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Engine Building: Huge Crank Resistance With Only 2 Pistons

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Best Answer Moshtaraq , 08 April 2021 - 12:20 PM

To answer this thread: I took it to the machine shop for a linebore and now it runs like a clock!

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#16 KTS

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Posted 10 March 2020 - 08:46 AM

are the conrod bearing caps all correctly matched to their conrod ? 



#17 Spider

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Posted 10 March 2020 - 09:58 AM

There's a whole host of possible causes here and it may not be just one.

 

With the crank free spinning in the block (and try it very slow too to feel for any tight zones), some causes that spring to mind;-

 

Piston to Bore Clearance

Bore parallelism and shape,

Bent Rod(s) and / or twisted

Tight Rings (what was the end gaps?)

Bore Finish

Ring types

Ring fit in pistons

Ovality of Crank Pins

Size and shape of Crank Pins

Size and Shape of Rod Big Ends

Thrust Clearance of Rods

Bores not perpendicular to Crank Centre Line (quite common)

Bores not straight (ie, 'bent' bores)

Bent Crank that was then re-ground

Over-torqued or under torqued Big End Fasteners - which may have been done previously

Heated Little Ends

Tight Little Ends

 

That's all that springs to mind, but there's many more possible causes than just these.



#18 Moshtaraq

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Posted 10 March 2020 - 06:09 PM

Disassembled everything and concluded that the pulley side bearing tightens up when going over 60Nm and the crank is not spinnable by hand anymore. Don't know how to fix this

#19 gazza82

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Posted 10 March 2020 - 06:22 PM

Check the shells aren't the wrong size.

Was it turning over ok before the rebuild?

#20 johnv

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Posted 10 March 2020 - 06:24 PM

You need to get the block and caps line bored?



#21 Moshtaraq

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Posted 10 March 2020 - 06:34 PM

I measured the mains and big ends with a micrometer before buying shells and went with the standard size, no undersized ones.
After installation I checked it again with plastiguage and they were right in the middle of the tolerance range. A thing I noticed is once I get it spinning it gets easier to spin, just getting it going requires a lot of force (something like 10Nm)

Edited by Moshtaraq, 10 March 2020 - 06:35 PM.


#22 timmy850

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Posted 10 March 2020 - 09:01 PM

Can you check the plasitgauge 4 times with the crank rotated 90 degrees each time while the other caps are torqued up? If the tolerance changes as you rotate it might be ovality or a bent crank as Spider has mentioned

#23 Moshtaraq

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Posted 10 March 2020 - 09:10 PM

Can you check the plasitgauge 4 times with the crank rotated 90 degrees each time while the other caps are torqued up? If the tolerance changes as you rotate it might be ovality or a bent crank as Spider has mentioned

I just checked the straightness (dont know the word, I'm not a native English speaker sorry :) ) of the crankshaft with a dial guage while turning it in the middle bearing and there was litteraly no movement whatsoever.

I'm thinking about getting my machine shop to take a look at it. Line boring/honing seems like a good idea. It's gonna hurt my student-wallet though :ohno:



#24 Earwax

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Posted 10 March 2020 - 11:30 PM

If you still have the old bearings, i would swap out the front bearing shell to see if improves. - 



#25 absx2

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Posted 11 March 2020 - 07:42 PM

I bet one of the main or rod caps is on the wrong way around or the two outer main caps come to think of it ?


Edited by absx2, 11 March 2020 - 07:44 PM.


#26 mini13

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Posted 12 March 2020 - 08:30 AM

with a main bearing that causes restriction this can be caused by poor fitting/loose dowels, its worth checking the tightness of these, and if they seem loose swapping them.

 

Swiftune do some billet dowls that are extra snug for a good price ( cheaper than std ones last time I looked)



#27 DeadSquare

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Posted 12 March 2020 - 09:37 AM

I bet one of the main or rod caps is on the wrong way around or the two outer main caps come to think of it ?

It is pretty difficult to incorrectly fit either of the two end main caps, without noticing it.


Edited by DeadSquare, 12 March 2020 - 09:40 AM.


#28 Rorf

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Posted 12 March 2020 - 11:03 AM

You need to do a trial build first - that is install the pistons and rods without the rings on to see that the pistons are not sticking in the bores due to bent rods or big end bearings not seizing up when the caps are tightened.

 

First strip everything, do not place oil between the bearing shells and the block - this must be scrupulously clean. Use normal 20W50 on the bearings before placing the crank in. Tighten main caps starting with middle one first. Check that crank spins easily after each main cap tightened down to final torque. If you have a problem the crankshaft needs to be checked and line boring might be required. Once this is sorted then install each piston and rod without rings and check if any tight spots. 

 

Once everything turns over easily, remove rods and pistons and reinstall with the rings on. Ensure the rings are gapped correctly and you use a good quality ring compressor.



#29 Moshtaraq

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Posted 12 March 2020 - 01:55 PM

When dissasmbling a couple of months ago I removed the dowels. Are they specific for every cap cause I just placed them randomly? I'll do a trail fit again tonight with all the advice you guys gave me. Crank is completely fine though, already checked that.

#30 mini13

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Posted 12 March 2020 - 02:35 PM

how snug were the dowels, they should be tight eniough that its not leasy to get the cap off without the bolts in ,often needing some gentle levering/ prying.







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