Author Topic: Drag bike engine Q's bore x stroke & pics, a lost thread  (Read 2718 times)

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Offline Don R

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Drag bike engine Q's bore x stroke & pics, a lost thread
« on: November 26, 2013, 08:30:37 pm »
 The 750 drag bike project I bought is under way, I rebuilt everything on the handlebars and re-created the shortened back fender in chrome.  The engine is out and After thanksgiving I'll be taking it apart to check it, repair the kick start  and see how big it really is. The stepson of the PO said 1200cc. That might be 1180 rounded off, we'll see. I can see sleeve through the fins.

 I plan to measure the bore and stroke and check bearings, if it's a stroker crank (And it does have silver aluminum rods) how suitable might the rods be for limited street use? A set of similar used rods sold at the auction so they have probably been changed at least once. The cam is an Engle, it's ported and currently has no electrical system except the ARD mag.

 What else should I look for while taking it apart? I'll check the trans, I have a freshly backcut F trans in case I need it. 

Strangely, it has a 17/44 sprocket set, maybe to keep RPM's down at top end? The PO did borrow parts off it for his customers bikes.
« Last Edit: December 28, 2018, 10:22:12 pm by Don R »
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Offline Jerry Rxman Griffin aka MuthaF'er

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Re: Drag bike engine Q's
« Reply #1 on: November 26, 2013, 08:36:47 pm »
Don,

Not sure, you may have been watching but a NOS set of RC stroker rods just sold on EBay. You may be able to use that pic to ID your's. Whatever you have the rods should be fine on the street if it is a drag bike (unless I missed your meaning).

Does your bike have an ignition disconnect? Bill and Sam were denied at tech but they were approved when they drilled a hole in the kill switch handle, ran a wire through it and attached it to their glove with an alligator/roach clip. 
As of today 3/13/2012 my original owner 75 CB750F has made it through 3 wives, er EX-wives. Free at last.  ;-)

Offline 754

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Re: Drag bike engine Q's
« Reply #2 on: November 26, 2013, 09:00:39 pm »
How much did the stroker rods go for.
 He said silver the RC aluminum are gold.
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Offline Don R

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Re: Drag bike engine Q's
« Reply #3 on: November 26, 2013, 10:04:42 pm »
 Yes, I can see the bottom of a rod through the hole at the dyno, it's silver not gold. 
 
 It does have a kill switch made from a set of points with a piece of rubber between them and a cord tied to the rubber. Pull the rubber and the points ground the mag.  I'd like the fctory kill button to work also, it's the only stock switch left.
 
The PO's stepson says he wants to buy the bike back after it's done. I'd love to get it to start and run so it can make an occasional "easy" trip through town. No one has provided a picture so far but the PO was married several times, pics have a way of being trashed. Black and chrome were the colors, that's easy.  It seems to be a 1973 K2.
« Last Edit: November 26, 2013, 10:10:32 pm by Don R »
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Offline Jerry Rxman Griffin aka MuthaF'er

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Offline Medyo Bastos

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Re: Drag bike engine Q's
« Reply #5 on: November 27, 2013, 06:07:23 am »
i bought them, i have a golden set as well

Offline Don R

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Re: Drag bike engine Q's
« Reply #6 on: November 27, 2013, 08:55:23 am »
That's a heartbreaker because while I was fixated on buying the sandcast bike at the sale a set of aluminum 750 rods sold for $40. Two vac. gauge carb balancers went for $85. That's OK, I got the two bikes. I let the KZ100 get away for $1000 and a 3 wheel late model goldwing was $18,500.
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Offline 754

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Re: Drag bike engine Q's
« Reply #7 on: November 27, 2013, 09:34:23 am »
Some of the silver rods are only 150 -200 new.. So if they were used you may not have missed anything
Maker of the WELDLESS 750 Frame Kit
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It's All part of the ADVENTURE...

73 836cc.. Green, had it for 3 decades!!
Lost quite a few CB 750's along the way

Offline Don R

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Re: Drag bike engine Q's
« Reply #8 on: November 27, 2013, 09:44:16 am »
Ok, I don't think anyone's been in this engine since the 80's. The guys that remember it say the PO always covered it up if they stopped by his shop. He eventually banished it to a new storage garage out back. 

 I'm assuming if this is a stroker, the rod length and piston pin height may be custom. I'm hoping to not turn this into a major rebuild, Buuuut, it always seems to get out of hand when I open one up. L0L. 
« Last Edit: November 27, 2013, 09:49:07 am by Don R »
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Offline Big Jay

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Re: Drag bike engine Q's
« Reply #9 on: November 28, 2013, 09:53:19 pm »
Your motor probably has MTC rods in it.   The rods on the e-bay site says they are 10mm short. I can't imagine you could swing a 20mm stroker in a 750.

Offline Bill/BentON Racing

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Re: Drag bike engine Q's
« Reply #10 on: November 30, 2013, 10:38:41 am »
I agree with Jay. I hope you have MTC aluminum ones, best next to RC Golden Rods if going alum. Have a very good set of MTC ones, also have them in Purple Rain, my turbo bike. Yoshi alum rods were never meant to see big cc's,heavy pistons etc. Pops biggest kit was 823cc, Bill
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Offline Don R

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Re: Drag bike engine Q's
« Reply #11 on: November 30, 2013, 02:24:37 pm »
 Thanks, good to know. We got an extra job to do on the funny car chassis (new motor plates the engine's tilted) we're front halfing so I'm waiting until we're done grinding and making metal dust to split the cases.
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Offline Don R

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Re: Drag bike engine Q's
« Reply #12 on: January 06, 2014, 07:02:01 pm »
OK. I got the head off and with my flintstone tools it appears to measure approx 72mm x 69 mm. around 1120cc?

Four of the nickel washers were totally puckered and an O ring was pinched under the copper head gasket. The bores look nice
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Offline Tintop

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Re: Drag bike engine Q's bore x stroke #'s
« Reply #13 on: January 06, 2014, 08:17:02 pm »
If it's a 69mm stroke it's likely a CB900 crank.  With a 72mm bore that would make it 1124cc.
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Offline Don R

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Re: Drag bike engine Q's bore x stroke #'s
« Reply #14 on: January 06, 2014, 09:03:26 pm »
I'm thinking it's an old custom crank or welded stroker. The dyno taper is on the left end and the right end is driving the ARD mag like a 750 crank would. They called it a 1200, close enough for me.
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Offline Big Jay

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Re: Drag bike engine Q's bore x stroke #'s
« Reply #15 on: January 06, 2014, 09:13:31 pm »
It is probably a welded 750. Does it have a roller cam chain?

Offline Don R

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Re: Drag bike engine Q's bore x stroke #'s
« Reply #16 on: January 06, 2014, 09:53:42 pm »
It is probably a welded 750. Does it have a roller cam chain?

It did look different. I'll check tomorrow. The stray O ring under the copper head gasket made #3 leak. I'm hoping it didn't run long enough to torch the deck. Pics in my build thread. The way the nickel washers were puckered in I'm wondering if it ran with the crankcase vent plugged or something. It was vented into a Hamms can when I got it.
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Offline Tintop

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Re: Drag bike engine Q's bore x stroke #'s
« Reply #17 on: January 07, 2014, 08:25:49 am »
What kind of primary chain(s) is it using, that would settle the crank origins.
1977 CB550/4 Cafe - Speed Warrior / BOTM 03/11
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Whittaker GBF Vintage Racing Sidecar (XS750 power) - ITG / 151's / CMR Racing Products (SOLD)
1976 CB400 SS - stock / BOTM 04/11 (SOLD)
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77 CB550 Cafe build
550/750 Filter Thread
Sidecar Rebuild Thread

Offline Don R

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Re: Drag bike engine Q's bore x stroke #'s
« Reply #18 on: January 07, 2014, 02:38:48 pm »
 When I get it split I'll report back but it's not a 900 crank, I can put the stock dyno rotor on it and plan to cut one down so I can start it with an onboard huyabusa starter. Hopefully the rest will be in good shape.
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Offline bear

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Re: Drag bike engine Q's bore x stroke #'s
« Reply #19 on: January 07, 2014, 04:28:43 pm »
When I get it split I'll report back but it's not a 900 crank, I can put the stock dyno rotor on it and plan to cut one down so I can start it with an onboard huyabusa starter. Hopefully the rest will be in good shape.

If it's a welded 750 crank it maybe worth getting it checked.
We ran a couple many years ago and they had a limited life with our application.
Check for out of true and cracks.

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Brian
« Last Edit: January 08, 2014, 03:58:53 am by bear »
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Offline Don R

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Re: Drag bike engine Q's bore x stroke #'s
« Reply #20 on: January 08, 2014, 08:53:00 am »
Thanks Bear, I will get the crank checked. The PO was one of those guys that loved building Hondas and going to the track to see how fast he could go but he wasn't a real drag racer that went every week. I'm hoping it's got low runs on it.
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Offline Don R

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Re: Drag bike engine Q's bore x stroke #'s
« Reply #21 on: January 08, 2014, 01:28:58 pm »
The one pic that shows the most. I'll post some more in my build thread. Does the rod look familiar to anyone? I'm wondering about torque numbers for the rod nut.


« Last Edit: January 08, 2014, 09:16:14 pm by Don R »
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Offline Tintop

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Re: Drag bike engine Q's bore x stroke #'s
« Reply #22 on: January 08, 2014, 01:35:05 pm »
Looks like weld beside the rod end (stroking work?), and the twin chains would make it a 750 crank.  Piston looks interesting too. 8)
1977 CB550/4 Cafe - Speed Warrior / BOTM 03/11
1980 CB750F (project)
Whittaker GBF Vintage Racing Sidecar (XS750 power) - ITG / 151's / CMR Racing Products (SOLD)
1976 CB400 SS - stock / BOTM 04/11 (SOLD)
1973 CB750 K - basket case (SOLD)
77 CB550 Cafe build
550/750 Filter Thread
Sidecar Rebuild Thread

Offline Don R

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Re: Drag bike engine Q's bore x stroke #'s
« Reply #23 on: January 08, 2014, 03:12:36 pm »
They brazed in the balls that close the oil passages. The weld on the rod journal is machined back down.
 See more pictures on my project shop thread "drag bike resto"
« Last Edit: January 08, 2014, 03:15:18 pm by Don R »
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Offline Don R

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Re: Drag bike engine Q's bore x stroke & pics
« Reply #24 on: January 08, 2014, 09:18:00 pm »
Where the blown head gasket was on #3 there was a stray O ring under the copper head gasket, it was obliterated but enough to make a leak.


« Last Edit: January 08, 2014, 09:20:25 pm by Don R »
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