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Old 10-27-2006, 12:15 PM   #1
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Default Buy New or Rebuild Existing Engine

just wondering what people's experiences have been with rebuilding wornout engines. Is it better to buy a new one or just replace rod,sleeve, and piston of the old engine. I've ran a N12LL3 for over 1yr and am pretty much pleased with it. Is there anything that I need to be concerned about in rebuilding the engine and is it worth it? I found RcMushroom to have rod, sleeve, and piston for $130 vs a new engine for approximately $300. This looks easy to answer based on numbers, but it looks almost too easy.
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Old 10-27-2006, 04:32 PM   #2
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I think that you have done well to get over a year on any engine. How many gallons of fuel have you ran thru it and what % of nitro?
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Old 10-27-2006, 04:53 PM   #3
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Quote:
Originally Posted by blanks596
just wondering what people's experiences have been with rebuilding wornout engines. Is it better to buy a new one or just replace rod,sleeve, and piston of the old engine. I've ran a N12LL3 for over 1yr and am pretty much pleased with it. Is there anything that I need to be concerned about in rebuilding the engine and is it worth it? I found RcMushroom to have rod, sleeve, and piston for $130 vs a new engine for approximately $300. This looks easy to answer based on numbers, but it looks almost too easy.
I perfer to buy new one (as your situation). Even you get new P/S/C, the crankshaft is still an used part and I believe the crankpin also worn out too since your sleeve has to be replaced.

$300 could go to a new Plus-12.

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Old 10-27-2006, 04:59 PM   #4
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Quote:
Originally Posted by blanks596
just wondering what people's experiences have been with rebuilding wornout engines. Is it better to buy a new one or just replace rod,sleeve, and piston of the old engine. I've ran a N12LL3 for over 1yr and am pretty much pleased with it. Is there anything that I need to be concerned about in rebuilding the engine and is it worth it? I found RcMushroom to have rod, sleeve, and piston for $130 vs a new engine for approximately $300. This looks easy to answer based on numbers, but it looks almost too easy.
Mad Max,
Don't forget to check your crankshaft, sometimes if the crankpin is worn out then you will need to replace it, also you want to make sure the bearings are also in good shape, if all parts totalled are still cheaper than a whole engine then go for the rebuild, so for the money you saved we can add it to the funding for the trophy girls.
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Old 10-27-2006, 05:11 PM   #5
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Mad Max,
Don't forget to check your crankshaft, sometimes if the crankpin is worn out then you will need to replace it, also you want to make sure the bearings are also in good shape, if all parts totalled are still cheaper than a whole engine then go for the rebuild, so for the money you saved we can add it to the funding for the trophy girls.
Sam,

Can you visually see that the crank pin is worn or do you have to use a digital caliper/micrometer? What amount of wear is considered "still good" versus "needs to be replaced"?
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Old 10-27-2006, 06:01 PM   #6
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Sam,

Can you visually see that the crank pin is worn or do you have to use a digital caliper/micrometer? What amount of wear is considered "still good" versus "needs to be replaced"?
I believe .001 is maximum clearance, and yes you need to use a micrometer to measure the I.D. bushing of the con-rod and the O.D.
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Old 10-27-2006, 07:26 PM   #7
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Hey guys. Let me put some "shade" on this topic.
We need some clearance between pin and connecting rod for first of all to have good lubrication (oil film has some thikness) and secondble we need to be able to put connecting rod on the pin when we assemble engine (this requirement make clearance slightly bigger then we wanted to) . On custom built engines clearance is way smaller (just good to have oil film there and have oil circulate little bit). But there is draw back-To assamble this kind of engine first we put connecting rod alone on the pin, then from the top we put piston with installed G-clip on front side (booster side) only, through the exhaust window we put wrist pin and then we put second G-clip also through the exhaust. It is pretty tricky procedure and required some skill and special tool which is not widely available. In this case usualy clearance designed to be 0.015-0.018 mm.
To make life easier we have to sacrafice clearance and for mass production engines clearance designed to be as follow:
In inital design of shaft and conrod every engineer put some clearance. I will give you sample of let's say 12 size engine. Crancksaft's pin 4.3 mm is laying in following tollerances 4.294-4.296 mm and hole in connecting rod is 4.325-4.330 mm, so clearance is in range 0.030-0.036 mm. This numbers give to us ability to lean connecting rod slightly and put it on the cranckshaft with piston install already on connecting rod.
As far as when we have to change crankshaft ( when pin conceder to be worn out) I would suggest to say if it measures 4.290-4.291 it is gone. Also needs to be measured if pin not eleptical. If make two mesurments ( 90 dereese to each other-vertical and horizontal positions) and find the difference-if it is greater then 0.008 then it is worn out and can't be use.
Visiualy we can see the following-if under the good light and magnifyed glass you find that pin has blue or dark brown spots ( regardless if it still measures right numbers)-then it is gone, if you can see that mirror on the pin doesn't look flat-then it is gone, if you can't see even mirror on the pin-it is gone.
I will try to make pics of my collection of cranksafts with wornout pins (with visible damge), but not sure if my camera can catch it. I will try anyway. If anybody has good camera and have worn out shafts-plaese let's halp each other and post your pics here. I was ale to catch with my camera only one visible damage
Yes to determinate correct number we need to have ID and OD measuring micrometers and it is pretty expencive tool. Good OD micrometer costs somewhere between $180-200 and tool to measure ID on the connecting rod is about $100.
I hope I was able to help somehow.
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Old 10-27-2006, 07:32 PM   #8
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As always Edward, you've been very helpful. What would we do without you??
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Old 10-27-2006, 11:07 PM   #9
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Quote:
Originally Posted by rmdhawaii
As always Edward, you've been very helpful. What would we do without you??
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Old 10-28-2006, 07:39 PM   #10
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Originally Posted by THE DOCTOR
I think that you have done well to get over a year on any engine. How many gallons of fuel have you ran thru it and what % of nitro?
Not quite sure on the # of gallons but I've ran 20% all year. I bought the engine on 10/22/05 and for some reason it is still tight. I spent a little over gallon on the break in process and after every run, I make sure that the piston sets near bottom dead center. I think I have at least 8-10 gallons through it.
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