# Main cam/crankbelt failure!!



## RaceBreedImport (Mar 28, 2009)

Hi

I had a mishap yesterday with my GTR32.. I run a OS Giken 3 liter engine with Tomei top end and about 700PS.
The engine have worked without any problems at all for a year, but yesterday the luck was over.. The car was standing still on idle, and the engine just stopped. My first thought was to try and start it again so I turned the key and heard the engine was not turning over right. Went out and opened the bonnet, and to my surprise saw the main belt was torn right off.. Mind you this is a cevlar belt.. When I got home I took the camcovers of, and exspected to see full clerance under each valve, to tell me that I have 24 bent ones in there. But as far as I can tell with the tools I have the clerance is not over 0.48 anywhere. I have turned the cams and checked clerance on all of them, and they are all the same, exept for the +-0.03 thats allowed..

My questions:
What can trigger this belt to mailfunction in this way?

Have any of you experienced something simmilar?

Can really the cams stop in sutch a manner that no valves have been bent?

Thanks

KE


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## NISFAN (Oct 11, 2003)

Some interference engines can survive a belt failure at idle. 

Hope for your sake that yours has too, although I would still strip down the head and double check each valve for peace of mind.

As for the belt failure, did it strip the teeth off it, or has it snapped? 

How old was the belt? could it have been overtightened, or too slack? etc etc.


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## chovden (Oct 16, 2003)

The same happened to me a few years ago in a s13 I had, had been driving around then came to a stop, it ideled for a minute or two, then just died.
Tried to start but it went araound way to easy so checked the cam belt and it had snapped.
Called around to a few experts and they al said you need 16 new valves..
Took out the plugs and used a boroscobe camera from work that can turn almost 180 degrees and checked everything, no marks on the pistons from valves hitting valves al went as thay sould.
So ended up replacing the belt and used the car as normal no problems.
So I guess you can be lucky.. Never found the reason the belt was just ripped straight of.


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## RaceBreedImport (Mar 28, 2009)

NISFAN said:


> Some interference engines can survive a belt failure at idle.
> 
> Hope for your sake that yours has too, although I would still strip down the head and double check each valve for peace of mind.
> 
> ...


I have borrowed a few tings to dubblecheck everything today. Im not too keep on taking the top end of really, so Im going to go in thru the plugholes today and take a look. 

The belt was changed this winter, and Im not sure if it was to tight or slack.
There are about 4 teeth missing from the belt? Do that tell anything?


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## RaceBreedImport (Mar 28, 2009)

chovden said:


> The same happened to me a few years ago in a s13 I had, had been driving around then came to a stop, it ideled for a minute or two, then just died.
> Tried to start but it went araound way to easy so checked the cam belt and it had snapped.
> Called around to a few experts and they al said you need 16 new valves..
> Took out the plugs and used a boroscobe camera from work that can turn almost 180 degrees and checked everything, no marks on the pistons from valves hitting valves al went as thay sould.
> ...


Ok, thanks!

I have borrowed a boroscope today

I hope I can just put the new belt on and start ofcourse


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## NISFAN (Oct 11, 2003)

OK, so was a new belt. All the more reason to try investigate why it failed. 

Perhaps check the idler pullieys, they can sometimes sieze. As you said you have turned the camshafts, I guess neither of them have siezed?

The thing that would worry me is if you still have the steel valve guides, a sideways force can crack them. If pieces of those go through your engine/turbo, they will be destroyed. 
And you really need to rotate the valves to see whether they are bent or not. I don't think a visual inspection via boroscope will tell you if a valve is only slightly bent (???).

As I said considering the spec of your engine, I would go the cautious route and take the head off.

Hope it was just a random belt failure, and all is ok with your engine.


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## Marlon88 (Sep 20, 2008)

You might got lucky and the valves didn`t hit. To check for bent valves put on a temporarily timing belt, take all the spark out and turn the engine by hand, if you feel resistance that means that the piston is hitting the bent valve. If it was me I would take the head off and inspect everything and not risk that 3.0 OS Giken bottom end....


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## David (Apr 25, 2003)

Marlon88 said:


> You might got lucky and the valves didn`t hit. To check for bent valves put on a temporarily timing belt, take all the spark out and turn the engine by hand, if you feel resistance that means that the piston is hitting the bent valve. If it was me I would take the head off and inspect everything and not risk that 3.0 OS Giken bottom end....


It is possible to have bent valves and the valves shut and still clear the pistons though, only in extreme conditions will a bent valve not close. The head i used for our RB30 build had 4 bent valves all closed fine and you could only tell by rotating the stems on a flat surface. But I suspect it would be enough to cause a poor seal, so I would recomend an individual cylinder leak down test.


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## RaceBreedImport (Mar 28, 2009)

David said:


> It is possible to have bent valves and the valves shut and still clear the pistons though, only in extreme conditions will a bent valve not close. The head i used for our RB30 build had 4 bent valves all closed fine and you could only tell by rotating the stems on a flat surface. But I suspect it would be enough to cause a poor seal, so I would recomend an individual cylinder leak down test.


I have just had a good long look at everything in a boroscope with a cam, and 180" rotation. There is no marks at all on either the valves or the pistons. I also rechecked the clearences with a digital gauge. In the manual it says you can have 0.38+- 0.03 mm on the intake side and 0.44 +- 0.03 on the exhaust.. I have 0.41 to 0.36 intake and 0.45 to 0.41 exhaust on all of them. If a valve was slightly bent could theese really be that spot on? I would think that if would at least be a few 0.05`s off??:nervous: I have a complete Tomei head by the way. Fully ported, dubble valvesprings,titanium retainers, and long bronze valveguides, 264 degree cams..

I think the next step will be to put on a new belt, and take compresion tests on all 6.. I do still worry about why it snapped in the first place dough, and will change the pullys as well.. I have even bought a spesial tool for the tightning of the belt today, so hope everything works out..


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## Marlon88 (Sep 20, 2008)

David said:


> It is possible to have bent valves and the valves shut and still clear the pistons though, only in extreme conditions will a bent valve not close. The head i used for our RB30 build had 4 bent valves all closed fine and you could only tell by rotating the stems on a flat surface. But I suspect it would be enough to cause a poor seal, so I would recomend an individual cylinder leak down test.


Yes it is possible to have a slightly bent valve and it will clear from hitting the piston. It will show as soon as you start the engine as it won`t idle right with leaking valve/s. I said to turn the engine by hand first is because if you turn the engine with the starter and you have bent valves you will make more and more damage, so it`s important that once you put your timing belt back always turn the engine by hand...


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## fabianGTR34 (Aug 6, 2006)

The belt tension is very important on RB engines.

Cam belt failure can be causes by several things:
Excessive wear can be caused by misalignment, sharp edges on the wheels, over tension.
A too sloppy belt can make noise and if it is too loose it will slip and you will bend valves. and in the worst case the valves lock up the cam and the belt will snap too.

Make sure you find the cause and not the consequence.. 

Since you are running a RB30, maybe some misalignment occurred? 

Good luck..


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## RaceBreedImport (Mar 28, 2009)

Marlon88 said:


> Yes it is possible to have a slightly bent valve and it will clear from hitting the piston. It will show as soon as you start the engine as it won`t idle right with leaking valve/s. I said to turn the engine by hand first is because if you turn the engine with the starter and you have bent valves you will make more and more damage, so it`s important that once you put your timing belt back always turn the engine by hand...


Yes, of course  to slip on a belt and start it will not bee on the list for the first things I will do 

I will make sure the belt is tightend the correct way, turn it by hand and the take a comp test of all sylinders before I even try to start it. I have talked to a lot of people about it today, and they all say that since the clerance between the retainers and the cams are the way they are, no valves can be bent.. you would see some more clerabce than is currently present 

Thanks for all advise dough gyes!!!


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## Marlon88 (Sep 20, 2008)

Like the other guys said, you have to see why it snapped. Have you inspected the belt to see any unusual wear? maybe something was rubbing against it? were you running the correct size longer belt for the OS Giken RB30 engine?


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## RaceBreedImport (Mar 28, 2009)

Marlon88 said:


> Like the other guys said, you have to see why it snapped. Have you inspected the belt to see any unusual wear? maybe something was rubbing against it? were you running the correct size longer belt for the OS Giken RB30 engine?


There is no marks that gives any clue to why i snapped.. My guess would be that it was probably on to tight, on idle you could sort of hear the belt "sing" a bit.. And yes the belt is correct, I am a OS Giken dealer, so all the parts was correct


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## freakazoid3 (Jan 19, 2008)

My belt sang a bit at idle aswell and when the engine was hot the belt was on way to tight.
Put the belt on a little loser and check the tension with your new tool when the engine is hot. As my kevlar belt doesn't stretch either and I loosened it up a fair bit so that the tension is right when the engine is at operating temperature  

Otherwise when she was hot the belt was about to snap due to the tension on it by parts expanding with heat and the kevlar not expanding at all :nervous:


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## rb30r34 (Jun 30, 2008)

General rule of thumb is when the belt is tensioned up you should be able to rotate it 45 degree either way with you thumb and finger.


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## rb26r32 (Sep 5, 2008)

good idea to get some new idler/tension bolts from nissan aswell


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## frostmotorsport (Aug 10, 2007)

rb30r34 said:


> General rule of thumb is when the belt is tensioned up you should be able to rotate it 45 degree either way with you thumb and finger.


yeah, but that's for a factory belt - I had to loosen my HKS belt off a ton - as it was ok cold but squeaked and squealed when hot - as Kevlar doesn't stretch like a standard belt does - and I had to do the same on my 4G63 too (also Kevlar, different brand)

I figure with these cambelts you have to go waaaay looser (when setting it cold) than seems safe

Dunno - there may be some tuner/builders on here that may be able to shed a bit more light on this.

Also, I replaced my tensioner bolt with a grade 12.9 cap-head bolt as the standard bolts are known to fail. If you are truly paranoid, you can remove the idler stud and replace that with a 12.9 bolt (or stud if you could find one) too.


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## RaceBreedImport (Mar 28, 2009)

frostmotorsport said:


> yeah, but that's for a factory belt - I had to loosen my HKS belt off a ton - as it was ok cold but squeaked and squealed when hot - as Kevlar doesn't stretch like a standard belt does - and I had to do the same on my 4G63 too (also Kevlar, different brand)
> 
> I figure with these cambelts you have to go waaaay looser (when setting it cold) than seems safe
> 
> ...


I have changed bought bolts to 12.9  I have had a idler stud failure before, and the result then was a destroid head. I was looking in the manual yesterday dough and have a few questions about the tensioner and the idler 

In the manual there is a lot of details about the tensioner, the spring and a special washer that goes in the back of it.. Since i have changed the bolts, the spring and the washer is taken of. Can this have any effect??
Can there be any addisional problems because I have changed the bolts?

I must admit that I think you gyes have a point with the heat beeing a problem, and I have to say I think the belt has been way tight on there. The problem with cevlar belts is that you cant really use the thumb and forefinger to bend it 45 degres. Its way to stiff for that.. Im a bit unshure how to best put the new one back on now. It says in the book that the initial pressure on the belt chould be around 196 nm (20 kg), and thats fine.. But how do you messure that??

Thanks again for your input!! much appreciated!!:clap:


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