Failure of Happy Time style 2-stroke Big End Connecting Rod Bearing

[Fabian]

It's all fairly self explanatory

The Chinese 2-stroke motorised bicycle engines that i have purchased from a Chinese distributor/retailer are all having similar problems with the big end connecting rod bearing failing in relatively short time.
They initally thought it was the crankshaft bearings failing so ordered high quality bearings to be fitted for their subsequent shipments but the engine failures continued.

When my first engine failed from a blown big end bearing and the second engine, now starting starting to make similar big end rattles, after being well looked after, had me develop a sense of frustration, to the point of wanting answers as to exactly which component of the bearing or it's construction is causing issues.

It just looks like the cage and the needle rollers are made out of very low quality material.

I've managed to find a supplier who can ship a high quality replacement K16x21x10 Needle Roller Bearing.
Once i've received the bearing, i'll have the crankshaft, replacement connecting rod and bearing assembly and piston sent to a reputable Kart engine rebuilder to have the crankshaft lightened, balanced and trued to finally achieve the goal of a reliable and smooth running engine.

Fabian

 

[Ol Shakey]

lightened crank

Hi Fabian,I would not lighten crank to much as it serves as a flywheel in these engines,balance it certainly,this wil help with most fatige failures,My motor has had minor balancing,runs much better than standard,2500 k's and revs to 6500 no problems so far

don

 

[Fabian]

Some Specifications for the Happy Time style 69cc connecting rod

 

[Fabian]

I've come accross a Suzuki connecting rod that is very close to the 69cc connecting rod.

Scroll down to Suzuki Connecting rods (page 252/253) and look for part number 03.3180
It's 14mm wide, which works out very well for the small end, but the big end would have to be machined down 2mm on each side from 14mm to 10mm.
The other obstacle to overcome is the 23mm big end diameter and 16mm small end diameter - a steel sleeve would need to be machined as a press fit for both the big end and small end to give the correct 21mm and 14mm for the original bearing sizes - can't see why this would be a major problem.

I have no idea what model of Suzuki motorcycle this aftermarket connecting rod is used in replacement of the original Suzuki part.
Still, it's a good option for a high quality item to replace the standard Chinese connecting rod.

http://www.pro-x.com/downloads/Technical.pdf


Fabian

 

[Fabian]

Ok, been busy doing research.

Looks like the Suzuki LT80 Connecting rod is perfect for the 69cc engine.
It has the exact 85mm centre to centre dimensions of the original connecting rod (perfect) and with some basic modifications: big end diameter sleeved down to 21 or 20mm depending on what bearing you choose to use, and machining the width of the big end down to 10mm.
The small end could be sleeved down from 16mm to 14mm if you want to use the original bearing "or" you could investigate this tempting idea:

using the Suzuki LT80 Piston which is 50mm.

This would require machining the original 47mm bore to 50.5mm and having the bore electroplated and honed to 50mm.
I will have to do more reseach on the exact dimensions of the LT80 piston to see if the writpin to piston crown is similar.
If it is similar, then you would not even need to sleeve the small end of the connecting rod and you would get the added bonus of having a beefier small end bearing.

Engine capacity would go from 69cc up to 78.5cc

Connecting rod: http://www.hotrodsproducts.com/index.cfm?fuseaction=cProducts.view&productid=221

Piston: http://www.mfgsupply.com/m/c/673M05000.html


Now that's an option, looking very much like Cherry Pie

 

[Fabian]

There is also something else i'm looking at: stroking the crankshaft.

Using an 18mm crank pin and line boring the crank pins holes in an offset manner, the stroke could be increased by 2mm.
This could require a slightly shorter centre to centre connecting rod ength but that can be achieved by offset grinding the big end sleeve.

Combine the extra 2mm stroke with a 50mm piston bore and engine capacity jumps to 82.5cc

 

[210061741]

Great work here.
How much dough is this gonna take.

I'm wondering what the real advantages of doing that would be.

What is the overall impact to the system?
More HP at the same RPM ?
Where would the Optimum Torque end up at ?
What would the max RPM be?
Would the crank need balanced to match the piston?

I seriously want to get to speed faster = higher torque
I seriously want to go faster = Better Gearing / Jackshaft

So if we can increase the effective Torque.
And increase the RPM abilities.
And make the motor last longer.
I'm sure we would have what we're after.

I spent the last week looking at high power high rpm motors.
Honestly after it all I think the HT is a more usable soloution.

I can't wait to see where you end up.
I know how much work is involved in searching out these things.

With great appreciation i will prolly follow you footsteps.

 

[Fabian]

Hi Rich

I am not looking for a huge amount of extra power; only need about an extra 25% to make the engine more usable. The nitrous kit will only be for overtaking purposes.
My logic is to increase engine capacity for added low and midrage torque.
I'll be using the original carburettor as this is an effective power restriction device because of it's low flow rate.

Having said that, the extra engine capacity will allow the engine to draw in more air at lower rpms, adding torque at those rpm's.
You won't have to rev the rings out of the engine to push the bike up hills.

Yes, you would need to get the crank balanced as a larger and heavier piston is being substituted for the original.
That doesn't worry me as i'm getting the crankshaft balanced and trued anyway.
What i do need to investigate, is the minimum allowable wall level thickness before cutting into the cylinder ports.
This will dictate what the maximum overbore can be.
The Yamaha YZ80 Wiseco piston comes in 0.5 oversize increments, i.e 47.5, 48.5, 49.5, 50.5
Im going to try for the maximum overbore and then have the cylinder impregnated with Nikasil for the hardest surface finish commonly available.

Obviously cost is an issue, but i'm more interested in reliability and smoothness, as i don't want my frame to crack from stress related vibration problems.

Just doing some numbers, the engine will cost somewhere between $700 and $900 when it's finished, including the cost of the engine and machining work and quality aftermarket parts.

I'm prepared to pay for quality and reliability and usability.

Fabian

 

[Fabian]

Handy Link:

Scroll down towards the bottom for a nice selection of crankpin sizes and pistons.

http://hosting.candlcompanies.com/2009 C&L Catalog for Vertex.pdf

 

[arceeguy]

.....I am not looking for a huge amount of extra power; only need about an extra 25% to make the engine more usable. The nitrous kit will only be for overtaking purposes......

....Im going to try for the maximum overbore and then have the cylinder impregnated with Nikasil for the hardest surface finish commonly available.

Obviously cost is an issue, but i'm more interested in reliability and smoothness, as i don't want my frame to crack from stress related vibration problems.

Just doing some numbers, the engine will cost somewhere between $700 and $900 when it's finished, including the cost of the engine and machining work and quality aftermarket parts.

I'm prepared to pay for quality and reliability and usability.

Fabian

Just my opinion, but I think that you'd be better off (and road legal in Australia) with a small motorcycle or scooter. You can buy a used postie bike (Honda CT-110) that would be super reliable and have better performance than any modified bicycle engine. I guess I am in the minority when I hop on my MB and putter around at 20mph. If I'm in a hurry, I hop on my motorcycle or get in a cage.

FWIW, it seems like our suppliers here in the US have not had any problems with lower end bearing failure.