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Old 30 Aug 2005, 11:32 (Ref:1394396)   #1
carsreunited
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Balancing

Just a quick question.

My bottom end is being rebuilt on the Dolomite Sprint after 2 bearing failures in 2 races!

When the bottom end is re-balanced, should this be done with the flywheel and clutch (or just the flywheel, or totally un-necessary to have both)? I'm not doing it myself but just wondered if its being done right.

Cheers,

Scott
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Old 30 Aug 2005, 12:18 (Ref:1394434)   #2
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I seem to recall that when I had mine balanced, I had the flywheel and clutch attached, and possibly some rods too, although I'm a bit fuzzy on that part.

Probably better to have them on while balancing, rather than off - as it's closer to the environment that the crank will be running in the engine I guess.
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Old 30 Aug 2005, 19:15 (Ref:1394762)   #3
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Balancing as you talk about it is mainly concerned about reducing engine vibration but to small differences in the mass of all the main rotating components. On a 4-cyl engine you should balance the crank, flywheel and clutch. On some vee engines it is also necessary to add some massed to the big ends to simulate the rod mass. It will make the engine a little smoother but almost certainly not stop you bearing failures.

The other part of balancing is a fundamental design aspect of the engine. You will notice that a crankshaft has a number of counterbalance weights on the opposite side to the big ends. Without the weights the big ends would cause centrifugal forces on the crankshaft rather like swinging a weight around your head on a piece of string. These forces cause bending loads in the crank and also load the main bearings.

If we assume that this lack of counterbalance produces a force 'F' for each cylinder. On a 4 cyl engine, main bearings #1 and #5 each have to carry a force F, # 2 & 4 carry no load whilst #3 carrys a load of 2F. This is why a 4 cyl engine normally fails the centre main bearing first.

Most modern 4 cyl engines have 8 counterbalance weights on the crank, i.e. 2 per big end, these are typically close to 100% counterbalanced so the rotating loads on the crank are generally quite low. Older engine tend to have 4 counterbalance weights, these type of engines are typically under balanced and the loads on the crank can become high, particularly if you rev it higher than its original design limit - the likely result of this is centre main bearing failure.

So how do you stop this? You need to reduce the mass of the big end pins of the crank as well as the big end of the con rod. The big-end pins can be bored out but you will need to be careful that you don't break though into the oilways. On the con rod, caps tend to have a lump of metal for balancing purposes, grinding this off is a good idea. Another way is to increase the mass of the counterbalance weights, you can bore holes in the weights and fill them with tungsten plugs which are much denser than steel.

So if your bearing failures are centre mains, then you need more counterbalancing.

High revs can also cause big end failures. The oil flows from the main gallery into the main bearings and then thro the internal drillings to the big ends. As the revs increase it becomes more difficult to get the oil from the gallery into the crank, eventually you will reach a point where oil can no longer be forced in and the crank will be starved and the big ends will fail. Higher oil pressure will delay the onset of this condition and will allow the engine to rev higher before failure.
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Old 30 Aug 2005, 19:16 (Ref:1394763)   #4
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Ideally you would balance the crank, flywheel and clutch cover together.

However, the bottom end would have to me massively out of balance to cause a bearing failure. This is more likely caused by insufficient oil pressure as a result of -

- inadequate pump supply.
- over large or oval bearing clearances
- drop in oil pressure due to heat thinning the oil
- oil surge

Are you happy that the rebuild is fixing your original problem?
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Old 30 Aug 2005, 22:40 (Ref:1395019)   #5
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Thanks for the replies.

It was number 2 bearing that failed 1st time and then number 1 on the last occasion.

I was revving the engine to 7000rpm, 500rpm more than a standard red line on these engines. I do not have an oil temperature guage yet so I don't know what temperature the oil was when the failures happened. I use Penrite 20/60 oil as recommended by the engine builder.

A bearing girdle is being made this time as others have stated that this has improved reliability on other Dolly Sprint engines.

The oil pressure always looked constant when I glanced at it. Generally 70psi at racing speeds, 40psi at idle.

So the concensus is that it is desirable to have the flywheel and clutch when balancing the crank but not critical?

Cheers,

Scott
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Old 31 Aug 2005, 05:03 (Ref:1395147)   #6
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To my knowledge racing clutches are balanced to zero at the factory so if this is the case its presense is not critical but the flywheel is.
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Old 31 Aug 2005, 10:45 (Ref:1395385)   #7
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Quote:
Originally Posted by tzei
To my knowledge racing clutches are balanced to zero at the factory so if this is the case its presense is not critical but the flywheel is.
This is my understanding too, otherwise you'd need an engine re-build everytime you changed the clutch.
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Old 31 Aug 2005, 12:27 (Ref:1395471)   #8
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I balance my crank, clutch cover, flywheel and front pulley as one assembly . . .basically anything that fits to the crank.

I'd agree with the other posts though, if it out it vibrates, bearing failure is down to crank / block machining and clearances and or oil supply
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Old 31 Aug 2005, 13:21 (Ref:1395522)   #9
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The normal way to balance is to start off by balancing the crank on it's own then add all the bits that would attach to it, building it all up and balancing them one by one. I have just had my engine balanced and they did the crank, flywheel, clutch, damper and front pulley although I dont know if you have a damper on your engine. Don't forget to supply them with all the bolts as well.
I had 3 engine failures, all out of balance related and found out it was the clutch cover (the only component common to all of them) which had bent mountings on the pressure plate putting it out of balance.
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Old 31 Aug 2005, 15:41 (Ref:1395598)   #10
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3 crucial aspects of accurate & repeatable balancing are location, location and location - i.e. how the part you are measuring the unbalance of is located on the thing which is spinning it.

When a clutch (or flywheel) is balanced on it's own, then it is balanced around the centre of rotation of the adaptor that it's mounted to. When it's fitted to the crankshaft then it's centre of rotation may be minutely different, with significant results.

Take the case of a clutch cover plate which is located on the adaptor (and later on the flywheel) by a number of dowels. The amount of unbalance measured and corrected for will depend on the accuracy of the position of the dowels on the adaptor, the accuracy of the position of the holes in the clutch cover plate, and the accuracy of the location of the holes on the dowels. Take the clutch off, turn it around and put it back on and you'll get a different amount & position of unbalance.

Similarly for the flywheel and the way it locates onto the adaptor it's balanced on and the crank it's subsequently attached to.

This is why you should balance the crank on it's own, add the front pulley & rebalance, add the flywheel & rebalance, and finally add the clutch and rebalance.

A part is only balanced around a particular physical centre of rotation - change that centre point (even by a thou) and it becomes (more) unbalanced. If you have a perfectly 5kg flywheel and it's centre of rotation moves by thousandth of an inch (apologies for mixing units!) then it'll become unbalanced to the tune of 127gmm.

Apologies for the ramble... as Denis says this sounds more like an oil supply problem.

Mike
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Old 31 Aug 2005, 20:20 (Ref:1395827)   #11
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7,000 isn't an exssessive amount of rpm, a well built sprint engine should be reliable at those revs with nothing more than a carefull build, how old was the engine in terms of bearings crank grind before tha first blow up? was the engine properly rebuilt/cleaned/new oil pump afterwards.

have you been suffering oil surge? that can very quickly ruin a set of bearings, if at all possible build the engine using lead copper bearings, they are far more tollerant of a bit of oil surge, or forien bodies in the oil than std tin type bearings.

ideally when building an engine balence the crank, then add the flywheel balence that and so on with pulleys clutch covers etc, but that wont be the cause of your problems,

i did 6 seasons on an engine that was reved to 8,000+ unbalenced, i say unbalenced , the origonal engine was, but over the seasons, it had a crank swap, different pistons a couple of rods changed and the flywheel changed/ lightened 3 times, in the and it ran a bigend, but by then the bearings were two and a half seasons old!
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Old 31 Aug 2005, 21:25 (Ref:1395891)   #12
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The engine had 30 minutes of running in on a rolling road, 2 hours of set-up and 15 laps of Snetterton on a track day when it spun number one bearing.

Then New crank, new conrod, new oil pump, new oil cooler and rebalanced.

2nd time, it had done 4 laps of Combe very gently, 3 hours of re-setting up on a rolling road (don't ask!), and then 16 laps of Brands Hatch indy.

Nearly everyone says it is an oil feed problem. The only two common denominators was that immediately prior to the failures is that I spun the car. Both relatively low speed (about 40 mph) and on both occasions I remember dropping the clutch as soon as the spin was un-recoverable but by the time I had come to a rest, the engine had stalled anyway!

Thanks for the advice everyone. Anymore thoughts are welcome.

Scott
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Old 1 Sep 2005, 07:48 (Ref:1396141)   #13
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have you got a decent oil pressure light that catches your eye and lets you know if the pressure drops to,say 20 or 30 psi? whats your sump like inside?if its a tried and tested race sump it should be ok,but most home brewed baffles look like they will work but they dont!,specially if the baffles are vertical. also check what the gap between the pick up pipe and the bottom of the sump is. lots of people braze half a small nut to the btm outside of the pick up to stop the gap closing. bet anything your problems oil surge!!
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Old 1 Sep 2005, 08:07 (Ref:1396156)   #14
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Its got a 30psi red warning light that sits where the glove box used to be. Its about 3 inches long so hard to miss when it comes on.

It is a newly baffled sump put together by someone who has built numerous other race engines without this type of problem. I'm at a loss to understand it. I'd sourced some decent NOS vandervell bearings on both occasions.
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Old 1 Sep 2005, 08:16 (Ref:1396166)   #15
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I bought a crank that was reground and after a massive internal explosion it turned out it had been ground off centre so I can vouch for the check, double check and triple check philosophy because I'd had everything balanced before building and it ran for 2 1/2 hours before it let go with little warning other than a small amount of vibration for 5 minutes or so

if your frying the odd bearing carsreunited I'd wager its an oil problem to the bearings, have you checked the block/end caps ? ie line boring ? if its out it may have something to do with it ?
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