Stretch con rod bolts.

[mattcfish]

Back to my original, um, topic. There seems to be some differences in thought between VW and Airhead tuners. VW tuners say stretch bolts bad (and I've proven it so), bushed rocker arms better than roller bearing type. Airhead tuners think the opposite.
When torquing con rod bolts on airheads, how should torque be applied to avoid over stressing the bolts. What torque values and steps should be used?

[vanzen]

What, No BMW Repair Manual ?

Big end bolts torque is stated plainly in the specs –
And that spec changes with the '81 models.
... may be that other variations exist throughout the years.

/7 through '79 spec = 48-52 Nm, 35.4-38.4 ftlb.

'81 spec = 50+2 Nm, 37=1.5 ftlb.

There will be no problem with a "stretch" screw, provided that screw
1.) is properly engineered for the application
2.) is used (and not re-used) as intended !

[mattcfish]

What, No BMW Repair Manual ?

Yes, but I don't trust everything in the manuals. I also remember reading a different procedure for torqueing the bolts on a 1070 kit. I can't find the German literature right now, but there were specific steps in torqueing. I seem to remember that you torque and untorqued then torqued more.

Big end bolts torque is stated plainly in the specs –
And that spec changes with the '81 models.
... may be that other variations exist throughout the years.

A /6 with aftermarket performance pistons may follow a different rule.

[Garnet]

Modern cars use "torque to yield" headbolts. They are touqued to a specific ft/lb spec and then turned another specified amount, such as one third or one half turn. (Given in degrees).

Perhaps the 1070 kit uses bolts like that, and that is why there are special instructions for it.

I would stick with stock new rodbolts and follow the instructions for them.

[Dave Backmarker]

Matt,
I feel your pain. I am a mechanical engineer and I've lived through my son owning a 1998 Jetta that has many, many very expensive stretch bolts in its VR6 engine. The best answer I have on why use stretch bolts is that they provide a very uniform torque without the use of a torque wrench.

As GSPD and others have noted here and I'm certain you know, tightening stretch bolts with oil or dry makes a huge difference. Also, the steps to get to the final torque must be adhered to precisely and should be done during one session. As any bolt stretches when torqued, if you let a stretch bolt "relax" over night between steps, you'll have a bolt that is tighter than you intended.

BTW, in the five engine swaps we did on my son's VR6 Jetta, we never once had a failed head or con rod bolt. Maybe that's because something else broke and we never put a huge number of miles on the engine! Seriously, the last rebuild was the magic and the car is running very well today. I guess our skills improved over time.

Best of luck with the Vanagon.

Dave
1975 R90S
2000 BMWR1100Rt

[vanzen]

Yes, but I don't trust everything in the manuals. I also remember reading a different procedure for torqueing the bolts on a 1070 kit. I can't find the German literature right now, but there were specific steps in torqueing. I seem to remember that you torque and untorqued then torqued more.

You will not trust the BMW manual's specifications for assembling a stock component engine,
but would trust information written here that might supersede it ?
BMW made a 1070 kit ?

GOOD LUCK, mattcfish !

[mattcfish]

Yes, but I don't trust everything in the manuals. I also remember reading a different procedure for torqueing the bolts on a 1070 kit. I can't find the German literature right now, but there were specific steps in torqueing. I seem to remember that you torque and untorqued then torqued more.

You will not trust the BMW manual's specifications for assembling a stock component engine,
but would trust information written here that might supersede it ?
BMW made a 1070 kit ?

GOOD LUCK, mattcfish !

Ureeka! Here's the procedure I was looking for for installing stretch bolts on a Siebenrock 1070cc kit.

The following instructions apply to both sides of the engine. Install on one side first, when finished -
continue on the other side. Follow BMW moments / torque / measures and all BMW installation
requirements. Use only the enclosed SPECIAL 12-cornered NUT in size 3/8 inch for installing the
ARP bolts into the connecting rods.
The attached ARP bolts for the connecting rods have to be "elongated" . This can be achieved by a
special device to 0.0054 to 0.0058 inches or via "multiple snugguing down" and "multiple
loosening" them as described below. The thread and the surface under the bolts head are greased
"from the factory" with molybdenum-fat. Molybdenum-fat is used because the friction while
fastening is 20-30% smaller than with normal engine oil.
2.2 Put new bearings in first connecting rod, and insert one conrod in engine according to BMW
installation instruction. Please note the correct mounting location of our asymmetrical
conrods. Pay attention to the clearance of the conrods+bolts. If you install the
connection rod the WRONG way, it touches the crankhaft´s balance weights when you
turn the crankshaft by hand half a revolution. This is obvious.
2.3 Tighten the conrod-bolts with 36Nm.
2.4 loosen both conrod-bolts a little.
2.5 Tighten again with 36Nm.
2.6 loosen again.
2.7 Tighten a third and final time with 36Nm. Now the connecting rods bolts are elongated and
properly assembled.

S

[mattcfish]

2.3 Tighten the conrod-bolts with 36Nm.
2.4 loosen both conrod-bolts a little.
2.5 Tighten again with 36Nm.
2.6 loosen again.
2.7 Tighten a third and final time with 36Nm. Now the connecting rods bolts are elongated and
properly assembled.

S[/quote]
Seems pretty complicated compared to just torquing and forgetting. Wouldn't torquing 3 times be the same as reusing a bolt?. I don't want to loose my new valuable top end because of snapped rod bolts.
I've already been down that path. I have the full PDF but the forum won't let me attach it.

[Garnet]

That method is for the ARP bolts that come with the 1070 kit. There is a reason for it, I can't tell you why.

Your top end is much closer to stock, in fact your new pistons may even be lighter than your old 900 ones. So stock rod bolts installed to BMW specs will work fine. Again BMW rod bolts have a good track record in stock and even modestly modified engines.

[mattcfish]

That method is for the ARP bolts that come with the 1070 kit. There is a reason for it, I can't tell you why.

Your top end is much closer to stock, in fact your new pistons may even be lighter than your old 900 ones. So stock rod bolts installed to BMW specs will work fine. Again BMW rod bolts have a good track record in stock and even modestly modified engines.

I see where we're all confused. I thought the stock bolts were ARP. Aren't they? Thats why we replace them everytime we change the rod bearings. I believe the new set I ordered from MI are ARP, better check again. Yes my new pistons are lighter, but they're going to be used much harder.
After what happened to the family truckster, I'm just a tad jumpy.