90 replies to this topic

[greens nice]

Now this come from experience with a flywheel coming loose too often in another car. We did almost all that you ahve done and found out that the crankshaft was cracked in No6 B/E journal and not far from falling apart with major consequences.

Before you go to doing more radical things get under the car and before you pull everything apart put a dial indicator on the outer clutch face of the flywheel and get a mate to turn it over slowly by hand and check the run out in the flywheel. (Take the plugs out first it will be easier to turn over)
There are specs in the workshop manuals as to what the runout tolerances are but for what you are doing it should be absolutely minimal.


If the run out is excessive it could be one of two things either a warped flywheel or a bent/cracked crankshaft.

Once the runout has been determined
pull the flywheel off take it to the engine builder and get him to check it for warpness/bend.
being an ultra lightweight flywheel there is every chance that this could be warped due to high revs and heat.

If the flywheel is ok then it is further in the engine.
Next is to check the end of the crank for run out

If there is any/too much runout in the crank then that will be what is causing the bolts to break/loosen.
The cause may not be that it is out of balance but the fact that it is either bent or cracked.

I would suspect no 6 B/E journal allowing the flange end of the crank to bend if the problem is in the crankshaft

cheers
peter

thanks piquet, youve given me something to think about.

The FJ ran around for 10 years with a cast iron flywheel, copper shim and red loctite, didnt rattle loose once. this is using 7/16 arp 351 clevo nascar bolts (dont have to cut them to size)
put a steel flywheel on it along with a roller cam. Used the copper shim use moly lube and did what arp told us to.
took 3 years to come off, so scrapped the copper shim but still used moly lube, came off in 4 meetings.
did the same thing over again and it took 2 meetings, 1 meeting and then eventually only 2 runs down the quarter before bolts loose (cant push the clutch in because they are touching the bolts). Maybe crank is cracked or bent?
blue motor crank btw

Edited by greens nice, 18 March 2010 - 08:12 PM.

[piquet]

before you go drilling and tapping the crank for the bigger bolts (which needs to be done with the crank out of the engine to get the most accurate results)
I would do all the other things that I ahve sugested first. They take time and careful consideration but don't cost anything as opposed to jumping in with both feet and costing money and it still may be a cracked/bent crank or a warped flywheel. And all you have to show is bigger boots that may not be able to be used.

A crack test of all components in an engine of the HP that you are talking about is very cheap insurance. Also consider lifing some of the more critical components such as B/e bolts and flywheel bolts. After all they do tend to cop a fair hammering and fatigue is a drivers worst enemy same is for engine internals
cheers
peter

[turbotrana]

I have had a crack in No6 blue mtr crank. The vibration used to be pronounced when you put your foot on the clutch and changed gear at 6000+ rpm. It would also vibrate the harmonic balancer bolt off (that I had drilled)

I also used to vibrate a lighten cast flywheel off and I ended up putting 4 x 10mm allen key bolts inbetween the other bolts. In total had 10 bolts holding the flywheel on. Never vibrated this flywheel off even with No6 crack.

In my next motor I put in an alluminium flywheel and a heavy B&M (dont make no more) harmonic balancer with a black mtr crank. I think the alluminium changes the harmonics as I only used the standard bolt holes on the back of the crank with aftermarket bolts and have never vibrated it off in over 15 years of 500rwhp turbo work. The motor is pretty smooth even around the renown 6000+rpm harmonic area.

[Stinga]

what about drilling the boltheads and lock wiring them as another locking method? but i agree you should address the problem of what is causing the bolts to come loose.

[Litre8]

I used to run the B&M Steel balancer on my old 202 and that, plus extra dowells locating the flywheel, seemed to help control the harmonics.

[Stinga]

what about drilling the boltheads and lock wiring them as another locking method? but i agree you should address the problem of what is causing the bolts to come loose.

hey mate,
thats what i did last time and i ended up breaking 3 bolts off.
think we may have found the problem tho. well im hoping anyways.

how big did you drill the holes?? what size wire did you use?

[rodomo]

how big did you drill the holes?? what size wire did you use?

Waste of effort IMHO.
If something is vibrating, all the wire might do is stop the bolts falling out.
It won't stop the components fretting against each other (due to the vibration), thus causing wear, thus effectively making the bolts loose.

[rodomo]

Oldjohnno made a post in a thread a while ago about harmonics in Holden 6 cranks.
Made sense to me.
I think if you can find that post (damned if I can) the crank DOES care.

EDIT: Found it!!!!!!!!

OK then....
I'm assuming you're familiar with crankshaft torsional vibrations. Quick summary goes like this: the crank is being continually subjected to impulsive forces from combustion and compression pressures as well as inertial loadings from accelerating and deccelerating the reciprocating bits. These forces vary in direction and magnitude and tend to make the crank motion somewhat jerky rather than spinning at a constant speed. Now the crank isn't perfectly rigid and is somewhat restrained at one end by the flywheel and the load but is relatively free at the front. Because of this there is some relative twisting forward and back between the ends of the crank. Providing this isn't excessive it's not a problem (say not much more than 1 or 2 degrees). Thing is though the crank (because of its springiness) has its own natural resonance or frequency that it wants to vibrate at, a bit like a guitar string. And if the frequency of the impulses fed into the crank matches the natural resonating frequency of the crank (or a multiple thereof) then things can get ugly. If left uncontrolled the amplitude of the torsional vibrations will jump dramatically. This isn't just a gentle buzz either, the vibrations can be violent enough to break the crank, or shear the flywheel bolts or shake the rim off the balancer. Incidentally, it's quite common for straight six crankshafts to resonate at a frequency that corresponds with 6000 - 6500 rpm. Provided you can stay above or below these critical speeds then vibration is usually negligible or at least manageable. Controlling the vibrations is a separate story. A good, understandable explanation can be found here.
Now for spark scatter. If the snout of the crank is vibrating torsionally then these will be fed into the camshaft as well via the drive gears. Incidentally the camshaft will have it's own natural resonance that may add to or detract from the cranks vibration. Driven from the cam is the distributor, which is trying to accurately time the sparks at the same time as having the living shit shaken from it. And the centrifugal advance mechanism is a set of springs and bobweights - a mechanism that is seemingly especially designed to be confused by torsional vibrations.. Spark scatter is the end result. Instead of all cylinders being fired at say 32 degs, some may be set off at 38 degrees while others are fired at 20.. Or maybe the trigger will become so confused it doesn't fire at all. The solution is to control the vibrations or at least try to isolate them from the cam and dissy (eg. belt drive cams as used in ProStockers). Ideally you'd do away with the dissy altogether and use a crank trigger instead, and this is what is commonly used in less ancient engine designs. Preferably position the trigger at the flywheel end where the vibration amplitude is slightly less than at the front.
The puzzling thing about the original problem is that the dyno is the ideal place to locate and rectify these sorts of problems. And it happened on a dyno??
Still reading? You need to get a life...

Edited by rodomo, 30 March 2010 - 11:23 PM.

[rodomo]

My interest was in the "spark scatter" in this thread:
http://www.gmh-toran...=1

[orangeLJ]

Thanks for the interesting read gents.

Definitely something you dont always think about.

Ive still got all the contacts saved to get billet cranks made by spool.....

[Heath]

A billet crank might solve your problem (theoretically can perfectly), but who's going to design it so that the counterweighting is actually adequately effective?

As far as a 'solution' for the average Holden 6 hotrodder, focus on a lightweight conrod & piston combination, gives you a good opportunity to go full floaters and get a superior rod ratio. That applies to either crank combo, particularly 202 though.