Big end bolts

[Burfcontrol]

First a question, then a bit of history as to why I'm in this position.

Has anyone reused big end bolts?
Did you have any failure as a result? How many miles have you done since reusing them?
I know it's a big no no. I understand they're stretch bolts and should only be used once, but I've been given the wrong ones from my dealer (I won't name because they've been very good over the years and I haven't had a chance to let them explain).
So is the risk that they'll shear or work loose? I've used loctite on them so if it's the work loose issue will I be OK?
I'm sure I'll end up going back in and replacing them, it's just the thought of another week in the car and tearing the bike down again having just spent a week with it in bits is hard to take right now.

The story so far....
Had an inlet valve/piston interface incident - left side, happened cruising at steady 65/70 mph on motorway. Got lucky, only bruised the piston and no damage to valve. Bike was still running and ticking over, very loud tappetty noise, oil pressure and temp OK.
Checked compression, good and same on both cylinders, so no sticking valves.
Calling various dealers had different theories as to cause. One suggested worn big end bearings (bikes done 100,000 miles, 1150 single spark, 2000) another debunked this theory saying they'd had bikes with completely worn big ends and no piston contact, which made sense as normal clearance would have to be very tight for this to have happened.
Called the oracle - St Eptoe - who as allways gave as good advice as he could without seeing or hearing the bike, suggested valve timing and common cause of this would be over reving. (Sorry if I didn't get this right Neil, it was last week and my memories not as good as it used to be ) I couldn't see how this would relate to steady cruising speed but it would explain the valve contact and steered me in the right direction.
Went to see a local engineer whose been working with Boxer cup team, we stripped the head and valve gear at his shop, no sign of any wear or pick up on any moving parts. Again theorising valve timing but couldn't figure out how it could advance at steady speed, would expect it to retard and usually jump a tooth at start up.
Put it back together without pistons and conrods and checked valve timing - lo and behold the left side was advanced about 30' - the right was spot on.
So retarded the timing by one tooth on the sprocket and rechecked -
Reassembled everything, fitted new big ends and rings at same time as I was that far into the engine.
Fired up, gently warmed up on centre stand, ran OK but still had louder than normal tappetty noise from left side - nowhere near as bad as original problem but still louder than should be.
Conclusion - rang the Boxer cup engineer who had asked me to let him know what I found. We now believe the left side cam chain tensioner is 'lazy' (his words) causing the tappetty noise and potentially allowing chain to jump ahead one tooth advancing timing and causing inlet valve to open around 30' before TDC.
I'll get a new one ordered tomorrow and see what happens when I fit it.

Cheers Jim

 

[cookie]

Has anyone reused big end bolts?

i have several times. no failures, but never in a bmw, and never stretch bolts.

 

[davegs]

Bolts

Dont even think about it these are very highly stressed components and if one let go would wreck the engine. Even if it was less damage what a job to strip it all down again just for the sake of a few car days.
Dave gs.

 

[ciderman gs]

also check you get the new style camchain tensioner. get new bolts if you have spent this much time sorting just do it once.

as somebody has said on here "there is never enough time to do a job once, but there is always enough time to do it twice"

 

[alimey4u2]

When building a performance engine, properly tightened rod bolts are essential. Rod bolt stretch measurement offers the assurance that all bolts are tight and will stay that way.

If you're building a high-performance or race engine and plan to use aftermarket high-performance connecting rod bolts, you may want to consider a tightening process that all major race engine builders use. Instead of tightening the rod bolts by achieving a torque value, the optimum approach for precision-tightening connecting rod bolts in these applications involves measuring bolt stretch during tightening. Aftermarket connecting rod bolts may vary in terms of their ideal torque by as much as 10 ft.lbs. from batch to batch due to variations in heat treating and materials. So if the concern is to arrive at bodi peak bolt strength as well as big-end roundness, the rod bolts should be measured for stretch. Even when using the best rod bolts available, measuring bolt stretch offers a much more accurate method of achieving ideal clamping loads as opposed to the use of a torque wrench.

Granted, when dealing with production engines that utilize torque-plus-angle specifications (for example, 20 ft.-lbs. followed by a 90° additional turn), you'll need to follow the OEM's procedure. Tightening connecting rod bolts using the stretch-monitoring method really applies only to performance engines that feature aftermarket high-performance rod bolts, where the bolt manufacturer has provided a stretch value reference.

Re-used connecting rod bolts in every engine I have built without failure, maybe BMW boxer engines are "special"..They all stretch to some degree Sir....

 

[pomm001]

Re-used connecting rod bolts in every engine I have built without failure, maybe BMW boxer engines are "special"..They all stretch to some degree Sir....

So called strech bolts are used in almost all engines for 'critical' joints it allows the engine design to use smaller bolts to achieve higher clamp loads
each time the bolt is tighten, it strectches slightly, ie the unengaged portion of the bolt will be longer after tightening than it was before, the material can only be taken through this process a couple of times , after that they will not exert the same clamp load into the joint and it may well fail.
( this is determined by experiment )
what you dont know is how many times the bolts have been used ! the bolts would have been tightened, once when the con rod was made, again when the engine was built, and maybe once more if there was any stripping during the engine build at BMW.
So you maybe using them for the third time ! so the odds are getting poor

So it you like to gamble that you big end bolts wont fail and sh*g your engine then dont buy new bolts

I think I would !

 

[alimey4u2]

Many thanks for that pomm but if they are "stretching" (not returning like a spring) doesn't that mean they are going past their elastic limit ?? Once past the elastic limit & on to plastic deformation, failure is inevitable. Help me out here, I'm sure I'm missing something...

 

[alimey4u2]

Googled it....

The replacement bolts your dealer supplied are referred to as s-t-r-e-t-c-h bolts, because they are PERMANENTLY stretched when properly installed. When stretched they actually "neck down" or have a reduced diameter and area where the metal has been stretched. This reduced area has experienced Plastic Deformation, and will not hold the same torque (tension) if applied again, it will in fact stretch more, (loosing tension) and often fracture. This is why stretch bolts must ALWAYS be replaced, and never reused.

Stretch bolts are used by car manufacturers for several reasons, usually none of what is reported in the hotrod magazines... First with stretch bolts automated engine assembly is much easier as the tightening of the bolt can vary by as much as 1/2 turn in most cases and provide the same clamp force. This is due to the fact that when a fastener is tighten into plastic deformation, it will not change it's effective tensile strength, (and clamp force) by very much over a given amount of elongation, (stretch). So when the pneumatic machine tightens the bolts in your engine, it can vary up to 1/2 turn and still have the same clamp force... at least in theory. Having watched the engines being assembled using this method, and viewing real-time direct readouts of clamp force verses degree rotation of the bolt, I can assure you reality and theory are not in 100% agreement!

 

[Burfcontrol]

Thanks for the replies - I guess I knew all along what the right thing to do was. I've got the new bolts and blanked off the weekend to fit them, ho hum.

I also got the new chain tensioner, thanks for the advice.

 

[pomm001]

Googled it....

Err, sort of