13 replies to this topic

[chrisproject]

Can anyone advise what benifits are gained if i have my crank balanced along with the flywheel? (Engine spec) i am rebuilding a 1275 which is bored out to 1293 and i have 10.1 compressin pistons with a piper 285 cam and isky followers.

Cheers

[Cooperman]

Balancing does make the engine smoother, but unless you want to rev to over about 6200 rpm a lot of the time in all honesty it is probably not really worth it. I'm sure others will disagree and I have to say that my 'S', which I rev to 6800 rpm quite often is all balanced.
It's a personal choice really and if you can afford it then it is not a bad thing to do. If doing it then you should also balance the con rods as they are a real source of out-of-balance forces, more so than the crank and flywheel actually.

[chrisproject]

Thanks for your advice, i will check my budget(the wife) and go from there. Do you recommend anybody who carry out the balancing?

[twrminisport]

MED Engineering.

[dklawson]

If you are doing a budget rebuild on a street engine you can certainly skip balancing. On the other hand, if you are building the engine yourself and planning on putting other go-fast parts in the engine as part of the build, skipping balancing is false economy. However, you typically do not balance just the crank. It starts with matching the weight and balance of the piston/con-rod assemblies, then the crank, then the flywheel/clutch mounted on the crank. It makes the engine smoother as stated above and that's a good thing if you are expecting to push the engine a bit.

[MRA]

Much much more importantly than balancing the dynamic assembly is ......

1) Ensure that the damper is in excellent health, any less then throw it out
2) Ensure that the flywheel runs true and in line, ie no shimmy
3) Ensure that if your crankshaft has been reground that the crank was straight beforehand, otherwise it will oscillate

Quite simply if any of the above are not 100% then don't waste your money on a balance.

Balancing DOES NOT cure c*** parts !

I have seen crankshaft that have been reground, but were bent to start with.... and what hangs off the unground parts of a crank ? Yes 10 / 10 .... the flywheel and the damper.

[bmcecosse]

Pretty much as above - the engine must be in spot-on condition, and you need to balance the conrods and pistons and check the crank has equal stroke, and the rods are all the same length and the piston heights are all the same......it's not just about 'balancing the crank'...... So saying - a fully balanced engine with a light flywheel is a delight!

Edited by bmcecosse, 29 June 2011 - 06:21 PM.

[Cooperman]

Everyone seems to be more or less in agreement on this one.
For, say, a race engine you need to do more than just balance everything. The best engines have their rods polished before balancing to relieve any potential stress-raisers and the crank is not just re-ground, it is 'stroked & indexed'. I know this was very expensive on the last 3 rally Mk.1 Cooper 'S's, but it was worth it I think. To explain 'stroking & indexing', (I know you know this already Martin!) there is an 'angular' and 'throw' tolerance with all cranks. Journals 1 & 4 should be exactly in line, as should 2 & 3. In practice, however, there is an slight difference of maybe a few minutes of arc. Also the throw, which determines the actual stroke length, has a manufacturing tolerance. 'Stroking & Indexing' removes these tolerances and, at the same time ensures that all main journals are exactly in line i.e. concentric.
This is necessary if you want to pull really high revs as with the 970 'S' which revved to 'gawd knows what'. It was surprising how far out a brand new and completely unused 1071 crank was when I built the 1071 'S'.
Martin, can you do this as the company in London I used to go to have closed down?
One thing it's more difficult, but not impossible, to correct is the centre lengths of the con rods. I think the nominal tolerance as designed was 0.0015", but I'm not absolutely sure about that. Do you do this tolerance removal Martin? When BMC used to build competition engines they selected rods with identical centre lengths, but we don't have that luxury now. This is not quite so critical, although it does make the pistons crowns come to different heights up the bore which is not ideal.

[MRA]

Yes Peter we can do stroking & indexing, however I must add that it doesn't remove the tolerance it only reduces or tightens it.....

Most machine shops can do this, although the best machines grip on the outer diameter ie the tail and sprocket / pulley end, instead of the inner centre drilling, or at least have an adjustable centre at both ends.

There are two different types of Grinder.... one where the head moves in and out as the crankshaft rotates, the other where the crankshaft is loaded wit the pins to be ground in line with the rotation of the machine.

0.0015" as high as that I was sure it was a lot lower than that, but I haven't got my book with me ??

To get better accuracy use a jig borer, which can get it closer, then followwith a hone, not one of those silly glaze busters that I recently found out a well known engine builder uses to hone his blocks ...... I mean a well known engine builder, who should know better

Still the main killer of crankshafts is people grinding a bent crank, because the nose & tail spin around out of centre...... need I say more.

[Cooperman]

Well, of course you can't have 'zero tolerance', but when I worked in Jig & Tool Design we used to work on a tooling tolerance of 10% of design tolerance. If/when I get around to building another full-on rally engine I'll get the crank to you for 'S & I'.
Can you also balance rods after they have been polished?
I was going from memory on the rod tolerance and as I said I could be wrong. But with BMC anything was possible!
However, when doing trial builds it is surprising how different the piston crown to block deck dimension can be. I've seen up to 0.004" and this is a combination of stroke difference, rod length and piston height above gudgeon pin centre-line. Modern pistons are pretty accurate one might guess with CAM techniques, but those old BMC/ARG/Rover rods and cranks are another matter.
It's not critical for road cars, but for competition where huge revs are involved it's another matter entirely. My son's 970 'S' would rev to around 8000+ (it was a rally car, not a full race car and the cam was a 286 with 1.5 rockers) and my 1071 with a 649 would go to 7500+ 'in extremis'. I don't like taking my 1310 'S' much over 7000 as it runs off the cam, but I have seen around 7200 and that's quite enough thank you! On those engines full and accurate balancing, stroking and indexing are, IMHO, very important.

[MRA]

I was making the point more for those who do not know..... it can be frustrating when you get drawings with dimensions like 599.999mm +/- 0.001mm it simply will not happen over such a length, often these drawings are created by a "designer" (Sorry to real designers) who just so happens to have learnt to use a cad terminal..

CAM has really only one place in Engineering, and that is to create repetitive cycles on complex parts... Think of it as mowing the lawn, if it was all straight then it would be simple, however if we introduce some very different curves then you need more control, it's pretty much the same with metal (apart from it smells different) CAM allows the system to create those complex paths that we see as very nice curvy things....

If you are simply milling a surface (block or head for example) then CAM doesn't come in to it.

Oh and yes we can balance conrods before or after polishing...

[dklawson]

Can you also balance rods after they have been polished?

Is it actually polishing? I am more familiar with rods being shot-peined to strengthen the outer surfaces and remove stress risers. Regardless, when I have seen rods balanced, they were only done for matching the assembled weight and the excess material was taken off the big end cap so the rod itself was left untouched. So Martin, you actually polish the rods?

I was making the point more for those who do not know..... it can be frustrating when you get drawings with dimensions like 599.999mm +/- 0.001mm it simply will not happen over such a length, often these drawings are created by a "designer" (Sorry to real designers) who just so happens to have learnt to use a cad terminal..

If that includes design engineers... I can put as many zeros as you would like on a tolerance. Most of us however are pretty aware that adding zeros adds cost. Most of what I do is dimensioned to +/- .005" until I get to dowel fits and bearing fits. In those situations we go for tighter tolerances but we still try to stay well within industry standards for what CNC tools can achieve. When you get down to tolerances at or below +/- .0005" you have to worry about things like the temperature the part will be used at and machined at.

[morley]

I would recommened getting the balencing done, i wasn't sure about getting mine done when i built my engine but i built my engine with the idea that i would gradually increase performance as and when, so i done it right first time and built it up with parts that i didnt really need for my spec but knew i would need if i ever upgraded [cenetr main strap, arp rod bolts etc]... and i'm now in the process of turboing so i'm glad i did those mods.


my engine builder found my crank to be out of balance by 17 grams, doesn't sound alot but when things are spinning at 6000rpm+...

to balance my crank and flywheel assembly was £60 and rods and pistons were £45, so a little bit expensive but worthwhile thing to do

[MRA]

We do both Doug, shot peen and or polish.

If the little end is heavy then it needs to be rectified, ie lightened.....

Balancing conrods is done on a highly accurate set of scales and should be done end to end, ie all little ends the same all big ends the same each rod identical in weight / mass.

any polishing must be down along the length,

.............

Yes Doug that is very true.

Design to requirement