Yamaha KT100 Can Rebuild Prices/Intervals

Michael Milanovich · Joined: 19 Mar 2010 Posts: 156

Finally making the move to 2 cycle racing after about a year hiatous of 4 cycle racing. I am curious to see whats the average complete rebuild price of a KT100 is as well as whats the rebuild intervals? From what I head is guys will do a top end every few races adn they are pretty easy to do. I am going to be looking for an engine sometime next month and I am trying to purchase something that I can be competitive with and not have to spend $800 or $900 on a used engine and go and have to spend another $500 to $700 to get a rebuild 3 races later.

John Mulvihill · Posted: Sun Jan 20, 2013 6:36 pm Post subject:

Hi Michael,

I come from a roadracing background so this may not apply to what you are doing.

6 to 8 hours on the main bearings (8 hours for double row). 12 hours on the crank bearing.

About 4 hours on ring (O&H Gold--see below). 8 hours on piston.

Monitor compression to check on sealing of ring. Check up and down play of main bearings.

John
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Mike Unger · Posted: Sun Jan 20, 2013 7:41 pm Post subject:

When we ran club stuff we would check pression when it was new and monitor as the season progressed. On my gauge we would rebuild when the compression was less than 130psi. That usually was around 10-12 hours.

We would rebuild top ends at least twice as often as bottom ends.

Typical cost of a top end rebuild is $400 to $500.
Bottom end and top about $800.

Of course now that we are running national level where a tenth of a second is the difference between pole and 20th we rebuild a little more often.

al nunley · Joined: 13 Nov 2006 Posts: 3035

We had a “good” compression checker and when the rings were fresh, our KT’s would pump 160 psi. Anything lower than that and we put in a new ring, maybe with a new piston.
Big end rod bearings last a long time. Main bearings don’t. We replaced them after three weekends at the track. They’re cheap and easy to replace, do it often. If the bearing gets loose, the crank starts jumping and that is hard on the crank.
The better the crank run out, (under .001”) the better the performance.
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If the data does not support the theory, get a new theory. (Al Nunley)
All else being equal; Compression is the Holy Grail.
45 years, in and around karting

Michael Milanovich · Joined: 19 Mar 2010 Posts: 156

Thanks Guys. I just wanted to get a general idea. I worked in power equipment throughout college and with 2 cycles, I always checked compression. I have a few leads on some engines that I might buy.

John Mulvihill · Posted: Mon Jan 21, 2013 5:04 pm Post subject:

Hi Michael,

FWIW and IMHO:

One of the wear and tear issues with a Yamahama is the relatively soft crank. We turn them at a higher RPM than Yamaha ever intended. As a result, the crankshafts wear eccentrically where they ride in the main bearings. The wear is progressive and tends to kill high RPM.

There is a fix. O&H makes hardened sleeves (20 mm ID/25 mm OD) that can be pressed onto new crank halves. The sleeves are used with 25 mm ID ball bearings. These are best used with new engines/cranks since there can potentially be issues using them on worn cranks.

When I blueprint a new motor, I always install the O&H sleeves and 25 mm ball ID bearings, but that's me.

If the motor has been run and the crank is worn, it might need to be replaced for best results.

Re cranks: If you buy crank halves individually, they might not match up closely (crank pin offset). Hopefully, Yamaha matches up the halves if you buy a whole crank.

FWIW: I use the heavier rods, as they last longer before the bottom end goes oval.

Thems' my opinions. Others are welcome to leap in here.

John
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JIM SILVERHEELS · Posted: Mon Jan 21, 2013 5:11 pm Post subject: Mulvy

Mulvy,
your getting like someone else we know with those crank savers.. now the Snap On compression gage. TEE HEE
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John Mulvihill · Posted: Tue Jan 22, 2013 8:55 am Post subject:

Hi Jim,

Only because I have a 5 gallon bucket full of dead Yamahama cranks in the shop. I think I can get 12 cents a pound for them at a scrapper (I'm holding out for 15......).

Depending upon a lot of variables, you could easily go through 1-2 cranks a year and the motor won't want to RPM when they wear. Lots of bucks compared to using the sleeves.

Many years ago (before I knew better) I tried running with worn cranks and it was slow. Add the cost of a couple weekends to the comparison bill.

So when I see a guy new to two-strokes asking about Yamahama's, I climb up on my soapbox.......

BTW, how did the hair removal treatments go?

John
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Walt Gifford · Posted: Tue Jan 22, 2013 9:26 pm Post subject:

John Mulvihill · Posted: Wed Jan 23, 2013 8:52 am Post subject:

Hi Walt,

I won't sleeve a used crank myself if it has more than one weekend on it. I use Loctite when I press them on. Never had a problem on a new (or virtually new) crank with the sleeves walking.

O&H has sleeves with locating pin holes and locating pins, but if the crank is already worn eccentrically????? Evil or Very Mad

I picked up a carbide drill bit for the locating pins just in case. It drilled a nice hole on an old crank.

I've seen them tack welded in place but I'm not sure if that is a great idea.

John
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JIM SILVERHEELS · Posted: Wed Jan 23, 2013 3:40 pm Post subject: Mulvy

John,
so when you pin it, do you lightly peen the pin in to expand it? Never did a sleeve job b4 and the pin sits under the bearings so it cant work it's way out? RED LOCKTITE?
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John Mulvihill · Posted: Thu Jan 24, 2013 9:59 am Post subject:

Hi Jim,

The small supplied pins appear to be hardened. I don't think that there is any way to peen them over.

As to which Loctite, its always been the compound I had on hand. Any Loctite engineers on here?

I'm guessing that on a new crank probably the "penetrating" or "wicking" which is thin. Red on anything worn.

Loctite also has slightly different products for press fit application as opposed to the threadlockers we usually see.

BTW, the sleeves are slightly tricky to push on as everything wants to pop out to the side when you first start to press. I made up a "tube" to push them on straight.

John

JIM SILVERHEELS · Posted: Thu Jan 24, 2013 12:20 pm Post subject: Mulvy

Mulvy,
thanks for the reply, you mentioned you've seen the collars tack welded. Never weld anything that was hardened, it will crystalize and break. Silver solder might work on hardened steel though, haven't done much since I smartened up and got away from welding 35 yrs ago.

How's the dance classes going?
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John Mulvihill · Posted: Thu Jan 24, 2013 7:03 pm Post subject:

Hi Jim,

Never said 'tacking' was a good idea. Just that I've seen it done. Additional problem was getting enough clearance for the seal.

Dance lessons are going fine, the pole just keeps whacking me in the wrong place......

John
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JIM SILVERHEELS · Posted: Fri Jan 25, 2013 3:31 am Post subject: Mulvy

John,
when you drill an already soft crank doesn't it really weaken the structure starting rupture. Had this happen to us yrs ago when I let a guy put those on for me, snap ! Glad about the dancing, thought you'd have given it up, but of course, ya ain't a quitter.
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