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Author Topic: CB750 K1 - piston shat itself pics included, now looking for parts  (Read 2425 times)

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Offline Ace

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CB750 K1 - piston shat itself pics included, now looking for parts
« on: January 07, 2011, 10:31:29 PM »
G'day all,

After my last topic about the #2 spark plug I decided to tear it down.  Just as well I did as the piston has broken apart.  A ring may have caused this, I'm not sure.  I have bits of the piston/ring embedded in the head so I think that's gone and it was original K1.  Pistons are wiseco 836 and on the up side I noticed that I had a good mid range cam by Ivan Tighe here in Australia.  Funny as I was going to put that cam in the bike.

Barrels don't look like they have any marks on them so I might get away with a hone.  Would another wiseco 836 kit fit in or do I have to get another set of barrels?

Who out there has a K1 head in bolt in condition hopefully ported/polished?

Should I tear down the rest of the engine and clean it all out?  Draining the oil revealed only tiny metal particles.  Might drop the sump pan.

Who out there had a set of barrels, pistons, head already to go and would part with them?

I'm trying to keep the original look of the entire bike so later model heads won't work.  Any help is greatly appreciated!!!

Thanks.
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1971 CB750 K1 - Sold
1978 CB750 F2 Supersport - Sold
1981 CB900 Bol d'or - Sold
2006 CBR1100 XX Super Blackbird - Sold

Offline dave500

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Re: CB750 K1 - piston shat itself pics included, now looking for parts
« Reply #1 on: January 08, 2011, 12:05:27 AM »
the piston land has failed,bad assembly or tolerances,check the dome hasnt hit the head,thats my bet,show us more  photos of the head,it might be salvagable.
« Last Edit: January 08, 2011, 12:07:11 AM by dave500 »
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Offline Ace

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Re: CB750 K1 - piston shat itself pics included, now looking for parts
« Reply #2 on: January 08, 2011, 12:09:02 AM »
The picture of the head isn't clear.  I'll try and get another one tomorrow.  25,000 miles since last rebuild in 2001.  I was tuning it at the time, gave a few blips to 7k and noticed a knocking sound from LHS.

I'm hoping I can hemi the head and also then port/polish and all will be good...fingers crossed.
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1971 CB750 K1 - Sold
1978 CB750 F2 Supersport - Sold
1981 CB900 Bol d'or - Sold
2006 CBR1100 XX Super Blackbird - Sold

Offline dave500

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Re: CB750 K1 - piston shat itself pics included, now looking for parts
« Reply #3 on: January 08, 2011, 12:13:25 AM »
with the other still in good condition pistons,less the rings reassemble it and "plasticene"the clearance from piston to head,carbon build could have caused that damage if the pistons run too close,breaking the land.if rings break they mostly stay in place and you get a smokey motor.

this is too close,its a 500 with XL125 pistons,i had this head relieved in this area,the valve to piston is good,its the outer edge,notice the plasticene peeled away where its real close to the head.
« Last Edit: January 08, 2011, 12:16:52 AM by dave500 »
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Offline Ace

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Re: CB750 K1 - piston shat itself pics included, now looking for parts
« Reply #4 on: January 08, 2011, 01:55:13 AM »
There doesn't look like there is a lot of carbon around the head or on top of the pistons.  I've seen worse with no damage.  I can see what you are talking about in your pic.

Here are some more pics of the head.
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1971 CB750 K1 - Sold
1978 CB750 F2 Supersport - Sold
1981 CB900 Bol d'or - Sold
2006 CBR1100 XX Super Blackbird - Sold

Offline dave500

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Re: CB750 K1 - piston shat itself pics included, now looking for parts
« Reply #5 on: January 08, 2011, 02:01:48 AM »
that head might be ok,itll need machining to fix the pock marks and all chambers to be the same,are you going to rebore it?machining the chambers will lower the static compression ratio,but boring will bring it up again somewhat,where in australia are you?
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Offline Ace

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Re: CB750 K1 - piston shat itself pics included, now looking for parts
« Reply #6 on: January 08, 2011, 02:39:28 AM »
NT.
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1971 CB750 K1 - Sold
1978 CB750 F2 Supersport - Sold
1981 CB900 Bol d'or - Sold
2006 CBR1100 XX Super Blackbird - Sold

Offline Terry in Australia

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Re: CB750 K1 - piston shat itself pics included, now looking for parts
« Reply #7 on: January 08, 2011, 02:48:57 AM »
G'Day Adam, I replied to your PM, but I forgot to say that you probably don't need to strip the engine right down, just pull the oil pump and clean it right out, flush the bottom end with a gallon of degreaser, flush the oilways and blow them thru with compressed air, and I'll send you up another head, no worries. Cheers, Terry.  ;D
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Offline UK Pete

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Re: CB750 K1 - piston shat itself pics included, now looking for parts
« Reply #8 on: January 08, 2011, 05:08:06 AM »
Take a look at this from a klr 650, this one let go big time at about 80mph






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Offline ekpent

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Re: CB750 K1 - piston shat itself pics included, now looking for parts
« Reply #9 on: January 08, 2011, 06:04:25 AM »
Holy Crap that 650 really cut loose. Were you riding it at the time.?
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