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

Pete_Muller

Moderator
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
 

Pete_Muller

Moderator
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

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Pete_Muller

Moderator
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
 

Brettm57

Member
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.
 

Pete_Muller

Moderator
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.

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sundog

Member
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.
 

tecbigdog

Site Supporter
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.
 

Brettm57

Member
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!
 

Pete_Muller

Moderator
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.

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