Go Back  R/C Tech Forums > General Forums > Nitro Off-Road
question about top dead center >

question about top dead center

question about top dead center

Old 05-09-2009, 06:36 AM
  #1  
Tech Elite
Thread Starter
iTrader: (5)
 
Join Date: Mar 2009
Posts: 3,660
Trader Rating: 5 (100%+)
Default question about top dead center

My engine has good compression with the plug in. But if i take it out and rotate the flywheel to tdc its like the piston almost stays there and the flywheel has play in it like it will move back and forth about a eigth of a inch while it seems the piston is not moving at all. Almost like its clicking back and forth at that point without any compresion. But i can feel just a tad bit before that point. This is with the plug out. Is this normal? sorry for all the questions.
dreaux is offline  
Old 05-09-2009, 07:46 AM
  #2  
Tech Elite
Thread Starter
iTrader: (5)
 
Join Date: Mar 2009
Posts: 3,660
Trader Rating: 5 (100%+)
Default

bumpity
dreaux is offline  
Old 05-09-2009, 07:50 AM
  #3  
Tech Fanatic
iTrader: (4)
 
Join Date: Jan 2009
Location: Connecticut
Posts: 775
Trader Rating: 4 (100%+)
Default

I think that a small amount of play is normal, seeing as I just removed the glowplugs and moved the flywheel at TDC on 2 different motors.
howtowakeakar is offline  
Old 05-09-2009, 08:13 AM
  #4  
Tech Fanatic
iTrader: (2)
 
Join Date: Feb 2008
Posts: 778
Trader Rating: 2 (100%+)
Default

How old is this engine?
LosiMadMan is offline  
Old 05-09-2009, 09:02 AM
  #5  
Tech Lord
iTrader: (84)
 
Davidka's Avatar
 
Join Date: Oct 2004
Posts: 10,733
Trader Rating: 84 (100%+)
Default

A tiny bit of play is unavoidable, if it's excessive then the rod bushings are worn. A usual running symptom of this is an engine that otherwise runs well but idles inconsistently and goes through glow plugs quickly. If you think the play is more than a tiny bit, replace the rod.
Davidka is offline  
Old 05-09-2009, 09:48 AM
  #6  
Tech Adept
 
rc-mike's Avatar
 
Join Date: Feb 2009
Location: Dickson TN
Posts: 130
Default

Originally Posted by LosiMadMan View Post
How old is this engine?
I don't think it is very old but look at his other threads and you will understand.
rc-mike is offline  
Old 05-09-2009, 10:40 AM
  #7  
Tech Master
 
Join Date: Aug 2006
Location: Oceanside CA
Posts: 1,075
Default

Little play is normal & is refered to as piston dwell time time in real car motors basicly relation of piston moving in a verticle plane & big end of rod moving in a mostly horizontal plane. When you build a hotrod motor most people look for longer dwell time for a torque application by using a piston with the wrist pin hole higher than stock so they can use a longer rod & increase piston dwell,so what I'm saying is its not always a bad thing. This is the motor you just had uh problems with correct? You can pull the back plate off & do the same test & look at where the rod & crank pin meet.If you can see the crank move inside the rod much you got problems & by move much I mean if you can see the rod gush oil out between the space between the 2 then suck it back in when you turn the other way thats the beginning of the end.V-spec crank pins have proven to be uhh less than hard enough there.The good news is once that starts you'll know within a few fuel bottles(500cc bottles)by exactly what Davidka described & soon wont be able to keep it tuned,only with my personal v-spec a new rod did not help much as the bronze bushing in the rod seems to be harder than the steel crank in a v-spec.

Be creative with the search function & you'll come up with more symptoms of this problem & see what the people did to fix it like tune was off or crank was replaced or they just freaked them selve out & motor was fine.You can ebay cranks between $35-60 fairly often if you need to,just make sure you need to before you do cuz like I said some movement is normal & I wouldnt have even said anything if I hadnt read some of your other posts.
butch man is offline  
Old 05-09-2009, 09:38 PM
  #8  
Tech Adept
 
chunk t's Avatar
 
Join Date: Nov 2008
Location: Elgin, IL
Posts: 225
Default

Originally Posted by butch man View Post
Little play is normal & is refered to as piston dwell time time in real car motors basicly relation of piston moving in a verticle plane & big end of rod moving in a mostly horizontal plane...
Nice butch man

I do all of my "degree wheel" timing via a spread sheet & measurements of the various engine components. On a spread sheet it's easier "see" the dwell time and track the piston's acceleration and deceleration as the crank goes through its rotation.
chunk t is offline  
Old 05-09-2009, 10:36 PM
  #9  
Tech Elite
Thread Starter
iTrader: (5)
 
Join Date: Mar 2009
Posts: 3,660
Trader Rating: 5 (100%+)
Default

conecting rod and crankshaft are fine. After all i put my engine through it is normal after all. I can stop being paranoid now
dreaux is offline  
Old 05-09-2009, 11:29 PM
  #10  
Tech Champion
iTrader: (17)
 
JAMMINKRAZY's Avatar
 
Join Date: Jun 2007
Location: Pittsburgh PA
Posts: 6,575
Trader Rating: 17 (100%+)
Default

It's an OS, they don't hold metal pinch at TDC like others do. The lack of pinch just means your engine is fully broken in. What you really want to check for is compression with the plug IN. That is the only thing that matters.
JAMMINKRAZY is offline  

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are Off


Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information

Copyright 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.