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.

 

[evelrod]

Just off the top of my head, I think it's pretty obvious that someone has already overtorqued this engine beyond simple repair. I'm not familiar with your engine, but if it is similar to a Fiat 124 then we are in business. One thing that comes to mind, aside the obvious pulling the threads out of the block (Cut to the chase and repair them ALL) is the need for new bolts. If they are stretched enough to bottom in the block drilling combined with even a smallish amt of oil, the "squishing out" you see is a result. I'm a bit surprised you have not twisted a head bolt.

Sounds like the previous rebuilder did not have a clue and pretty much screwed you in the process.

Rod

 

[franzh]

I had a '80 Mazda 2.0L SOHC engine in my sons vehicle. We bought it with a blown head gasket. During teardown, I noticed that the bolt head seemed to turn before they came loose from torque. When I measured them, they were over 8mm longer than spec. They were bottoming out in the block before they were tightening the head. These were not torque-yield bolts.
The threads in the block were also jammed. I cleaned out approx 10mm of bolt sealant from the bottom of the threads. Chasing and cleaning the threads and installing a hardned washer under the headbolts allowed the head to be torqued to spec, and the engine lasted until the vehicle was wrecked about a year later.

In retrospect, the head has been removed at least once before, maybe twice.

Franz

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[desertfox]

hi Deltona

Yes its a good idea to re-new head bolts but I am not clear from your post as to whether the bolt failed or the internal thread, I presume the bolt.
Very often head bolts are taken to 90% of yield stress during assembly and because torqueing isn't the most accurate method of achieving preload that can get to the yield point.
If your internal threads are failing then sound like there getting to much load.
Have a look at this sites might give you some back ground

http://www.roymech.co.uk/Useful_Tables/Screws/index_screws.htm

http://www.roymech.co.uk/Useful_Tables/Screws/Preloading.html

http://www.boltscience.com/index.htm

desertfox

 

[Deltona]

Thanks for your pointers Rod/Franz and yep, the 124 does have the Lampredi Twincam too, just an earlier version. The ones I work on are the later turbo 16v type with twin balancer shafts which were used in the integrales and Fiat Coupe (16vt).
You are also correct in that I am trying to sort out someone elses mess, but as cheaply as possible because the customer is now having to pay twice and things being as they are I can't turn the work down.
The last engine builder either didn't notice or didn't want to know that a lobe on each of the cams was partially missing too! It was certainly like that when he rebuilt it as there was a shim the size of my doorstep in the corresponding tappet bucket....

The head bolts aren't too long, I checked this before fitting. The block has gone through a thorough process of:

Dip in de-grease/strip tank, powerwash
Dip in de-rust tank, powerwash
Core plugs removed
Skim block top
Rebore & hone
Clean out all threads with tap etc, etc

I wonder if the threads have been weakened by excessive copper grease, like some kind of hydraulic lock? If the bolts are beyond their elastic limit then would they pull the threads out of the block rather than snapping?

 

[desertfox]

hi Deltona

Normally the bolts should break before stripping the internal threads however you mention the block is CI which is a brittle material maybe you haven't enough thread engagement and that would cause internal thread stripping, what grade are the bolts are they the right grade for the head.
If you can post some figures like bolt size and grade,torque,thread engagement we might be able to assist you further.

desertfox

 

[evelrod]

It doesn't matter if the bolts ARE the correct length...it only matters that they are not bottoming out or "hydraulicing"...block threads CAN be damaged in such a condition as well as broken head bolts. "Copper grease" ? I have used CopperKote antiseize on occasion but not on head bolts. I can see a problem with that.

Interesting...I have had a myriad of problems with Fiat...broken timing belts/idlers, seized spark plugs and such. Never had problems with excessive cam lobe wear.

I don't normally use oil as a thread lube, too easy to get too much and end up with the "squishing out" you mention. I use a small smear of ARP moly lube on mine with the appropriate tq reduction recommended by ARP.

Rod

 

[Deltona]

Hi DF,

I'm afraid I don't know much about the spec of the bolts (beyond what I can see and measure of course). I just assumed they were standard, mainly because they look it and have the 'Ribe' fitting on the head.
I have ordered some standard bolts which will come tomorrow, at least then I will have another element which I know is correct.

Rod, when I said I knew the bolts were the correct length I meant that I have compared them to others and checked the thread hole depth in the block, there is plenty of end clearance.
The worn cam lobe problem is prevalant on the 16v engines, rare or never on the 8v heads. The 8v turbo engines are generally more robust than the later 16v ones.
I recently stripped an engine down with 5 of it's 8 lobes one one cam missing!

 

[desertfox]

Hi Deltona

How much of the threaded portion of the bolt is engaged in the head?
You need to have the bolt engaged completely in the female threaded portion, if the bolts are stopping short of the end of the tapped hole that would suggest why there failing.

desertfox

 

[Deltona]

Hi Desertfox,

Thanks for persisting, I fitted some new standard bolts today and they all held fine. They were the same length as the others.
It would be interesting to know why what has happened did, but without extensive testing which I have no funds or time for we can only guess.
With no real knowledge I am kind of thinking that they were old, had undergone too many cycles, become non stretch and lost their ductility......

Deltona.

 

[desertfox]

hi Deltona

What size of bolts are they? any markings on the head of the bolt? also can you tell me thread engagement and torque setting?
Now were the bolts failing or the internal threads? The reason I ask is that normally a well designed joint would have the bolts fail before the female threads,if the bolt and female threads are different materials the thread engagement is usually greater than one bolt diameter.
Materials of the same strength only need about one bolt diameter thread engagement.
Now if the bolts are tightened to 90% yield stress using torque figures its very likely they have yielded due to the error with torque settings when first assembled and so therefore they should be changed when the head is stripped down otherwise they can fail if reused.
If your female threads are failing it could be the bolt isn't filling the thread on assembly there could be a lot of other reasons for the female threads to fail be thats one of them which is why I askd about thread engagement.

desertfox

 

[desertfox]

Hi Deltona

Nice little article on cylinder head bolts:-

http://www.enginebuildermag.com/Art...sealing_starts_with_proper_head_bolt_use.aspx

desertfox

 

[Deltona]

Hi Desertfox,

I don't think there is much to be gleaned from any info I can get on these bolts but:

M10 x 1.25.

Length of thread in contact with block: 20mm.

No markings on them whatsoever.

Torque figure is difficult to define, but sequence is: 20Nm, then 40Nm, 90 degrees, 90 degrees. This gives an ultimate torque of about 65 - 70Nm

As per my original post, the threads in the block are failing.


Thanks for the excellent link, I shall digest it (and the rest of the site/forum) when I have some quality time, it's been a long week!

Deltona.

 

[LionelHutz]

It sounds like they were torque to yield bolts which are a 1-use bolt.

 

[desertfox]

Hi Deltona

I am not sure I follow your tightening sequence but is this what you mean? ie:- if you set the wrench to 20Nm and tighten up and then turn an additional 90 degrees, set the wrench to 40Nm tighten up and then another 90 degree turn.
I have looked at several sites for thread engagement and used your 60Nm as a figure for the tensile force calculations.
Page 23 of this link is the formula I based my calculations on:-

http://gltrs.grc.nasa.gov/reports/1990/RP-1228.pdf

Minimum grade of screw for 60Nm would be a grade 10.9

http://www.roymech.co.uk/Useful_Tables/Screws/Screw_loads.htm

I used a friction factor of 0.1 and used the formula:-

tensile force = torque/(0.1* bolt dia)

that gave a force of 60000N in the screw

using this in the formula from the NASA link for thread engagement and transposing to find engagement length I get a thread engagement of 41mm.
No need for alarm just yet as I have made estimates for the

Cast Iron strength of 150N/mm^2

Friction Factor = 0.1

In addition there is a huge error on correlating torque values with tensile load but it gives an idea of how to avoid the thread in the tapped hole stripping.


desertfox

 

[Deltona]

Hi DF,

The torque sequence is just as I wrote it: 20Nm, then 40Nm, 90 degrees, 90 degrees.

Thanks,
DT

 

[desertfox]

hi Deltona

Well thanks for that, what I don't understand is how you can add the torque's to get 60Nm also what does the 90 degrees mean.
If you set the torque wrench to 40Nm you can't get more than 40Nm thats what I am struggling with.

regards

desertfox

 

[Deltona]

Ah, I see. It seems the extra two 90' turns give (practically speaking and not theoretical) an approximate increase of 25 - 30Nm, let me explain:

Set the tool to 20Nm, tighten bolt. Set to 40Nm, tighten bolt. Set tool to 90' (or use a 90' angle indicator, set square etc) and turn bolt through 90'. Repeat. Having now given the bolt a complete 1/2 turn (or 180') in two stages the job is done. This is as per factory manual.

I arrived at the 25 - 30Nm figure by putting a spare bolt on a spare head and block, tightening to the above sequence and then trying to move it again, it took around 65 - 70Nm to move the bolt further.

Hope this is clearer!

 

[desertfox]

Hi Deltona

Okay thanks that sequence is what I was thinking in previous posts thanks for the clarification just out of interest whats the length of the bolt?

desertfox

 

[Deltona]

Hi Desertfox,

Sorry for slow reply, Christmas and all that got in the way.....

Total bolt length to underside of (bolt) head is 173mm.


On a similar note:

I don't think a torque plate is totally necessary when boring and honing these cast iron 4 cyl blocks as they are very rigid, but I like the machinist to use one. I have one which is circa 15mm thick (have to check to be precise) and have some short HT bolts to bolt it down with, is it possible to calculate what torque setting I should use for these to make them equal to the eventual much longer ones?
Maybe difficult as we don't know the strength of the original bolts.....

Thanks again.

Deltona.