Why would someone spot weld the pin to the crank discs on a KT100 crankshaft assembly?

Peter, are you the gentleman that was making the brass threaded inserts for the KT 100 cylinder? Are they available? I recently did a 8mm TimeSert - I did not think those brass versions were available. I just discovered this forum.

Yep -- shoot me a private message.

PM
 
I see no signs of cracking. Prior to set up I did smooth out the welds so they would more or less flush with the plates as they were touching the sides of the vee blocks as I rotated the crank. Any axial movement would cause some error when indicating on the tapered surfaces.

When I true a crank, I will usually get it "quite close" in my truing stand, and then for the finish check, I have a set of brand new very high quality FAG bearings that I put on the crank, and then rest those bearing in the precision ground V-blocks. I may bump it around a touch while doing that final check (needless to say: I remove the bearings before smacking the crank).

Oh... final *sanity* check is always in the cases... where I just do a quick check after the lower end is assembled.

As an FYI: a copper hammer will not mark up the crank, whereas a brass one is usually a bit harder and will typically leave small dings in a Yamaha crank (they are not has hard as something like an IAME crank).

This is the exact hammer I use:
https://www.amazon.com/gp/product/B0001P0Y32/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1

PM
 
I've been meaning to take a picture of my crank truing stand -- finally did it today.

Per my message above, this is what I use to measure for getting a crank "close", and it's about as handy as any I've ever used.

I still do the final measuring with high-quality bearings on the crank, and it sitting in precision ground V-blocks.

PM


stand.jpeg
 
Thanks for the photo Pete! What crank bearings do you recommend for a Yamaha? I have a couple I'm going to tackle a rebuild on in the future.
 
Believe it or not, I always ran the OEM steel cage Koyo bearings in my own engines. (The ones that are standard/stock in a new Yamaha). They are actually a very nice bearing.

When I was in the engine building business, most every customer insisted on a "better" bearing (more accurately: what was *perceived* as a better bearing). Consequently, I had tons of new Koyo stock bearings in my shop that I had removed from brand new engines/cases that I was blueprinting.

So... that's what I have always used.

The perception in the engine building business is that a "looser" bearing is faster (in other words: a C4 bearing instead of a C3). There are a couple of issues that pertain to that: for starters, the specification for internal clearance in ball bearings has a significant "overlap". So much so that a C3 bearing on the "loose" side of its tolerance actually has more internal clearance than a C4 that is on the "tighter" side of its tolerance. So right off the bat: a bearing needs to be measured for internal clearance in order to "really know" what you have.
The next issue is: if an engine case is accurately machined (bearing bores perfectly in-line, and back faces perfectly parallel), then main bearings with more internal clearance are not required, and may actually be a detriment to how precisely the engine is rotating.

That said, the nicest bearings I ever "felt" were any of the FAG bearings that are actually made in Germany. Difficult to find these days.

PM
 
I'm very fortunate. I know a hell-of-a-lot of 'Hacks' that have won a LOT of National events. Sundog, you don't have a clue about what you are talking about........
Of course it was all because of the welded crank pin. That should fail a tech inspection btw. I've built my share of point championship engines. tecbigdog, I don't value your opinion.
 
Of course it was all because of the welded crank pin. That should fail a tech inspection btw. I've built my share of point championship engines. tecbigdog, I don't value your opinion.
Wow... It only took you 16 days to come up with that snappy response and I don't give a rats behind whether you value my opinion or not.
 
Believe it or not, I always ran the OEM steel cage Koyo bearings in my own engines. (The ones that are standard/stock in a new Yamaha). They are actually a very nice bearing.

When I was in the engine building business, most every customer insisted on a "better" bearing (more accurately: what was *perceived* as a better bearing). Consequently, I had tons of new Koyo stock bearings in my shop that I had removed from brand new engines/cases that I was blueprinting.

So... that's what I have always used.

The perception in the engine building business is that a "looser" bearing is faster (in other words: a C4 bearing instead of a C3). There are a couple of issues that pertain to that: for starters, the specification for internal clearance in ball bearings has a significant "overlap". So much so that a C3 bearing on the "loose" side of its tolerance actually has more internal clearance than a C4 that is on the "tighter" side of its tolerance. So right off the bat: a bearing needs to be measured for internal clearance in order to "really know" what you have.
The next issue is: if an engine case is accurately machined (bearing bores perfectly in-line, and back faces perfectly parallel), then main bearings with more internal clearance are not required, and may actually be a detriment to how precisely the engine is rotating.

That said, the nicest bearings I ever "felt" were any of the FAG bearings that are actually made in Germany. Difficult to find these days.

PM
Pete, Thanks so much for that response! I believe that explains what I've been thinking for a while now. What few Yamahas I have rebuilt I put the "better" bearing in, and it seemed like the bearing was worn out in no time. With just a few outings at the track I could wiggle the crankshaft up and down slightly in the bearings. I sat out kart racing completely for 30 years, but I didn't remember the bearings wearing out so quickly back then. My next rebuild will be with OEM bearings! Thanks again!
 
Brettm,

When you go to rebuild your engine(s), it's a good idea to measure the bearings bores to see how much interference each bearing will have. Very early permanent mold cases (what I would consider the "best" castings) had a LOT of main bearing interference -- often upwards of .0020". Ideally, those older style cases should probably be at about .0012" interferences, and hopefully no more than about .0014". The die-cast cases (what Yamaha has sold for the past ~25 years) can... and probably should be... maybe no less than .0013", and probably no more than .0015"-.0016".

Less interference can be run... however there is one thing to be aware of if running minimal interference: When running a "pipe" setup with a fairly radical exhaust pipe that requires 10,000+rpm slip, the engine/clutch needs to cool a bit before re-starting the engine and going back out onto the track. What happens is that with a setup like that, the clutch can get *very* hot, and when the engine is shut down and the karts sits for just a couple of minutes, the heat from the clutch can soak through the crank and really heat that side of the case (along with radiant heat from the clutch directly into the case from its close proximity). If the engine is re-started when the PTO side of the case is excessively hot, that is when a bearing will spin in the case. A few extra minutes of cool-down between runs never hurts.
(once again though: typically happens only if running on the loose side for bearing interference).

Also excessively tight bearing bores can put a significant "squeeze" on the bearing, and some of the internal clearance of the bearing will be taken away when the interference is excessive.

PM
 
A proper tig weld would need very little added metal . as you well know .
Knife edge balancing would hardley show a difference .
 
Well, here is what i did. I figure I have nothing to loose by trying to straighten them. I know I will eventually have to buy a new crank when the rod and bearing need replacement. I bought a 2 lb brass hammer at HF, and created a brass wedge from some rod stock. I made a sketch to understand the error component with the two indicators in phase. The out of phase component is more difficult for my simple mind so I made a little mock up of a crank assy. I got the idea from a guy on YT. That enabled me to figure out where to whack the crank half to correct the out of phase error. I approached it slowly, as it is kind of weird smacking something to straighten it. Having two indicators set up makes it easier to keep track of the relationship between the halves. I indicated as far out on the tapers as I could to maximize the error. The crank saver sleeves are in the Vees and I put some oil on on them to help make rotation easier. After about 45 minutes I got the indicated runout on each side within 0.0005 inches, which is good enough for me!

So I feel much better about running this crank and I got a little experience tweaking the assembly. Hopefully the pictures will show up in the post.

View attachment 22806View attachment 22807
The problem I have found with our cr250 stroker cranks is once they look the like photos above they always seem to return back to that shape.
 
Welding in crank pin = hack. If the pin is too loose, welding won't fix it. Also welding the pin usually makes it go out of wack.
My thoughts exactly.i have crankshaft ocd.i sent one to 2-different shops bc the first guy i lost faith in him.and i told LAD when i sent it to him,he text me back said it was .006 out both ways
 
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