BBK question

[KSR Moto]

I would like to know what exactly puts the most strain on the crank when having a BBK installed

1. Piston weight
2. High compression
3. Power output
4. High rpms

Or everything above?

[PirateLabs]

Jul 25, 2018 4:18:34 GMT -5 KSR Moto said:

I would like to know what exactly puts the most strain on the crank when having a BBK installed

1. Piston weight
2. High compression
3. Power output
4. High rpms

Or everything above?

My opinion is all of the above.  Others will have differing opinions maybe but I think the piston weight and high rpms mixed are probably the worst 2 things when put together.  Brent posted, a long time ago, some photos where he weighed three pistons, 2 from  BBK kits and a stock one.  Of course the 80cc piston weighed the most but then, there was an 80cc aluminum  one (airsal I think?) that I believe weighed the same as the stock piston.  I remember looking around and that I could only find that in the 80cc size and not the 72 cc size.  In other words, he had a piston for 80cc that weighed the same as a stock piston.  Maybe i can find that old topic after work if needed?  I like that lighter piston but really wish it was in the 72cc size because I would still have those other factors working against the stock crank.

Bill

[scooterpimp]

Im going with compression, putting excessive loads on crank bearings ,& rod..etc.

[humanshield]

The answers above are as good as any. But I'll add this...

I don't think this has an exact answer as posed. Reason being, it depends on which BBK, the head used, cam or no cam etc.
I would remove number 3 because it's basically a function of the #2 and #4 (and displacement, cam etc)

1). Piston weight - Yes (if using a BBK with a significantly heavier piston)
2). High Compression - Yes (If the compression ends up very high)
3). Power Output - (Kinda, but this is related to one or more of the other 3 choices)
4). High RPM's - Typically when stock, these motors were not designed for High RPMS (most of them anyway, a few were)

Wikipedia shows the following.....

139QMB 49.5 cc (3.02 cu in) 2.95 hp (2.20 kW) at 7,500 rpm 39 mm × 41.4 mm (1.54 in × 1.63 in) 10.5:1

On most stock QMB139's, you're on the backside of the power curve after 7,500rpms.

If you go to YouTube and search "High RPM GY6" or "GY6 12K RPM", you'll find few (if any) results.  it's NOT common.

But which one specifically? Not sure that could be determined exactly. All of them contribute to the stress on the crankshaft.
Each one of the 4 affect the bearings and crank on every stroke (except compression on exhaust stroke).....but compression more so on the power stroke.

Good question.....hard to pin down a single answer imo.

Put another way, if you want to avoid crankshaft / bearing failure when installing a BBK.......
1). Select a kit with a piston as near OEM piston weight as possible
2). Keep compression down to reasonable levels

3). (Power output will self adjust according to the #2 and #4)

4). Don't over-rev your motor.

That said, if you find an exact answer, please let us know.

[90GTVert]

I think to know precisely what is the worst, you'd have to know all specs and probably have to have at least a degree in engineering and some free time on your hands.

What most of us know is that all of those will induce some level of stress to various components. If you wanted to know which is harder overall on the scooter, I'd say #3 because high power output will stress darn near everything. That's not the question though.

Keep all of those specs within reason if you want it to last long. To me, everything you do to increase any of those over stock will increase stress on components. A lot of this stuff is a compromise and you have to figure out where you want to fit. There's a reason that race machines travel on trailers.

Max Reliability <<<<<<<<<<|||>>>>>>>>>> Max Performance
Stock Engine    <<<<<<<<<<|||>>>>>>>>>>       Race Engine

That's a bit over-simplified, but the basic idea.

[KSR Moto]

Great answers and explanations,exactly what I was looking for😀

[scooterpimp]

Usually bbk in 47mm (80ish cc) & smaller are less abusive on stock crank bearings etc. Have a couple 80 cc bbk that i did for people hitting 1500 & the other a bit higher. One done @ 300 miles other done at 150 miles. But they are easy on them. (Unlike me) just my 2 cents worth.

[golfmech]

47mm with A9 cam on a stock head is a good combo. i paid $30 for the bbk and $15 for the cam thats only 3/4 of a tank of gas in my superduty lol.

[GrumpyUnk]

My vote is for the weight of the piston. 10,000 times a second, or 8,500 or 8,000 or 7,500 ... it must accelerate from stop at the bottom of the stroke, accelerate to full speed, then slow down to zero. Then accelerate again to top speed, then back to zero. That's one cycle.
The weight/mass accelerated produces force: f=ma. Force equals mass times acceleration. You added more mass with a BB piston in almost every case(not seen the 47mm that matches a 39mm mass), thus increasing the force, even at lower rpms. With less than stellar components, {connecting rod, bearing, crankshaft}, the change likely exceeds designed spec. Unless the machine was really designed for 72cc displacement.
If you figure out the acceleration needed to cycle at 8,000 rpms, 0-max-0 within (8,000rpm/60sec/min)=133.333 cycles per second. Take the stroke, do some calculus(I have forgotten how) and you can get the instantaneous force needed to accelerate that mass. The numbers will astound. How the numbers increase with increased mass will astound further. Any budding engineers out there?
tom

[jbjhillbilly]

I had forgotten about these. NCY makes a GY6 piston with the skirts cut down to reduce mass/weight. Even though it's for a 150cc 4t, the concept should be the same for 49cc-sized engines.


Anybody want to give it a go?

[GrumpyUnk]

That looks like a racing piston, and likely resembles the pistons currently used in current 'performance' vehicles.
I think the piston crown would need to be cast with more 'meat' than a similar piston with full skirting. I am sure there is a tradeoff.
What does it look like from the bottom? The metal around the wrist pin boss looks to have lost a lot of weight, so I bet that's where the major decrease is centered. El cheapo BBK pistons have full metal near the outer diameter of the piston near where the retaining clip is fitted to its groove. That whole area looks to have gone on a significant diet. Neat.
tom

[jbjhillbilly]

www.shop.theruckshop.com/images/1407520970079-1677389562.jpeg

[ratdog]

Of course it’s really all of the above.  Or “3” if you need to stick to your list.

weight of the piston is really not a killer, it the “out of balance”  that is doing the job.  If you took out the crank, and had it balanced to the new piston, I’d bet you would get less wear and tear then the stock piston.  Of course no one in their right mind is going to pay for something like this to be done on a moped, so we will never know.

higher RPM is a killer no matter if you go to a big bore kit or stock.  One of the biggest killers with higher RPM I’d to keep the rocker arm on the Cam.  With the stock valve springs the point where the rocker arms are jumping off the top of the cam lobes can come fairly early.  I haven’t seen any better valve spring kits in my searches, so I’m guessing that is sort of a “hard limit”. I’m sure this is why the CDI all seem to have the same max advance on the spark.  I’m guessing that the riders that mod the pick up to give a bit more advance, and therefore more RPM, kill their engines fairly rapidly

a personal opinion is that more people hurt their engines from bad assembly then anything else.  Of course I haven’t looked at them all, but I’m shocked that more don’t stress using assembly lube.  When your putting one of these together, you should be using a ton of assembly lube.  Of course on the bore, but also on the piston and rings.  But the most important one that no one seems to mention is the cam.  Those cam lobes should be thick with assembly lube as well as the bearings.  The cam assembly is small enough, I think I’ll just let it some in an oil bath until I’m ready, then put on the assembly lube!

IMHO, you should “break in” the motor with the idle set too high.  These engines don’t pump a lot of oil just sitting there at low RPM. I’ll bet a lot of people damage their “new” engines by just letting them sit there and idle in the first few hours of running.  Of course you should do a oil change soon after the build.  I would tend to to think you should think in terms of “hours” instead of miles for this first oil change. I’m thinking, about 5 hours of operation.  I would also suggest a oil plug with a magnet in it.

the other “assembly” step I have never seen in the vids is checking the ring end gap.  You can do this very quickly before you start the assembly.  I would also look at the condition of the cylinder walls.  If they don’t have a nice cross hatch patterned to them, get a hone and make a nice cross hatch pattern.  IMHO it is 100% necessary to have a fine cross hatch to get your rings to break in for best compression.

im willing to bet that most of these BBKs are installed without using a torque wrench.  I think it 100% needed to do the torque correctly.