PK 80 connecting rod big end bearing failures

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crmachineman

crmachineman

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May 24, 2012
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Hi everybody, it's Fred from CR Machine.
EEEK!

There appears to be a bunch of defective crankshaft assemblies coming from PK 80 bottom end assemblies which I have been purchasing from BGF. The last 3 units I have received from them have defective crankshaft assemblies. The defect is that the big end hole in the connecting rod was made too large. It is supposed to be 21MM, but it measures out to 21.13 MM.

It's entirely possible that the complete engines coming from BGF could have the same defect as well!

Until I start getting good connecting rods, or start making oversize crank pins, my engines will have to have the newer super rat crankshaft. Unfortunately that crankshaft has less volume, and doesn't produce as much power as the larger 5 piece cranks.

I do not know how many are out there, but there could be a lot. How many vendors are selling these defective units, I can't say, but I will try to notify BGF of this problem.

This excessive clearance is a big problem;
One of my motors I sent to a customer, had one of these crankshafts, and it failed within a few hours of operation, so it's a serious problem!

Thanks,
-Fred
 
biknut

biknut

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Sep 28, 2010
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I bought 3 motor kits from BGF with bad cranks last year. I was thinking it was the small end on mine that was the problem. It kept eating the small end bearings, even though I was using higher quality Japanese bearings. Close inspection revealed the small end of the rod wasn't machined very well. It vibrated badly out of the box too.

The second motor, the crankshaft was so badly out of true that the main bearings wobbled in the cases enough to wear the cases out in 600 miles. After the holes where the bearings are pressed in got loose enough the flywheel started scraping the crankcase making a god awful noise.

I dissasembled the third motor out of the box. The crankshaft was so out of true I never bothered to put it back together.

I now assume all the BGF/LEB motors are bad.

I like my Flying Horse motors a lot better.
 
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crmachineman

crmachineman

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May 24, 2012
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Right. So Biknut has experience problems with the small hole on the connecting rod. Right now, the last three engines I received from BGF were fine there, but the big end hole was oversize by about .005"! The problem connecting rods appear to be a copper color.

What a mess!!!

-Fred
 
biknut

biknut

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Even if you manage to get a motor with a good rod, I'm thinking the crankshaft will be badly out of true, and balance.
 
crmachineman

crmachineman

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May 24, 2012
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Yes, you are right, they always seem to be misaligned from getting forced together. Aren't they all like that? I know the GT-5 engines you get from Gasbike have distorted cranks too. They are both out of round too. I guess the only ones that aren't out of round are the super rat engines which use the later 3 piece crankshaft, but they are probably distorted like all the rest.
-Fred
 
BigBlue

BigBlue

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Nov 29, 2011
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Fred,

Sorry to hear of your problems, but it seems as the years go by, these engines get crappier and crappier. Mishandling (i.e., tossing onto a pile or into a box) the crankshafts after assembly can also cause problems. Hopefully your problems will be rectified by GBF.

Keep of the good work,

Chris
AKA: BigBlue
 
biknut

biknut

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I bought 4 cranks from 3 different venders. 2 were the kind with bolt on fly weights, and 2 were the three piece type. All of them sux.

I'm starting to think it's best to just not look too closely. That being said, my latest motor, a Flying Horse 80, when I took the head off before installing it on my bike, the crank looked pretty good compared to all the others I've checked. Since I put it in service, it's proven to be the second smoothest motor I've sampled out of only 7 motors. The 3 smoothest motors were all Flying Horse, but 2 of them were 50s.

I have one more new Flying Horse 80 on the shelf that I planning to use on my next build. If it's as smooth as the others, that tears it for me.
 
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crmachineman

crmachineman

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May 24, 2012
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New Hampshire
Hello everybody,
Well, the story gets more and more interesting. Don Grube has been very nice to write me a few long emails, telling the ins and outs of this engine business. According to him, there is a low cost competitor who appears to have a bit lower manufacturing standards. Rick for BGF buys from this particular competitor of Grubee who makes this "PK80" motor. I guess that is the right name for them. Well, anyway, they are 40 MM stroke engines (70cc) compared to the 38MM stroke engines you get from Grubee (66cc). I do like the power and torque of the PK80, however, obviously the connecting rod problem must be dealt with. I'm not terribly worried about that, as I have already ordered material for making my own oversize crank pins.
I think Rick from BGF will not say too much to me about this problem. I'm sure he is loosing sleep over it. The rumor is that he purchased a container of these bottom end units from this cheap manufacturer. Too bad; it's such a small detail, and the bottom end units would be perfectly fine if they had a better payed/skilled workforce in the shop. In the real world of machining, to sell a defective part, like a connecting rod hole being .005" too big is just a bit mind boggling! I offered to buy these no good units for a greatly reduced price, but he has not gotten back to me about it. I'm afraid He is stuck will a big bunch of bad goods. Either he sells them to me, and I fix them, or he continues to sell them to unsuspecting customers, knowing the units will fail within hours of operation. It's a real bummer; just imagine being in his shoes! He has two choices: sell them to me at his cost, making zero dollars, or continue selling them to customers, and have his reputation wrecked. Both cases and not so good for him. If I was him, I would be pissed off at that factory!
Maybe I should start buying my engines from Grubee.... Any suggestions where I should get my engines from?

Biknut: you mention this flying horse 80 engine, who did you buy that engine from?

Thanks a lot,
-Fred
 
crmachineman

crmachineman

New Member
May 24, 2012
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New Hampshire
Huffydavidson said:
Fred your so good at what you machine at your shop ,have you thought about machine us a crank ?I bet if you did it be a dandy .
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Aww man! I've already designed one; a stronger design per the request of Dave Rust of Arrow motorized cycles. I'm not sure when or even if I will actually make them. Cost is the main problem. My feeling is that I don't want to exceed $400.00 for a motor. I just don't know if I can make the cranks efficiently enough to able to keep the price of my engines under $400.00

The cranks would probably be in the neighborhood of $100.00 each!

At that price, I must ask myself: "how many will I sell per year?" Maybe a couple dozen? So, is it worth a month of my time building all of the tooling and all the machining?

-Fred
 
biknut

biknut

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Sep 28, 2010
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I would be willing to pay $100 for an accurate crank. IMO the crankshaft is the weakest link in a cg motor. It's the main reason why one motor is good, and the next one is total crap. All the other common problems these motors have, pale in comparison.

Give me a good crank, and I can make any cg brand into a good, dependable motor. The problem right now is, if you get a motor with a good crank, you're just lucky. I doubt more than 20% of cg motors have a good crank. That's probably even high.
 
Huffydavidson

Huffydavidson

STREETRACER/MANUFACTURER
Jan 29, 2012
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I talked to D.RUST and he was willing to sell me a Balanced crank for.$80.00 so Fred a$100.00 for the quality of your work is not unreasonable .you can put me on the waiting list .I got your money today pal .
 
crmachineman

crmachineman

New Member
May 24, 2012
259
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New Hampshire
biknut said:
I would be willing to pay $100 for an accurate crank. IMO the crankshaft is the weakest link in a cg motor. It's the main reason why one motor is good, and the next one is total crap. All the other common problems these motors have, pale in comparison.

Give me a good crank, and I can make any cg brand into a good, dependable motor. The problem right now is, if you get a motor with a good crank, you're just lucky. I doubt more than 20% of cg motors have a good crank. That's probably even high.
Click to expand...
Well said Biknut,
I couldn't have said it better myself.

-Fred
 
D

Drewd

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Jul 25, 2008
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May wish to consider a Dax engine. I bought one about 4 years ago and it is still running. At first it wasn't very powerful but after a very long and painful break in period, it has a lot more power than my newer "PK80" I purchased from pirate cycles. If Dax engines are stll the same as mine, I strongly suggest you consider them for your upgrades.

As for BGF, don't waste your time. He doesn't care and will continue selling a known crappy product and won't stand behind them.
 
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