Topic: cb750 aluminum rods stretch help!!

[73nancy]

ok guys first post in the racing forum!! i just recently picked up a k3 750 race bike from a older gentleman that built and raced the bike a few times. the specs on the motor are big bore 911 race cam don't know what it is. 13:1 compression with dyna ignition, aluminum rods. other parts in engine not sure on. So the bike was road raced a few times before he found it was to heavy for him and built a stock class cb350. i was told that the motor ran great but he said but near the end of the races the rods would stretch and close the plug gap. he said no damage was done it would do it just enough to make the motor almost quit. my plans for this bike is i want to convert it to a drag bike i have a extended swing arm and a air shifter from the same guy. the bike has been sitting since 03 and i have yet to mess with it yet. so my question is what should i do about the plug problem. i was thinking shimming the plugs out but i don't know how far since it has good plugs in it? any help from you guys would be great would love to get this bike to the track at some point down the road. never drag raced before always wanted to. thanks ryan

[Retro Rocket]

Change the rods, its not the spark plug you should be worried about, its the VALVES, hit one of those and its all over. CycleX has super rods for around 600 USD

http://www.cyclexchange.net/Cranshfts%20and%20Connecting%20Rods%20Page.htm

It will save you a lot of grief further down the track...

[Big Jay]

If you are not running nitro, you don't need aluminum rods.

[turboguzzi]

aluminum natural fatigue limit + unknown engine history = steel rods.

a no brainer IMHO

[73nancy]

The guy I bought it from owned a honda dealership been working on bikes a long time. The told me they were $800 rods about 15 years ago. Hate to just take them out. The work he did to it is not backyard so I trust that it was built bad. For a drag bike would it stretch like that compared to a road racer which is run a lot longer? Is there anything I can do or should I drop compression down

[754]

At one point in the late 70,s or early eighties NGK came out with a new plug, may have been the U tip.
 I put them in my 900cc single cam Honda, and the pistons hit the plugs closing the gap, put normal plugs back in, and it was fine..
 the ?900? turned out to be an 836 using cut down CB 350 pistons, they do have high domes.
 Just mentioning it ..if clearance is quite close (due to parts used) shimming or changing plugs might make a difference..

[Sam Green Racing]

Ryan, if it happens near the end of a race when the motor will be stinking hot, could it be that the rods are expanding due to the very high temprature and not actually stretching. I would have thought that if the rods stretched they would stay stretched and not return to normal size and ready to race again.
Thoughts anyone

Sam.

[mec]

they return to normal size only if they are made from rubber.

mec

[Sam Green Racing]

What about ELASTIC then.

[73nancy]

I will call the guy to see what details I can get from him.  I don't want to blow it up. He did tell me he bought big bores for all his bikes since it way just as cheap as nos pistions back then. I might try to shim the plugs up a bit to start the bike see what the motor sounds like. Do most big bore pistions have valve reliefs? What would be the draw back of moving the plug up? Is there a plug that might work that is one of those weird center fire ones. Thanks for the help guys

[turboguzzi]

nobody is saying that the guy didnt do a good job.... but as you are stating yourself, they are giving already "some problems". so maybe space the plugs just to hear it running, but seriously, with 13:1, would lift the head and have a good look at piston to head and piston to valve clearance with a plasticine check.

taking an engine apart for no apparent need, specially on a 750 sure sucks. welding broken cranckases sure sucks more

[73nancy]

nobody is saying that the guy didnt do a good job.... but as you are stating yourself, they are giving already "some problems". so maybe space the plugs just to hear it running, but seriously, with 13:1, would lift the head and have a good look at piston to head and piston to valve clearance with a plasticine check.

taking an engine apart for no apparent need, specially on a 750 sure sucks. welding broken cranckases sure sucks more

yeah i know what your saying it does suck to have to do it though. so i think im going to shim the plugs start the bike let it idle see if there is anything else it might need while i will have it apart. then pull the engine. So my question would be to you guys plasticine how does it work? can you check it with the engine out turning it over by hand?  Also 13:1 is that common for a lot of 750 race motors? thanks a bunch guys really helpful.