PDA

View Full Version : Dr. A Tech Alert- injector bolt torque


abittenbinder
02-26-2013, 05:24 AM
Here at the Global Sprinter Research Center I am always eager to investigate Sprinter related technical issues.

Group members have noticed that Mercedes Benz has a published torque spec for the injector hold down bolts that seemingly differs significantly from the long standing DC published specs as shown in DC workshop manuals as well as on the instruction sheets that DC at one time included with replacement injectors.

The extended threaded shank length, 85.83mm long, 6.0mm dia., 8.8 grade, factory hold down bolt, hold down pawl and injector seal ring are identical part numbers for both the 5 cyl. and 6 cyl. engines.

Using my 'test' 647 Sprinter cylinder head, which is permanently mounted to one of my work benches, I have recently performed tests related to hold down bolt torque.

This dedicated 'test' cyl head has been quite useful in my repair tooling fabrication.

As many of you know I have developed in-house tools and fixtures for removal of broken hold down bolts, repair to stronger than new stripped hold down threads, and various custom black death repair tools for my in-house use.

This test head features my custom carbon steel hold down threads making it ideal for these hold down bolt torque experiments because data is not compromised by any aluminum thread deformation or failure.

After careful measuring of bolt length (before and after torquing) and injector seat seal thickness to 0.01mm tolerance, and using a calibrated Snap-on electronic 1/4 inch drive torque wrench set to display in/lbs and accurate to 0.1 in/lbs, I have the following observations to report:

TRIAL 1- A fresh, factory hold down bolt torqued to 62 in/lbs (approx 7 Nm) and then an additional 90 degrees, results in 0.08mm crush of a fresh factory seal ring.

The Sprinter's copper seat seal ring features a double convex cross section and the clamping force induced 'crush' creates narrow sealing flats on each side of the ring.

Monitoring the bolt torque during the 90 degree rotation reveals a peak of 180-190 in/lbs before full 90 degrees is achieved and remains at this level all the way to 90 degrees. This peak/plateau signals bolt yield has occurred.

TRIAL 2- A new seal ring and a fresh, factory hold down bolt torqued to 62 in/lbs. (approx. 7Nm) and then an additional 90 degrees X2 (FULL 180 degrees), results in the same 0.08mm crush of the seal ring as well as a steady 180-190 in/lbs torque reading during angle tightening.

Being a stretch to yield, non-reuse, bolt it was not surprising to see permanent elongation. Elongation was approx. 0.30mm for each increment of 90 degrees of tightening rotation (after the 62 in/lb initial torque).

TRIAL 3-A fresh hold down bolt tightened to failure. The bolt tolerated several additional 90 degree sequences PAST the initial 62 in/lbs and 2x90 degrees.

It has previously been reported that fresh hold down bolts have failed when several group members had torqued to 62 in/lbs and then 180 degrees (mistaking 1/2 turn for 90 degrees).

I now suspect this occurred because of bolt bottoming in the base of the blind bore. Bottoming can occur because of debris at bottom of the blind hole.

CONCLUSION:

Tightening the fresh hold down bolt and seal ring will produce the same  clamping force (defined by seal ring crush thickness) regardless of which of the 2 torque specs are used.

The desired residual bolt stress (to achieve essentially infinite cyclical fatigue life) is achieved by both specs but the 2X 90 spec does allow for less care and precision during the tightening procedure.

Torque spec #1 (62in/lbs +90) is certainly less risky if contamination may be lurking at the bottom of the very deep blind bolt hole. I suspect it is also somewhat less risky if the aluminum threads are not in 'as new' condition.

Be sure you test your cyl head's bolt hole threads by using a wire brushed  used hold down bolt with an indexing paint mark, turning in by hand while counting turns, to assure threads are clean and bore is unobstructed to full depth.

This is especially critical when performing black death surgery.

Doktor A   

owner
02-26-2013, 06:29 AM
Would you expect the copper washer to crush a bit more after it has been subjected to actual use - vibration and heat cycling etc? Do you have any there that you could measure?

I guess the key point is that you must always carefully clean the bolt hole. Then clean it again. Which is what I have always done on my 7Nm+90+90 OM612 ML270CDI.

Oilburner
02-26-2013, 01:08 PM
I was thinking, if is possible to make similar test with glow plugs. Using two common type of glow plug, Beru and Bosch, simulate removing and installing to find out, at what torque glow plug is prone to break.

abittenbinder
02-28-2013, 05:30 PM
Be sure you test your cyl head's bolt hole threads by using a wire brushed  used hold down bolt with an indexing paint mark, turning in by hand while counting turns, to assure threads are clean and bore is unobstructed to full depth.

This is especially critical when performing black death surgery.

Doktor A   

After several people contacted me for more details, I see I should have been more specific regarding my bolt hole thread advice as shown above.

I recommended using a used wire brushed bolt because you should not disturb the factory applied dab of yellow upper thread sealing wax (located on every new bolt) until final assembly.

No, that yellow material is not an encapsulated thread locking compound, it is a sealing wax cleverly designed to prevent bolt seizure resulting from moisture or other contamination entering the threads in the bore.

A test hold down bolt will hand turn approx. 19 full turns into a properly cleaned bore without the injector hold down pawl in place.

Doktor A

abittenbinder
03-05-2013, 07:55 PM
I was thinking, if is possible to make similar test with glow plugs. Using two common type of glow plug, Beru and Bosch, simulate removing and installing to find out, at what torque glow plug is prone to break.

Glow plugs are a unique case.

The hex is part of a thin outer hollow shell which is often corroded at the site of failure.

Failure can occur when the glow plug's threads are seized in the head and high torsional force is applied in an attempt to unscrew the plug.

It may not be useful to determine torque to failure of a new glow plug's shell, because when a technician with proper removal tools NEEDS to remove a glow plug, his 'best practices' choice is to heat engine to operating temp, while perhaps using his favorite penetrant or thermal shock spray, wait for a short hot soak to achieve maximum head temp and then apply torque until the plug either moves or breaks.

The seized glow plug removal tools actually require the glow plug's outer shell to be sheared before the tools can be utilized.

The point I'm trying to make here, glow plug breakage is not the catastrophe of a sheared injector hold down bolt (which requires custom tooling and special skill to repair), IF YOU HAVE THE PROPER GLOW PLUG TOOLS.

Most broken injector bolts fail during installation when the cylinder head's injector hold down threads and bore have been improperly prepared. Preparation is quite difficult in that narrow and deep mine shaft.

Doktor A

surlyoldbill
03-05-2013, 09:24 PM
Case in point to support Dr A on injector hold down bolts:
During replacement of my #1 injector, I was installing the hold down bolt and it sheared before even contacting the pawl. Likely I did not clean the threads very well, my tap did not reach the entire depth of the well.
Clean the well well. You can use an old bolt with vertical slots cut on the sides as a thread chaser to reach all the way to the bottom.

(I was able to drill out the sheared section, and install a helicoil. Using a 1/4" spacer on top of the pawl because I could not reach the full depth of the well with tap for helicoil. Holding very well for the last 20k)

Turbo John
03-06-2013, 01:38 PM
Is there a recommended injector ( thread or) maintenance schedule. I have no black death with an engine of 75k and all looks normal.

Oilburner
03-06-2013, 01:46 PM
Is there a recommended injector ( thread or) maintenance schedule. I have no black death with an engine of 75k and all looks normal.

I would enjoy the ride without disturbing it.
IMHO.

abittenbinder
03-07-2013, 04:02 PM
Is there a recommended injector ( thread or) maintenance schedule. I have no black death with an engine of 75k and all looks normal.

For my Million Mile Club fleet clients, I have performed scheduled injector removal and copper seat seal replacement at regular 180k intervals.

Keep in mind that these are deep pocketed businesses that cannot tolerate unscheduled downtime and accumulate lots of miles very quickly. I do not necessarily recommend this for the average Sprinter owner.

Doktor A

Turbo John
03-07-2013, 04:21 PM
For my Million Mile Club fleet clients, I have performed scheduled injector removal and copper seat seal replacement at regular 180k intervals.

Keep in mind that these are deep pocketed businesses that cannot tolerate unscheduled downtime and accumulate lots of miles very quickly. I do not necessarily recommend this for the average Sprinter owner.

Doktor A

I cant tolerate having a situation like that either. Its nice to know that you do this every 180k to head off future problems. Million miles is quite a feat and the only way that will happen is by more than routine maintenance.
Thanks for letting me know this..

dad2ts
11-02-2016, 08:17 PM
I'm not sure if anyone will see this, nevertheless this seems like a good place to ask.

I believe I have read that Antiseize is not recommended but..

I really want to put Ni Anti-seize on the injector hold down bolt and injector body.

I'm not sure that the it will do any good on the injector and may actually cause corrosion of the aluminum since there will be a galvanic potential. I'm not sure the anti-seize will do anything at all to help with the buildup of carbon around the injector due to blow by. So I my not be doing any good and actually cause a problem. But then again, I could be wrong on both counts.

I prepared for reassembly by making sure that the hold down bolt will easily thread into the hole with the head dropping beyond the top of the injector pawl. I did this for each hole prior to reassembly. Thanks to the forum for the heads up.

My thinking on the hold down bolts is that this design is simply a lever that pushes down on the injector to crush a copper seal. Smart engineers have figured out the yield strength of the bolt and matched it to the material properties of the copper washer to produce the desired crush/seal across the surface area of the seal face. In the process of crushing the seal and the engine thermal cycles the copper work hardens. Because the bolt is tensioned below its yield point the residual strain acts like a spring and maintains sealing pressure.

So, with this understanding, I want to lubricate the threads, so that the elongation ( 1 mm per turn) actually occurs. Because if the bolt binds somewhere along the thread length that 62 in.lbs plus 90 degrees only results in a torsional deformation. And of course that doesn't sound good to me. Since there is only vibration to cause the bolt to loosen and the oils get quickly burned off. Why doesn't this line of reasoning make sense?

Gaspiping Sprinter
11-02-2016, 08:48 PM
I just finished changing the copper washers and tie down bolts and I put copper anti seize on both the injectors and the bolts. The only way the anti sieze will blow by around the injector is when the copper washer fails ( Less likely to happen with new washer ,new bolt and cleaned out bore and tie down hole) and black crude pushes it out of the way. As a matter of fact I tried putting one old copper washer to see what happens to the anti seize material I applied to the body of the injector and it was blowing out in no time .
I followed Vic's instructions in his Write up thread , the tie down hole must be cleaned beyond good enough or else you risk breaking the bolt .
When all was set and done I used dishwashing soap mixed with warm water in a spray bottle , got the truck running and sprayed that mix around the injectors and saw no bubbles indication of no leaks found .

dad2ts
11-03-2016, 01:37 PM
Thanks for the confirmation.

I like the idea of a bubble test for the blow-by.

I used an old hold down bolt and fashioned a tap out of it with a dremel. With the injectors and pawls out. I cleaned each hole so that the bolt would go finger tight to a depth below where it would hit the hold down pawl. I was so mad that I bottomed out, broke, the valve cover breather bolts and had to buy two covers, that I am quite paranoid about the cost of ignorance.

I think this forum is half the reason we enjoy working on the sprinter so much.

Gaspiping Sprinter
11-03-2016, 09:12 PM
Sorry to hear about your trouble and hopefully everything will work out well for you at the end .

dad2ts
11-06-2016, 05:25 AM
BTW, I used anti seize on the injector and hold down bolts. They turned smoothly and I could swear that you could almost feel the copper washer set during the 90 degree beyond 62 in-lb turn. No bubbles found in the leak test. Drove around all day, ran beautiful. I have confidence in the injector replacement etc.

Thanks all

michaelyares
11-14-2016, 12:44 AM
TRIAL 3-A fresh hold down bolt tightened to failure. The bolt tolerated several additional 90 degree sequences PAST the initial 62 in/lbs and 2x90 degrees.

Any idea at what torque the bolt snaps? Is the 2x90 preferred to provide a stronger and longer lasting clamp force?

Also, do you know of a domestic retailer of the Febi ceramic grease? (Or is there an equivalent anti sieze at our local auto parts store?)

Thanks for the great info.