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Checking Compression, With Head Off?


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#16 8tyman8 OFFLINE  

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Posted January 30, 2013 - 11:24 PM


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#17 1978murray OFFLINE  

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Posted January 31, 2013 - 08:47 AM

i can tell if a motor has compression with head of. by plugging the cylinder with hand and crank it over. if ur hand gets sucked down hard and gets pushed back up with good force. then u have compression if not ur rings are shot



#18 Guest_rat88_*

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Posted January 31, 2013 - 09:42 AM

i can tell if a motor has compression with head of. by plugging the cylinder with hand and crank it over. if ur hand gets sucked down hard and gets pushed back up with good force. then u have compression if not ur rings are shot

But can you tell the difference between 70psi and 100psi? It may "feel" good at 70, but it probably wont run.



#19 olcowhand OFFLINE  

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Posted January 31, 2013 - 09:58 AM

What is the most hilarious is that he was speaking in all seriousness when he said "it's got great compression"!   :(   And a 2 stroke being rebuilt & not cracking the cases & pressing the crank for new rod & big end bearing....ridiculous!  Those big end bearings can feel smooth as silk but actually have the needles in pieces.....seriously, they can!  Looking into the big end rod slits can even show the needle bearings look like new, but the rod will be pitted.  Calling an engine rebuilt without rebuilding it is a joke.



#20 DougT ONLINE  

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Posted January 31, 2013 - 10:08 AM

Come again? You lost me at....."Seems to me....."  :worshippy1:  Isn't math great. The things you can prove or disprove with simple numbers.

 

He was talking about compression ratio. That is the comparison of the total volume in the combustion chamber when the piston is at the top and when its at the bottom. That's not how to tell the actual compression developed. Seems I've seen a tool that was like a suction cup that applied vacuum to the piston with the head off and acted like a leakdown detector. That's about the closest thing I can think of to check compression with the head off. I hate to admit it but I bought a parts engine years ago that felt like it had good compression. When I took the head off there was a rubber ball stuffed in the top of a busted piston. I've often wondered how long it took to get a ball just right so it had the right "feel" to it????



#21 MH81 ONLINE  

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Posted January 31, 2013 - 06:17 PM

He was talking about compression ratio. That is the comparison of the total volume in the combustion chamber when the piston is at the top and when its at the bottom. That's not how to tell the actual compression developed. Seems I've seen a tool that was like a suction cup that applied vacuum to the piston with the head off and acted like a leakdown detector. That's about the closest thing I can think of to check compression with the head off. I hate to admit it but I bought a parts engine years ago that felt like it had good compression. When I took the head off there was a rubber ball stuffed in the top of a busted piston. I've often wondered how long it took to get a ball just right so it had the right "feel" to it????


Doug, did you "return" the rubber ball to its previous owner??? I am thinking I would've at least tried...

#22 HowardsMF155 ONLINE  

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Posted January 31, 2013 - 06:28 PM

I couldn't tell from the clip, is it possible that the remark about compression was dubbed in later?



#23 DougT OFFLINE  

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Posted January 31, 2013 - 06:29 PM

Doug, did you "return" the rubber ball to its previous owner??? I am thinking I would've at least tried...

No, it was a Portland show purchase. I didn't take it apart until I got home and didn't know the person I bought it from. Just another lesson from the school of hard knocks.


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#24 UncleWillie ONLINE  

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Posted January 31, 2013 - 09:36 PM

This is why I don't watch any of those shows. Just recently the storage wars people got busted for seeding the bins. The guy on the pawn shop show always knows someone who can identify and value whatever piece is brought in and he knows it off the top of his head. After 5 minutes of watching I can feel my IQ leaking out of my head and I turn it off.


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#25 8tyman8 ONLINE  

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Posted January 31, 2013 - 10:55 PM

But can you tell the difference between 70psi and 100psi? It may "feel" good at 70, but it probably wont run.

If its a Briggs it will run with Carb cleaner .. even if the compression is low heck i have seen them run with 30-40 psi they run like crap but they still run



#26 8tyman8 OFFLINE  

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Posted January 31, 2013 - 10:58 PM

I couldn't tell from the clip, is it possible that the remark about compression was dubbed in later?

around the 0:35 mark ..



#27 Tennblue59 OFFLINE  

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Posted February 02, 2013 - 09:54 AM

You can get the compression ratio (kind of) by measuring the water (oil/fluid/milk/whatever...) displaced between bdc and tdc, but will be off because of the volume in the head chamber.

 

And I guess if all the water runs out the bottom of the crankcase around the rings before you can turn the crank, that is an "indicator" that you compression will be, well, kinda low.....

 

Back in the motocross days, we used to do a couple top end rebuilds between bottom ends, but seals and bearings on the bottom end needed to be done regularly, or you are just wasting money and time.....

 

As for the "reality shows", I put them up there with WWA wrestling for realism....... :wave:



#28 IamSherwood OFFLINE  

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Posted February 02, 2013 - 10:59 AM

i can tell if a motor has compression with head of. by plugging the cylinder with hand and crank it over. if ur hand gets sucked down hard and gets pushed back up with good force. then u have compression if not ur rings are shot

 

In no way, am I making fun of your post here, 1978murray. :wave:

 

Apparently "Rick" from "American Re....." must have read your post.

He was spotted leaving a tatoo parlour, with freshly done #'s on the tops

of his fingers. Now he'll be able to tell us how many pounds of compression,

depending on how many fingers wiggle as he cranks them engines over. :smilewink:

 

fingers.jpg



#29 ol' stonebreaker OFFLINE  

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Posted February 02, 2013 - 11:06 AM

  The only question I have is if the compression is in question why not test before the head is off?

                                                           Mike



#30 DougT ONLINE  

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Posted February 03, 2013 - 02:55 PM

  The only question I have is if the compression is in question why not test before the head is off?

                                                           Mike

Because you have to get the nuts out first!         bigrofl.gif






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