Author Topic: Cobra motor disassembly and rebuild  (Read 107620 times)

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Offline Bill/BentON Racing

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Re: Cobra motor disassembly and rebuild
« Reply #25 on: December 16, 2015, 08:43:42 PM »
With ring package, 220 grams, pin 39 grams.
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Offline dragracer

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Re: Cobra motor disassembly and rebuild
« Reply #26 on: December 16, 2015, 09:06:49 PM »
Dude, you're finally making me happy. Should have pulled the whole damn thing apart to begin with. Remember, when you buy a used engine, you go into the purchase knowing its just a parts motor. I don't care what the previous owner tells you about its condition. I'm speaking from experience here. And know you've gone through this at least two times already - got it now??

Offline gschuld

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Re: Cobra motor disassembly and rebuild
« Reply #27 on: December 16, 2015, 09:24:15 PM »
With ring package, 220 grams, pin 39 grams.

Good deal.

Were the pins held in position with Teflon buttons?

George

Offline dragracer

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Re: Cobra motor disassembly and rebuild
« Reply #28 on: December 16, 2015, 09:31:50 PM »
So what size bore???

Offline gschuld

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Re: Cobra motor disassembly and rebuild
« Reply #29 on: December 16, 2015, 09:41:45 PM »
They are RC Arias 73s Frankie.  Stamped:

 RC ENG
   73H

under the crown.

George

Offline dragracer

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Re: Cobra motor disassembly and rebuild
« Reply #30 on: December 16, 2015, 10:01:44 PM »
Those Arias 73mm pistons I have are 190grams. Didn't weigh the rings or pins. In fact the 72mm KZ pistons I have are only 208grams with the rings, pin was 18 grams. Wisecos. What a difference the years make in piston design and material weight.

Offline gschuld

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Re: Cobra motor disassembly and rebuild
« Reply #31 on: December 16, 2015, 10:07:11 PM »
Pin was 18 grams?  Misprint I hope... 

FWIW, the old ring packs for the RC and MTC pistons average 23 grams per piston.

George

Offline Bill/BentON Racing

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Re: Cobra motor disassembly and rebuild
« Reply #32 on: December 16, 2015, 10:33:51 PM »
Stocker around 40 something . Lightweight ones usally about 10 grams less.
BentON Racing Website
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Over 35 years of experience working on vintage motorcycles, with a speciality in Honda SOHC/4 with a focus on the CB750 and other models as well from 1966 - 1985.
______________________________________
1993 HRC RS125 | 1984 NS400R | 1974 Honda CB750/836cc (Calendar Girl) | 1972 CB 500/550 Yoshi Kitted 590cc | 1965 Honda CB450 Black Bomber | 1972 Suzuki T350 | 1973 88cc | Z50/Falcons Pit Bike | 1967 CA100| 1974 CB350 (400F motor)...and more.
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Offline dragracer

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Re: Cobra motor disassembly and rebuild
« Reply #33 on: December 16, 2015, 11:30:08 PM »
Pin was 18 grams?  Misprint I hope... 

FWIW, the old ring packs for the RC and MTC pistons average 23 grams per piston.

George

I'll weigh it again tomorrow but I'm pretty sure that's what it weighed.

Offline NickO

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Re: Cobra motor disassembly and rebuild
« Reply #34 on: December 17, 2015, 01:19:50 AM »
Some epoxy filler from the inlet port broke loose and jammed in the chamber?

Offline Bill/BentON Racing

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Re: Cobra motor disassembly and rebuild
« Reply #35 on: December 17, 2015, 05:09:07 AM »
Close. It's a piece of foreign material. We use them all the time......Let's see engine out,been in engine room for a couple months , plugs out....hmmmmm checked with boroscope and blew air in plug hole... should have opened a valve and blew through there. The air moved it that you could see. So what is it? Pic later, Bill
BentON Racing Website
OEM Parts | Service | Custom Builds
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Over 35 years of experience working on vintage motorcycles, with a speciality in Honda SOHC/4 with a focus on the CB750 and other models as well from 1966 - 1985.
______________________________________
1993 HRC RS125 | 1984 NS400R | 1974 Honda CB750/836cc (Calendar Girl) | 1972 CB 500/550 Yoshi Kitted 590cc | 1965 Honda CB450 Black Bomber | 1972 Suzuki T350 | 1973 88cc | Z50/Falcons Pit Bike | 1967 CA100| 1974 CB350 (400F motor)...and more.
______________________________________
See our latest build 'Captain Marvel' CLICK HERE

Offline MRieck

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Re: Cobra motor disassembly and rebuild
« Reply #36 on: December 17, 2015, 05:24:21 AM »
I never liked that. Going to replace em with better ones, better head nuts also and cylinder stud kit. New cam chain and primary's etc, new mains also. This 1/2 ready for a bath. I cleaned my lens. BF chew toy closest so far!! Closest guess wins a John Stein Drag Racing book!
I have to say the big ends of #2 & #3 rods weigh a noticeable amount less after grinding those steel rod bolts down. Whoever built it should have cut all the bolts to the same size. Why bother balancing the crank?
 This is the kind of stuff I have noticed when disassembling old performance motors.....a lot of sloppy, get it done attitude work.
  Maybe it is ticky tack but it bugs the crap out of me and, as Frank said, demands the complete disassembly of any of these things.  ::)
« Last Edit: December 17, 2015, 05:26:37 AM by MRieck »
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Offline bear

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Re: Cobra motor disassembly and rebuild
« Reply #37 on: December 17, 2015, 05:39:29 AM »
I have to say the big ends of #2 & #3 rods weigh a noticeable amount less after grinding those steel rod bolts down. Whoever built it should have cut all the bolts to the same size. Why bother balancing the crank?
 This is the kind of stuff I have noticed when disassembling old performance motors.....a lot of sloppy, get it done attitude work.
  Maybe it is ticky tack but it bugs the crap out of me and, as Frank said, demands the complete disassembly of any of these things.  ::)

Yep gotta agree. You might be able to bolt on HP, but you can't bolt on reliability.
That comes from doing the little things right.
The older I get the faster I was.

Offline MRieck

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Re: Cobra motor disassembly and rebuild
« Reply #38 on: December 17, 2015, 06:07:36 AM »
Pin was 18 grams?  Misprint I hope... 

FWIW, the old ring packs for the RC and MTC pistons average 23 grams per piston.

George

I'll weigh it again tomorrow but I'm pretty sure that's what it weighed.
2 teflon buttons weigh 10 grams Frank. ;D
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Offline gschuld

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Re: Cobra motor disassembly and rebuild
« Reply #39 on: December 17, 2015, 06:10:26 AM »
You might be able to bolt on HP, but you can't bolt on reliability.


That's a good one ;)

George

Offline dragracer

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Re: Cobra motor disassembly and rebuild
« Reply #40 on: December 17, 2015, 06:31:02 AM »
Pin was 18 grams?  Misprint I hope... 

FWIW, the old ring packs for the RC and MTC pistons average 23 grams per piston.

George

I'll weigh it again tomorrow but I'm pretty sure that's what it weighed.
2 teflon buttons weigh 10 grams Frank. ;D

I'm probably way off the mark on my pin weight then. Lol. A mind is a terrible thing to waste. Blame it on old age.

Offline NickO

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Re: Cobra motor disassembly and rebuild
« Reply #41 on: December 17, 2015, 07:01:58 AM »
I never liked that. Going to replace em with better ones, better head nuts also and cylinder stud kit. New cam chain and primary's etc, new mains also. This 1/2 ready for a bath. I cleaned my lens. BF chew toy closest so far!! Closest guess wins a John Stein Drag Racing book!
I have to say the big ends of #2 & #3 rods weigh a noticeable amount less after grinding those steel rod bolts down. Whoever built it should have cut all the bolts to the same size. Why bother balancing the crank?
 This is the kind of stuff I have noticed when disassembling old performance motors.....a lot of sloppy, get it done attitude work.
  Maybe it is ticky tack but it bugs the crap out of me and, as Frank said, demands the complete disassembly of any of these things.  ::)

You and I sound like we could put the world to rights Mike, the older I get the more pedantic I become? I look forward to sharing a beer sometime maybe....who knows?

Happy Christmas BTW.

Offline NickO

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Re: Cobra motor disassembly and rebuild
« Reply #42 on: December 17, 2015, 07:03:18 AM »
Close. It's a piece of foreign material. We use them all the time......Let's see engine out,been in engine room for a couple months , plugs out....hmmmmm checked with boroscope and blew air in plug hole... should have opened a valve and blew through there. The air moved it that you could see. So what is it? Pic later, Bill

Discarded ear-plug maybe (or a Leprechaun)?

Offline madmtnmotors

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Re: Cobra motor disassembly and rebuild
« Reply #43 on: December 17, 2015, 07:29:09 AM »
Almost looks like one of the valve keeper wedges? Dropped down the spark plug hole during assembly?
TAMTF...


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Offline 754

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Re: Cobra motor disassembly and rebuild
« Reply #44 on: December 17, 2015, 09:58:45 AM »
Looked a bit like a dowel to me , maybe like the camholder size...
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Re: Cobra motor disassembly and rebuild
« Reply #45 on: December 17, 2015, 10:03:19 AM »
Close. It's a piece of foreign material. We use them all the time......Let's see engine out,been in engine room for a couple months , plugs out....hmmmmm checked with boroscope and blew air in plug hole... should have opened a valve and blew through there. The air moved it that you could see. So what is it? Pic later, Bill

Roach clip?? 
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Offline Medyo Bastos

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Re: Cobra motor disassembly and rebuild
« Reply #46 on: December 17, 2015, 10:11:53 AM »

[/quote]

Roach clip??
[/quote]

maybe if he was in denver


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Offline gschuld

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Re: Cobra motor disassembly and rebuild
« Reply #47 on: December 17, 2015, 10:13:18 AM »
Close. It's a piece of foreign material.  We use them all the time......, Bill

Roach clip??

LOL.  Damn that was funny... ;D

George

Offline MRieck

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Re: Cobra motor disassembly and rebuild
« Reply #48 on: December 17, 2015, 10:23:41 AM »
I never liked that. Going to replace em with better ones, better head nuts also and cylinder stud kit. New cam chain and primary's etc, new mains also. This 1/2 ready for a bath. I cleaned my lens. BF chew toy closest so far!! Closest guess wins a John Stein Drag Racing book!
I have to say the big ends of #2 & #3 rods weigh a noticeable amount less after grinding those steel rod bolts down. Whoever built it should have cut all the bolts to the same size. Why bother balancing the crank?
 This is the kind of stuff I have noticed when disassembling old performance motors.....a lot of sloppy, get it done attitude work.
  Maybe it is ticky tack but it bugs the crap out of me and, as Frank said, demands the complete disassembly of any of these things.  ::)

You and I sound like we could put the world to rights Mike, the older I get the more pedantic I become? I look forward to sharing a beer sometime maybe....who knows?

Happy Christmas BTW.
Thanks Nick and the same to you. I don't think I'm up to the job of putting the world to rights but a motorcycle engine is a task I can handle. ;D
 And I shouldn't just say "older" performance engines.....I have seen some real bonehead stuff in modern engines too ;) ;D
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Offline dragracer

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Re: Cobra motor disassembly and rebuild
« Reply #49 on: December 17, 2015, 10:25:36 AM »
Come on Bill, go ahead and let us in on the truth. I know you were waiting for someone to beg you. Lol