connecting rod failure diagnosis: eric??

Joined
Aug 14, 2001
Messages
21
Likes
0
#1
what is the most common failure mode for a big end conrod bearing?

we just had a brand new crank installed, and only got 4 hours out of it! prior to this we lost the original crank to the same failure, but no upper end damage.

in this case it appears the crank migrated to one side (to the right) until it contacted the case. piston skirt is seized and plating damaged at the finger. (to replace or rebore, ah that is the question)

could not find any contamination in the intake tract at all.

we have not changed anything. did plug readings at mid and top, both excellent. no breaking up during operation at all.

32:1 maxima superM, sunoco ultra 94 (same for 2 years)

dont want to lose another!

thanks,
 

EricGorr

Super Power AssClown
Joined
Aug 24, 2000
Messages
708
Likes
1
#4
Sounds like a classic case of "pinched crank". Installing a new crank is a delicate matter. I use this method;
1) Get a chunk of aluminum rod stock that is near the same diameter as the outside diameter as the main bearing's iinner race.
2) Heat the aluminum with a propane torch for 5 minutes. Place the piece on the inner bearing race in order to transfer the heat and expand the race.
3) Ooops I forgot to mention to put the crankshaft in the freezer for 5 hours first so it contracts.
4) With the crank journals contracted and the bearing race expanded the crankshaft should drop into the right side main bearing without any trama.
5) Repeat the bearing race heating procedure for the left side case.
6) Kaw makes a special tool that wedges and supports the inside of any crank so the flyweights don't shift out of parallel during installation. We call that "pinching" or "wedging".
7) Now try to install the left side case onto the crank. I prefer to use the Suzuki crank tool which allows you to draw-on the case half without impacting the crank.
8) Once the cases are bolted together, the Suzuki draw-on tool and a case-splitter can be used alternately to align the crank in the cases. Use a feeler gage to measure the gap between the flyweights and the cases.
9) Aligning the crank in the cases also serves to reduce the side load on the main bearings and reduces break-in tiime plus insures durability.

I demonstrate this method of crank installation in my Two-Stroke Engine Rebuilding video, its the best $25 investment you'll make in your home workshop.

Regarding Kaw cranks, I don't think they could find a cheaper bearing to hang on the big end. My suggestion, have someone rebuild it with a Hot Rod kit. That bearing is far superior to any OEM alternative and the rod is forged rather than cast like OEM or IMS rods. I know its tempting with Kaws just to replace parts like the crank and cylinder because they're so cheap, but thats 2-fold and you really get what you pay for.

Good luck, Eric:)
 
Joined
Aug 14, 2001
Messages
21
Likes
0
#5
thanks eric.

now that we have two "spares" i will be having them assembled and trued with the IMS kits. we used one on his 60 and the longevity was amazing.

what do you recommend doing with the scored cylinder? he runs the 85cc and supermini classes. lots of options!
 
Joined
Jul 6, 2001
Messages
6
Likes
0
#6
Regarding oem and IMS connecting rods, these rods are forged,not cast. The Hotrods are forged, too, in the same country as the IMS rods, Taiwan.
IRONDOG
 
Joined
Jan 25, 2001
Messages
1,396
Likes
0
#7
Have to be a little careful at the crud end of the market. ‘Long’ brand rods have long been known for having terrible bearings & the rods were merely adequate. Mine snapped quite nicely in half cutting the engine to bits yet the clearances were just fine. Hmm . . .
Was a bit disappointed when I found in a Hotrod kit that they had re wrapped a Long brand rod.

OK so this isn’t for MX stuff I think they just sub out the playbike stuff, but worth looking at what you are buying for old banger bikes if you are intending to thrash them.