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Old 04-24-2012, 07:40 AM   #1
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Default Repairing-resizing worn cranks

Is there anybody that offers a service to retrue crank pins and rebush used rods to fit. It seems a shame to spend 100 on a crank and 50 on a rod if somebody can offer this service for say 40 to 50 dollars. I know some racers have access to machine tools and this would be a nice money maker in this crap economy. How many out of spec cranks do you have sitting in your pit box?
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Old 04-24-2012, 08:02 AM   #2
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Is there anybody that offers a service to retrue crank pins and rebush used rods to fit. It seems a shame to spend 100 on a crank and 50 on a rod if somebody can offer this service for say 40 to 50 dollars. I know some racers have access to machine tools and this would be a nice money maker in this crap economy. How many out of spec cranks do you have sitting in your pit box?
it really isnt a case of just re bushing a Rod, or resizeing a crank pin.
these little engines pull serious RPM and the pistons are a tapered fit. so stress levels on the alloy in the rod and the metals in the crank are also to be considered. i for one wouldnt trust a rebushed Rod, as the alloy or ROD it self has been stressed. generally i do not rebuild my engines, i may put a set of bearings in and a few Rods in its life span, but once you start replacing Cranks due to being worn out, then its had a fair bit of fuel. you may find you will struggle with a rebuilt engine for tune, after putting through alot of fuel , or some other issues with crank cases etc. if you look after your engines, and replace a rod or 3 in its time, then when the time comes, it may be better for your racing or track time to just replace an engine complete. nothing worse than trying to chase an issue with an engine after a rebuild or 2 and waste that money when it could almost buy you a new engine in the first place. i think the time and effort put in to re bush a rod alone would be worth more than just buying a new one ready to put in.
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Old 04-24-2012, 08:23 AM   #3
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Anyone that has the capability to do so would also have the ability to cheat. And no, I don't care to elaborate on that
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Old 04-24-2012, 08:29 AM   #4
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Anyone that has the capability to do so would also have the ability to cheat. And no, I don't care to elaborate on that
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Old 04-24-2012, 08:39 AM   #5
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it really isnt a case of just re bushing a Rod, or resizeing a crank pin.
these little engines pull serious RPM and the pistons are a tapered fit. so stress levels on the alloy in the rod and the metals in the crank are also to be considered. i for one wouldnt trust a rebushed Rod, as the alloy or ROD it self has been stressed. generally i do not rebuild my engines, i may put a set of bearings in and a few Rods in its life span, but once you start replacing Cranks due to being worn out, then its had a fair bit of fuel. you may find you will struggle with a rebuilt engine for tune, after putting through alot of fuel , or some other issues with crank cases etc. if you look after your engines, and replace a rod or 3 in its time, then when the time comes, it may be better for your racing or track time to just replace an engine complete. nothing worse than trying to chase an issue with an engine after a rebuild or 2 and waste that money when it could almost buy you a new engine in the first place. i think the time and effort put in to re bush a rod alone would be worth more than just buying a new one ready to put in.
I think the same way. It would be a better spent money to buy a new engine.

I have changed the crankshaft of a OS VZ-B of mine, after two rebuils (conrod, piston, sleeve, bearings), but that thing had almost 15 gallons run through. It lasted for 7 gallons more (the carburetor body that looks like a block, but that is actually a two-piece eventually dismantled), and I don't regret that money, because it was my first "racing engine", but nowadays, I'd recommed buying a new engine other than changing a worn crankshaft.

And I think that even if re-bushing the rod and recovering the crank pin is less expensive and well executed, it's not as reliable as a new set.

Just my two cents.
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Old 04-24-2012, 11:39 AM   #6
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Default Rebush

I agree and disagree. With that service for 50 and a pinch for 20 that's a whole lot less expense than dropping 400 for a new engine.Years ago I used to race on road and there was a guy in our club that routinely resized and rebushed his engines. After talking with him on the subject he seemed to think that it was no big deal. He also told me that he consulted with Ralph Burch on the use of rebushed rods. All the problems that they had at the time was that the bushing would sometimes spin. That was corrected by pressing a retaining pin right in next to the bushing and the aluminum. Problem solved. I for one would like to have the option.
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Old 04-24-2012, 11:56 AM   #7
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In case of an RB .21 engine there comes a time you can use Novarossr R model rods (5mm) because the C-rod (from RB) is onle a few hunderds of a mm larger.
If you can find a serius grinder you can go to the P-rod type, I believe that one is 4.7mm, when thaking away that much material I think you have also tangen away the hard layer.....
On the other hand with +/- 5mm Novarossi pins you can also get a P-rod and with the right tools drill it out to the crankpin size.

I know some people have drilled out the pin and pressed a new pin, with good machines and tools it is simple to do so. From own experience I can say you have to find the right material pin because a simple needle bearing pin sounds that simple but has no flex and will break within a few tanks of use. A shaft from a good material drill or milling bit works like a charm but to be sure it will stay on its place you have to laser weld it from behind.
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Old 04-24-2012, 12:44 PM   #8
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Default resize

I measured my bad cranks and the worst are just about .003 out of round. That's in thousandths. (3 thousandths). I don't know how deep the hardening process is for the cranks. But it has to be more than just a couple of thou.
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Old 04-24-2012, 07:34 PM   #9
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You do realize that once you re-grind the crankpin the rod bushing must get matched to that specific crank? And that you cant just buy a rod made for that motor after that, you have to buy a rod with a bushing matched to that crank? You do realize that? Also, where did you come up with those dollar figures? Just out of thin air?

People seem to think that jusy because someone has the equipment it means its easy and cheap to make. Its actually more expensive to make 1(one) than many, you have to setup the machine to each individual crank. You have to know how to run a machine shop/machine in order to understand how that works. Just because it seems convinient to YOU doesnt mean it will convinient to make.

IMHO its just not a good idea.
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Old 04-25-2012, 03:50 AM   #10
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Default Resize

Thanks Roelof for the helpful information. I didn't realize there were stock undersize Novarossi rods. I guess a reamer could be used to enlarge the bushing a couple of thou.
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Old 04-25-2012, 03:52 AM   #11
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You do realize that once you re-grind the crankpin the rod bushing must get matched to that specific crank? And that you cant just buy a rod made for that motor after that, you have to buy a rod with a bushing matched to that crank? You do realize that? Also, where did you come up with those dollar figures? Just out of thin air?

People seem to think that jusy because someone has the equipment it means its easy and cheap to make. Its actually more expensive to make 1(one) than many, you have to setup the machine to each individual crank. You have to know how to run a machine shop/machine in order to understand how that works. Just because it seems convinient to YOU doesnt mean it will convinient to make.

IMHO its just not a good idea.
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Old 04-25-2012, 04:23 AM   #12
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Thanks Roelof for the helpful information. I didn't realize there were stock undersize Novarossi rods. I guess a reamer could be used to enlarge the bushing a couple of thou.
I believe there is also an E-rod with a crankpin of about 4.9mm

They are not undersized but only to make differences between brands and models.
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