Balancing a bottom end

[Dave McCulloch]

Having followed the recent post from KingK, one of the few valid points which seemed to come out was that balancing the bottom end of an engine was "a good thing". As I understand it this involves static balancing on the pistons and rods by matching the weight of all 4 pistons by shaving material from the bottom of the piston where the gudgeon pin is supported; balancing the rods "end over end". Then dynamic balancing on a special machine of the rotating bits - ie the crank and flywheel.

 

How necessary is all this on a K series which will be rev limited at about 8,000 rpm, built using standard 1.8 crank in a scholar block (using the 1900 pistons), with standard 1.8 rods reamed out to be fully floating and lightweight flywheel?

 

If it is necessary (or at least desirable) is the static balancing something which is DIY with a set of electronic kitchen scales, so it's only the crank and flywheel which need sending off for balancing. And how much is this likely to cost.

 

The other point I'm struggling slightly with is that it's all very well balancing the engine itself, but when you're driving along the crank is connected to the gearbox, prop etc - surely any out-of-balance in these parts will just be transmitted back along to the crank?

 

Thanks for any guidance!

 

Dave

[mahatma]

Mulling over the KingK article again, I was thinking the exact same thing, but going as far as the effect of an unbalanced wheel ... surely this would have the same effect?

 

Andy

[Joe 90]

Dave,

In my experience, a balanced engine makes the car nicer to drive. If the engine is in bits, you can get it done for £100, and it's money well spent. If you are going to have to tear down the engine, then the cost becomes excessive. I haven't really asked how well places can manage to balance bits, so I just usually go with someone reputable. Kitchen scales are much too crude for DIY balancing.

 

As for the other rotating bits, you are probably aware that wheels and tyres are balanced. Prop shafts are usually balanced by the addition of small weights. The tolerances on gear box internals are so tight they don't need balancing.

 

SEP field working, not spotted in 103,000 miles. Some photos on webshots, updated 21 Sept

[oldbutnotslow]

Oliver is right, it takes away any harshness in the engine which in turn will allow you to enjoy higher rev limits, always providing or course that the rest of the internals are up to the job, without damaging vibrations. You say that you have a lightened flywheel. If this is the case I would definitely balance the bottom end whilst you have it in bits. Part of the mass of the flywheel as standard is to dampen the effects of internal vibration caused by poor manufacturing tolerances. The car will rev much more freely and will feel very much smoother. Make sure that you have the crank Flywheel and clutch assembly balanced as one unit otherwise you are wasting your money.

It’s a job for professionals. Don’t be tempted to start to shave bits off pistons and rods or grind lumps off the crank, as you will bu++er it all up very quickly.

I used to build Escorts to rally back in the 1970s and always had the bottom end balanced. The engines tended not to go bang as frequently. You don’t say what you use the car for. This may also determine your need. If the car spends its life on a track where you are using sustained revs, between 6k to 8K, as a regular feature it will certainly be beneficial. If on the other hand you Blat about the lanes with the occasional use of high revs you may see little benefit.

 

 

Grant

 

Black and stone chip

here

[V7 SLR]

Report from my balancing:

 

Crank:

Before = 125g/mm. After = 41g/mm.

 

Added front pulleys:

Before = 396g/mm. After = 27g/mm.

 

Added flywheel:

Before = 190g/mm. After = 30g/mm

 

Added clutch cover:

Before = 347g/mm. After = 68g/mm.

 

The clutch cover only has a minimal amount you can remove without damaging it, but what got to me was that the front pulley-system affected imbalance so dramatically.

 

One of my locals had his system balanced which led to an immediately noticably smooth revver engine. More *willing* to rev too. There was no way I was *not* going to balance mine after hearing his and once I'd seen my balance report I was more convinced than ever.

 

They matched my pistons/rods too. This is for my Scholar 1900cc conversion too.

[simonbell]

Well worth the money if your engine is already in bits. I had the duratec bottom end balanced before rebuilding

 

Simon Bell - Caterham 7 Duratec R

I`ve seen the future.....and it`s powered by duratec Check out the website here

[Dave McCulloch]

Thanks all

 

The car is used almost solely on the road - though I do intend to do a few track days once I've stopped spending all my money on engine parts!

 

Dave

[Mick Day]

Dave

 

Have a look here. I know this is for an air-cooled VW but the principles are still the same. This is what I think Simon Erland was trying to get across. Use quality componants & get them balanced. In effect everything which goes around needs to be balanced. Those with a small diameter such as prop-shafts (should be dynamically balanced by the manufacturer anyway), gearbox's & diffs probably don't have much effect. Remember wheels are balanced but we don't balance the hubs or brake discs.

 

Mick

[Tony Martyr]

You may be missing the point that the clutch is a torsional damping device and the balance of components after the flywheel are damped by the soft components all the way to the tyres

[tiddy1]

got my crank balance at Rolls-Royce where I work (aero engines that is) the guy who did it for me was very apologetic as he said he did not have enough time to finish as well as he would have liked but he hoped 7g/mm was ok!

 

nice job it seams

 

simon

[Joe 90]

The torsional damper in the clutch will damp torsional vibrations. This is unrelated to "out of balance" vibrations, so all components should still be considered for balancing.

 

Can I get my bits done at R-R please? 7gmm, wow.

 

SEP field working, not spotted in 103,000 miles. Some photos on webshots, updated 21 Sept

[tiddy1]

All our aero engines get balanced to this level or better, but when you spin bits up to 50,000 revs then you have to, a bit like turbo's

 

Simon