I hope you gentlemen could enlighten me regarding an issue with my DOHC Pajero 3.5 V6. I have searched the archives a bit, but found nothing.

A couple of days ago my Pajero lost power assistance for the steering while driving and the charge light came on. As I was very close to home I limped the short distance home down the road. At home an inspection revealed all auxiliary drive belts to be tight and in place (there are three belts: air con compressor driven off crank pulley, alternator/water pump driven off crank pulley and power steering driven off water pump pulley)

What has happened is that the crank pulley, which is located onto the front of the crank sprocket (the one that drives the timing belt) by a pin and clamped by the crank bolt, has "spun" relative to the crank sprocket. The crank bolt was still tight and the crank sprocket was still securely keyed to the crankshaft, but both the crank sprocket and crank pulley was damaged beyond repair in the process.

How is this joint (crank pulley onto crank sprocket) supposed to work? Is the auxiliary systems drive torque supposed to be transferred by friction generated by the crank bolt sandwiching the whole shooting match together (pin only a locating device) or does the pin actually transfer the torque (it seems puny)?

What do you guys think was the cause of the problem and how can I make sure that it does not happen again after the repair?

I hope my explanation is clear enough. If not I'll try again.

Gerrit

It sounds as if the crank sprocket kept turning while the crank pulley was held stationary?? The ruined sprocket and pulley would have been where the two inside faces were turning on each other?? From memory of my timing belt replacement, and consulting my FSM (not very detailed at all), it appears the crank sprocket sits against a rear flange on the crank shaft and is indeed sandwiched between this flange, an outer flange and the crank pulley. The torque transfer is from the key on the shaft to the pin and the torqued friction of the crank bolt. There is no taper on the crank shaft. I don't think there is very much torque to worry about on the crank pulley (the pin being location and some torque transfer) and I don't know how it could have seized, other than one of the accessory items seizing. Can you turn all the driven accessories? Could the A/C compressor have seized with the clutch engaged? I would think if the water pump seized it would have made a terrible noise first?? Maybe someone else here can offer more info.

I don't know how the 3.5L pulley is constructed, but if it's like the 3.0L, there's a key in the crank nose that receives the sprocket and the pulley (vibration damper). As you said, I think the pulley on the 3.5L is located with a pin. Some torque is transferred through the pin and some through the bolt-provided friction - whether there's also a keyway in the pulley, I don't know. In your case, what sounds to have happened is the rubber between the pulley hub and the outer ring has spun - but I don't see how that would mess up the crank or the sprocket.

I would think that if the sprocket had spun on the crank, you would have trashed all of the valves, since the 3.5L is an interference engine.

It's possible that the bolt loosened enough for the pulley to back out just far enough for the pin come out of the locating hole and spin, leaving the bolt fairly tight - or it's possible that the pin sheared, allowing the pulley to spin against bolt friction.

In any event, you'll need to take it down to find out what happened.

Frank

The only keyway is for the sprocket not for the pulley. The locating device on the pulley is a roll pin of sorts.

Hi Guys,

Yep it all happened just like pcc said. The crank sprocket turned behind the crank pulley. All the driven accesories are free, so my take on it is that the root cause must be the crank bolt. Threads on this bolt seem to be somewhat damaged (overtightened maybe?). My theory is that the bolt was overtightened, thus reducing the clamping force just sufficiently that the crank sprocket could shear the roll pin off and spin relative to the crank pulley. Comments?

I have obtained the spares from the local agents (hugely expensive down here in South Africa, I might add), but am concerned about getting the correct crank bolt part number, especially after reading this
tale of woe. Could anyone advise the correct crank center bolt part number for the 3.5 DOHC (I understand that there was a recall on the crank bolt for the 3.0, but not the 3.5 DOHC. the part numbers for the 3.0 seem to be MD172725 or MD138069)

The recall on the 3.0 also mentions oiling the threads and clamping contact area of the bolt prior to tightening. Should I do this? I am afraid of overtightening the new bolt as well.

Thanks for all your trouble.

Regards

Gerrit

I would suspect that the bolt was not torqued enough rather than too much. Both the bolt and the crankshaft end will be hardened steel. If the bolt was over tightened you'd have to go a long way to stripping the threads. It could have been cross threaded, which would have given a false torque reading, but the interference threading should have prevented the bolt from backing out. If you thought the bolt was tight it may have been just cross threaded. If the threads are damaged then you may have a major problem. The bolt is easily replaced but if the threads on the crankshaft are damaged, <img src="/forums/images/graemlins/scared.gif" alt="" /> !!! I would inspect the threads in the crank very carefully. You could even run a tap to make sure they are clean and to somewhat repair any damage. I don't know if the crank bolt recall was an issue with your vehicle, you may want to check with the dealer. You may also want to use this opportunity to do a timing belt change and crankshaft seal. Best of luck.

Hello again pcc,

I hear what you are saying about the amount of effort required to overtighten. The three or four revolutions of thread closest to the joint plane (the ones that would have taken most of the clamping force) on the old crank bolt definitely are damaged, though, and it was difficult to get the bolt out.

A thread repair specialist has looked at the threads in the crank stub and pronounced them fine. He did run a cleaning tap down the hole for good measure.

Regards

Gerrit

If you spun a crank sprocket on a 3.5L DOHC, you should have bigger problems than a crank bolt - if your 3.5Ls are the same as those in the States. As I understand the 3.5L DOHC, if you turn the crank without synchronous rotation of the valvetrain, you'll bend the valves.

Frank

It does not appear from the explanation that the crank sprocket slipped while the crank kept turning. I would do a compression test just to make sure. It is strange though that if the crank bolt loosened off enough to cause the pulley pin to break and the pulley to stop turning, that some damage would not have occurred on the sprocket key and cause that to slip as well. If this is the case I think pampaskat is very lucky or prays to the right god. I guess the pin is in fact "puny" and not up to much torque if the bolt is not torqued properly.

I don't think my automotive gods would be so kind as to prevent valve damage if that happened. <img src="/forums/images/graemlins/shiner.gif" alt="" />

Frank