crankshaft broke again??

[dazzed]

hey guys , i keep breaking crankshafts in my 240z and im getting used to it as this happens every 2 years approx. but why? has struck me , for the record i have had 4 crankshafts fiited to the engine in 8 years is this normal or what? why does this keep happening? the crankshaft just snaps the engine still goes just makes a banging noise have to drive slow , have a new crankshaft fitted and its ok, give it 2 years the same thing happens again, whats going on? up untill the crank has broken each time it has ran fine, it is only standard bore an stroke L28 , standard rods pistons just has 74 nissan comp grind cam and triple carbs. i have spoken to a few people and this hasnt happened to them ,does this happened to anyone else and if so what did you do to stop this from happening as im starting to think maybe this old crap dinoseur of a motor just might get replaced with a rb motor not sure yet . 

[zr240]

HI

 

Do you run the std harmonic ballancer? Do you do and sprints/ track days? Does it always brake in the same place? How do the bearings look when you take them out?

 

Ash

[C.A.R.]

And what is your RPM redline?

[24OZ]

have you had the crank tunnel line bored to make sure it's spot on?

[Zedman240®]

One thing I'm always sure I get done no matter what is to get the flywheel and clutch balanced as an assembly. Just a thought...

[Steve thomas]

http://www.swmotorsport.com/

Buy one of these guys BMW based Balancers and your problems are over.

[dazzed]

HI

 

Do you run the std harmonic ballancer? Do you do and sprints/ track days? Does it always brake in the same place? How do the bearings look when you take them out?

 

Ash

i run a bmw style ballancer custom made as this prolonged the life of the crank it didnt stop it from breaking, most of the use was in sprints, bearings were always refitted as there was nothing wrong with them, the crankshaft always breaks at number 6 main bearing journal

[dazzed]

And what is your RPM redline?

the 74 grind cam falls of pwr around 6300rpm, as i can feel it fall off pwr i ussually just grab another gear but the engine will freelly rev to 7200rpm, just revs dosnt make anymore power not that i go there reguarly, 4th gear end of main straight phillip island 6500rpm, cheers lurch. dazzed

[dazzed]

have you had the crank tunnel line bored to make sure it's spot on?

no i havnt , motor was a low km jap import motor, this was considered and disscused, as the bearings showed no sign of abnormal wear, i guess you would never know unless it was checked, which meant a total engine rebuild everytime it happened whereas only crankshafts fitted in order to reduce time and cost factors, considering its only a dunga datsun dinoseur tractor engine cheers 240z .dazza

[dazzed]

One thing I'm always sure I get done no matter what is to get the flywheel and clutch balanced as an assembly. Just a thought...

this was carried out by a reputable engine shop for the first 2 times it happened, the cranks were linished and inspected carefully before they ballanced them, so gave up on that and just keep throwing cranks at it , cheers zedman, dazzed

[dazzed]

http://www.swmotorsport.com/

Buy one of these guys BMW based Balancers and your problems are over.

it maybe what i have fitted now but i didnt get it off them, prior to having this one fitted i had a steel billet stock nismo competition ballancer, remember thes cranks that have broken are only stock not treated or anything flash like that , now that the bmw style ballancer has been fitted i havnt done any sprints yet but it will be interesting to see how long this crank lasts, i guess eventually something has to give they cant last forever pretty old stuff , chances are that each crank fitted has had the crap flogged out of it already, so we will see how long this one lasts, thanks for that link steve, cheers dazzed

[zr240]

thats strange then as I ran a bmw ballancer on my l28 race motor and it had a std l28 crank and never broke it??

 

Sounds like you have done all the right things??

 

Ash

[Scoota G]

So let's paint a picture here.

 

A crankshaft does Umpteen thousand kays in another motor and then sits around for a while and then gets transplanted in your donk, does a couple of years service and then says no dice.

 

It sounds like there may be a fault with you block.

 

I would be very keen to see the engine apart if i were you and see tolerences measured.

[24OZ]

no i havnt , motor was a low km jap import motor, this was considered and disscused, as the bearings showed no sign of abnormal wear, i guess you would never know unless it was checked, which meant a total engine rebuild everytime it happened whereas only crankshafts fitted in order to reduce time and cost factors, considering its only a dunga datsun dinoseur tractor engine cheers 240z .dazza

 

it sounds to me like the block is the common problem

if you don't want to go to the trouble of line boring it just use another block....surely you couldn't be unlucky enough to have 2 that are out of alignment

[Steve thomas]

There must be other issues with this,I have some hi reving L's out there that are over 10 years old that do many races with the BMW setup without problems inc Championship winning cars.

What Flywheel/Clutch do you have?

[24OZ]

There must be other issues with this,I have some hi reving L's out there that are over 10 years old that do many races with the BMW setup without problems inc Championship winning cars.

What Flywheel/Clutch do you have?

welcome aboard steve,'bout time

[nat0_240_chevZ]

Do you have a spiggot/pilot bush installed in the back of the crank???

Why has the main bearings bore, NOT been tunnel bored after the first crank, its surely worth less than multiple breakages.

Is the entire rotating assy balanced?

Are the bores all wearing evenly?? could be inconsistant drag due to out of round on cyl 5 or 6??

What does the break look like, can you post some pics? also very close up of the fracture area  on the crank??

Is there any noted damage on the piston face/tops??

There is something obviously causing it as my old l24 would rev happily to 8500 rpm all day long for the 3-4 yeas i owned it, but its day was done when i melted a piston in #1 after the dissy clamp let the dissy free spin!!!

answer some of the above and it should start to guide you to the root of the snappage, id say fatigue crack without even looking at it. but that is the obvious anyways.

 

nat0

[620Z]

I tend to agree with Nato. I reckon it's something not lining up exactly and eventually wearing and breaking. I would be pulling the whole engine out and get the block completely checked for alignment around the crank etc. (Presuming you didn't want to get a new block for which I would still do the same anyway). As my engine builder says everything must be exact (With in 1-2 thou) to get the most power and avoid terminal problems. My block went of to the (Block) engine grinders 3 different times for checking different things for alignment before it was assembled. And this was for no apparent problem. My bloke said that the standard cast blocks were OK for standard cars etc but once you get up into the higher end of performanse you need better tolerences than what come off the production line. This (New) engine builder of mine was extremely fussy and was well aware of getting tolerences and measurements exact for a race engine you want to extract maximum power and rev to extremes. Todate never had a problem and it rev's to 7500rpm no problem which is not bad for a V8. Was your crank forged?

My 2cents worth from a layman.

[dazzed]

welcome aboard steve,'bout time

sorry ive lost ive just about lost count of the times it has happened , i used to run a nismo competition ballancer as i knew someone who ran one for years without a crank fault, tho with the crank i now have in it i have fitted a bmw style pulley so im yet to find out what happens and how long it lasts , i must admit the motor does feel a bit smoother since fitting it then againit might just be my imagination

[dazzed]

There must be other issues with this,I have some hi reving L's out there that are over 10 years old that do many races with the BMW setup without problems inc Championship winning cars.

What Flywheel/Clutch do you have?

this has been pointed out before as well, chromoly light weight flywheel im pretty sure its a nismo flywheel, and a low profile std clutch cover 3200 lb calmping force its high but its the only clutch ive had in that hasnt blown itself to pieces in 5 minutes  and its been refitted each time a crank has been replaced

[dazzed]

thats strange then as I ran a bmw ballancer on my l28 race motor and it had a std l28 crank and never broke it??

 

Sounds like you have done all the right things??

 

Ash

last  i recall on the datrally dyno the rear wheel kw was 138 140  and im pretty sure back then i only had 240z carbs on it , yeah must have i didnt even have triple carbs then, and it hasnt been dynoed since mmm maybe just too much pwr for std cranks? how much can a std crank stand up to ?? there must be a limit, maybe ive found it

[dazzed]

Do you have a spiggot/pilot bush installed in the back of the crank???

Why has the main bearings bore, NOT been tunnel bored after the first crank, its surely worth less than multiple breakages.

Is the entire rotating assy balanced?

Are the bores all wearing evenly?? could be inconsistant drag due to out of round on cyl 5 or 6??

What does the break look like, can you post some pics? also very close up of the fracture area  on the crank??

Is there any noted damage on the piston face/tops??

There is something obviously causing it as my old l24 would rev happily to 8500 rpm all day long for the 3-4 yeas i owned it, but its day was done when i melted a piston in #1 after the dissy clamp let the dissy free ,spin!!!

answer some of the above and it should start to guide you to the root of the snappage, id say fatigue crack without even looking at it. but that is the obvious anyways.

 

nat0

yep , even went to the trouble of fitting a roller bearing in the cranks , the whole lot was ballanced as an assembly each time, i would assume that if the main bearing bores were out of alignment the bearing shells would show signs of scuffing bearings were perfect, we changed them last time just for the hell of it even tho they didnt need replacing,  ive always like the way a std 2.4 revs short throw crank  makes a difference not as much mass getting flung around in the wash, i think that for the cranks to have lasted as long as what they did is fantastic for what they are and what ive been putting them through, thing is i dont know of anyone else this has happened to

[dazzed]

I tend to agree with Nato. I reckon it's something not lining up exactly and eventually wearing and breaking. I would be pulling the whole engine out and get the block completely checked for alignment around the crank etc. (Presuming you didn't want to get a new block for which I would still do the same anyway). As my engine builder says everything must be exact (With in 1-2 thou) to get the most power and avoid terminal problems. My block went of to the (Block) engine grinders 3 different times for checking different things for alignment before it was assembled. And this was for no apparent problem. My bloke said that the standard cast blocks were OK for standard cars etc but once you get up into the higher end of performanse you need better tolerences than what come off the production line. This (New) engine builder of mine was extremely fussy and was well aware of getting tolerences and measurements exact for a race engine you want to extract maximum power and rev to extremes. Todate never had a problem and it rev's to 7500rpm no problem which is not bad for a V8. Was your crank forged?

My 2cents worth from a layman.

some say that a good motor is a loose one i guess you would have to do a fair bit of testing to determine how tight or loose pistons in bores should be or the old naa sheel be right mate whack er in , i agree, you get to a certain point and all the std figures go out the window , this is commonly called the point of no return ,and checking and double checking everything still dosnt gaurantee it  wont fly to      pieces a 7500rpm v8 sounds like it been put together well

[nat0_240_chevZ]

the the std nissan revs to nearly that and its still singing at this speed, glad i got one going in now!!! pics are on nico.

yes troppo motors do have loose piston-bore clearance but this is also to reduce thermal expansion that causes excessive bore pressure as well as reducing cyl-bore drag.

but tight in the sense that craig was mentioning was positional/concentric tolerances of the crank mainbearings, not piston to bore.

maybe the new balancer that you have noticed a slight improvement from will do it, but in saying that wasnt it #5 where it broke???

try agtr clutch which is uprated ie an osgiken single or double, they fit the L easily too.

dowelling mAaybe a suggestion...

but if it feels smoother it probably is.

also what is the all of the separate mainbearing clearances like after it breaks?? @ 0, 90 180 & 270* not just @ 1 pos'n?

nato

[620Z]

Thanks Nato for clearing up what I was trying to say as a lay person. Yeah I was talking about the crank area. My block went to the point of lining up the crank main bearing shaft area to the piston bores. He said some people call that "Blue Printing" the motor. Apparently people have different ideas on what that means.

Obviously by making everything true or with closer tolerences than the standard cast block then it will enhance the engines performance and minimise wear and possible destruction.