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Discussion Starter · #1 ·
Ok so hear me out. I’m new to the motorcycle world, honestly only rode a handful of times. I recently purchased an 03 shadow spirit 750 with a nasty tick for $300. Bike is in decent shape except for that. I’m a mechanic by trade so I tore the engine down to find a bent rod. I started searching for a used engine found one 750 from an 04 went and found it locked up so I obviously didn’t buy. I have found a bunch of 600cc engines from similar year models and since I don’t have any attachment to the 750cc, is it possible to put the 600 in my chassis with minimal effort?
 

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1983 Honda vt750 Shadow
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Usually not an easy swap with many these V twins since some are 45 degree cylinders and others 52 degrees. And some are shaft drive and some are chain. Then there is the electric parts that have to go with the engine.
Not impossible just not easy.
Some engines on ebay.


 

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1999 Shadow 750 ACE VT750CD3 Modified
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Actually I have considered this the other way around, stuffing a 750 engine into a 600 frame.
I don't know for sure I think it should work the VT600 and VT750 are in the same family.
I'm guessing it will bolt up, the exhaust will probably require a rethink on the rear cylinder, the 600 is 4 speed and the 750 5 speed. The twin carb will probably work OK, the single carb 600 is a bit of a dog and probably wouldn't like pushing around 50 lbs of extra weight. You would definitely need the ICM from the 600 and you would need to rework the harness to suit.

There is probably other stuff that will pop out and bite, but I think @swifty2014 is right it is doable, but really if the project is worthwhile it is worth sticking the correct 750 engine in there.
 

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Ok so hear me out. I’m new to the motorcycle world, honestly only rode a handful of times. I recently purchased an 03 shadow spirit 750 with a nasty tick for $300. Bike is in decent shape except for that. I’m a mechanic by trade so I tore the engine down to find a bent rod. I started searching for a used engine found one 750 from an 04 went and found it locked up so I obviously didn’t buy. I have found a bunch of 600cc engines from similar year models and since I don’t have any attachment to the 750cc, is it possible to put the 600 in my chassis with minimal effort?
When you mentioned the "bad tick", the first thing i thought of was that you'd bought a bike that had eaten a dipstick. Have you by chance inspected the primary drive gear and the driven gear on the clutch basket?
 

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looks like 98 to 07 VT750DC ad 98-03 VT750CD used the same engine cases
 

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Discussion Starter · #8 ·
When you mentioned the "bad tick", the first thing i thought of was that you'd bought a bike that had eaten a dipstick. Have you by chance inspected the primary drive gear and the driven gear on the clutch basket?
291304
The rod on the right was causing the piston to slap the crankshaft you can see the marks on the crank. I’ve looked at the clutches and everything seems to look okay, I’m new to bikes but to my eye the only thing I found was the rod bent
 

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1999 Shadow 750 ACE VT750CD3 Modified
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That is pretty typical of these engines if they get hydrolocked.
There is enough grunt in the starter to do that kind of thing.
 

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Dang...!
 

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2002, Shadow Spirit 1100
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That is pretty typical of these engines if they get hydrolocked.
There is enough grunt in the starter to do that kind of thing.
I was wondering what might have bent the rod. Hydrolock would do it. Either gas passed the petcock and filled the cylinder or oil in the cylinder from being layed or stored on its side for a long time.
 

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1999 Shadow 750 ACE VT750CD3 Modified
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I was wondering what might have bent the rod. Hydrolock would do it. Either gas passed the petcock and filled the cylinder or oil in the cylinder from being layed or stored on its side for a long time.
It is a pretty rare in these engines as it has to be a perfect storm usually caused by fuel or even rarer, coolant. The 750 ACE and Spirit with the twin carbs have a low mounted electric fuel pump and the float bowl is below the throttle bore on the carbs. So the perfect storm, there is more than half a tank of fuel, the petcock is left open on the fuel tank, the fuel pump is stopped in such a position it allows fuel to flow through, the float valve on the carb is stuck open allowing the float bowl to overflow, the intake valve is partially open. The coolant problem I have only seen pictures of one "barn find" example with old acidic coolant and a rotted head gasket.
 

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Discussion Starter · #13 ·
It is a pretty rare in these engines as it has to be a perfect storm usually caused by fuel or even rarer, coolant. The 750 ACE and Spirit with the twin carbs have a low mounted electric fuel pump and the float bowl is below the throttle bore on the carbs. So the perfect storm, there is more than half a tank of fuel, the petcock is left open on the fuel tank, the fuel pump is stopped in such a position it allows fuel to flow through, the float valve on the carb is stuck open allowing the float bowl to overflow, the intake valve is partially open. The coolant problem I have only seen pictures of one "barn find" example with old acidic coolant and a rotted head gasket.
you know I did find that the petcock leaked fuel past it when I was removing the tank and I didn’t ask how long it had sat but the guy I bought it from got it from the previous owners widow. However, he did say it backfired and then started tapping and he just listed it for sale
 

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Ouch... Slapped the CRANK? At least the bearing surfaces are okay, or you’d have mentioned it.
I have a 2003 vt750 was running fine in the morning then that evening went ride was leaking fuel it started tapping noticed the leak changed the oil still tapping then it locked so from what you said it can be rebuilt
 

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1999 Shadow 750 ACE VT750CD3 Modified
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It is not worth rebuilding if a rod is bent other than an educational endeavor.
It is a selective assembly parts were selected for tolerances and weights when the engine was built.
Crankcase, crankshaft and rods. Grab a copy of the service manual to see what is involved: Manuals See what I mean?
Parts need to match and Honda warehousing for the VT750 other than rod bearings these days is very much luck of the draw whether a part will match the original build.
Add up all the prices including gaskets.
It is generally cheaper to replace with a verified good used engine and less down time.

That is the plan for mine, when it blows up. Bolt 'er in, swap everything over.
 

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What he just said. When rebuilding an engine costs a couple hundred dollars and some time, unless you like doing it for fun, and a new engine is a thousand or more, it makes sense. But if you can grab a good used engine online or from a scrapyard for a couple hundred dollars...
 

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If you get a new Honda made connecting rod the only thing that I would be concerned with is the weight of the rod. It would need to be the same weight as the original rod, may be better to buy two rods so they match.

My knowledge is from automobile engines so there may be specific bike knowledge that I don’t have. With car engines unless you are seriously racing a rod swap is a non issue.

As for cost, this is dependent on each persons ability and available tools needed for the job. If you don’t have the general knowledge and specific tools for the job then an engine swap makes more sense. The OP will probably not have a problem with this and personally I would rebuild the motor from the bike or swap out an identical motor.

Using something different can be done if you have the patience, knowledge, needed parts and the time to do it.

Of course if you would rather ride,,,just buy a new bike.

Eric


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