every now and then this situation occurs.

I wouldn't use shorter bolts for clamping the torque plate it might well strip your cast iron threads and it can affect the stresses in the block look at this site:-

http://www.goodson.com/technical_support/tech_library/cylinder_boring_tips.php

desertfox

 

[Deltona]

Hi Desertfox,

I've never had this happen before and neither has another specialist I spoke to, I think unless we got the actual bolts tested we will never know, the fault definately lies with either a previous mechanic or the bolts. I think they must have been tightened (or over tightened) too many times. Lesson learned: Don't risk it, buy new bolts. I only re-fitted as the customer said the head was to come off again for a gasflowed one to be fitted and the manual said that they could be re-used about 4 times.

In some ways the best way to pre-stress a block is to torque a scrap head on with bore holes already machined through it then bore the block through this.
However, I have been warned against this method by the machine shop because the further away the cutter is from where it is driven from the less accurate it becomes.....

Deltona.

 

[desertfox]

Hi Deltona

Yes best way is to have new bolts as you said, it could be that if a previous mechanic over tightened the bolts, then the female threads may have been damaged at that point and unfortunately for you when you re-assembled they failed.
How did you get round the failed female thread? just out of interest.
I agree with your machine shop you need to keep the cutting tool as short as possible.

regards

desertfox

 

[Deltona]

Hello Desertfox,

I used an off-the-shelf thread insert - or two anyhow, one on top of the other to get the correct length.
Someone brought in an 'Emergency job' a few weeks back of a similar kind (different circumstances and engine) and I bored it out oversize, made up an insert from an M14 bolt bored and tapped out and did it that way. I have a good quality lathe and a sturdy old pillar drill in my workshop so can perform basic tasks like this.
I work on a type of rare and sought after old car with many parts unobtainable so have to be a little resourceful from time to time.

Deltona.

 

[desertfox]

Hi Deltona

Thanks for the info, I assume that you haven't had issues with temperature and the threaded inserts when the engines built up and running.

desertfox