22 replies to this topic

[Carl]

Anyone have experience with a bent crankshaft nose?  I have a SNC in a recently rebuilt engine (2 weekends) and it appears that the end is bent.  The harmonic balancer is very wobbly, but not loose.  Removing the harmonic balancer, the timing gear and bolt head are also very wobbly.  How can this happen?  Is there possibly something going on that does not involve a bent nose?  I threw the alternator belt during the last weekend, but am thinking that was a result of the wobbly balancer/pulley, but maybe it is the other way around?

[Carl]

I can send a video of the crank in operation if that is helpful, but cannot upload it to the forum

[Carl]

Here is a link to the video

 

https://youtu.be/wloMFsodrYU

[Tom Sager]

Possibly the key on the nose of the crank that secures the cam gear has failed.  Not unusual on a 1.6 engine.  If you are lucky the key and gear are the only items damaged. 

[Steve Scheifler]

Agreed. That isn’t something bent, it is almost certainly a case of the bottom timing gear being sloppy on the crank, and some combination of the key and keyways in the crank and gear being damaged. If it was just rebuilt it was almost certainly assembled wrong this time, or already damaged and not checked carefully before assembly.

[Carl]

The "not bent" sounds promising, but it does seem like the bolt head is moving around.  The bolt is tight.  Seemingly at least 100 ftlbs.  The builder is a reputable guy, so it will be interesting to see what things look like when the bolt gets removed to inspect the crank, keyway and timing gear.  Since the engine is so new I didn't want to do it myself, in case there is some question about the assembly or parts.  

 

I just have a difficult time seeing how the nose could get bent, since it is only driving the alternator and water pump and valves.  Does that actually ever happen?

[Steve Scheifler]

Not unless you leave the engine lying on the railroad tracks or drop it from several hundred feet.

[Carl]

That is kind of what I was thinking, but based on looking at the video on a cell phone (at the track this weekend) the builder is thinking bent crank.  He has not seen it in the metal yet, so he may change his mind.  Or I may really have bent a crank nose in my last two weekends at the track. 

 

Has anyone had a bent crank nose?

[Tom Sager]

Like Steve said it would take a gigantic force upon the nose of the crank to bend it. 

 

Have a look at the video linked below start at 10:55. 

 

Hopefully your crank nose doesn't look like this one.

 

The repair suggested in the later part of the video wouldn't be recommended by any SM engine builder for a race car. 

 

[davew]

My guess would be that the key was installed wrong. There is a small chamfer on one end of the key. With the keyway at the 12 o'clock position the chamfer must go in first and down. The key can go in 8 ways. Only 1 is correct. Any damage to the keyway should be a replacement crank. The various fixes are ok on a street car, but not a race car at 7000 rpm. I would also recomend ARP crank bolts.

[Carl]

Took things apart and found wear on the key, keyway, and ID of the lower timing gear.  In the video, the rotational and "wobbling" motion of the timing gear with the worn key installed can be seen (not bolted to the crank).  The rotational movement is over 12 degrees.  When I flipped the key to engage an unworn part of the key the movement was about half.  For my information how snug a fit is the lower timing gear with a normal key and keyway? 

 

https://youtu.be/sftRUn_LE2o

 

I am positive that the crank keyway had at least some wear before the rebuild, but not noticed at that time.  I understand that the correct fix is a new crankshaft and am not completely comfortable with the conditional fix of using loctite (with new key/gear/crank bolt), but am willing to go with that for the time being.  The gaps that need to be filled are pretty small and since I have seen (at work) similar repairs done on some large machinery, I am hopeful (and reasonably confident) that it will be successful.  If not, I plan on demanding that the crank be replaced (with me paying for the new crank, but not labor) which I believe the builder will accept at that point.    

 

For future reference can anyone suggest how many hours it should take an experienced Miata mechanic to R&R the engine and replace the crankshaft?  

 

 

Attached Files

•  Worn Keyway and Key - small.jpg   224.63KB   0 downloads
•  crank keyway wear - small.jpg   188.14KB   0 downloads
•  Key in Timing Gear.JPG   58.84KB   0 downloads

[dstevens]

If you've got a street car mechanic on it they'll likely use the something like All Data for the labor estimate or the factory book.  (the NAs don't have time estimates in the book)   That's what the dealers and shops pay on the labor.   For like cars All Data lists between 16 and 21 labor hours for a crank r&r with clearancing the bearings.  The race pro guys here could do it quicker than that.  It'd take me 10-14 hours total I'd reckon, the big dogs less than that but that doesn't mean they'll charge any less.

[Bench Racer]

Carl, if you remove the engine and replace the 16-20 hours will be considerably less. 

[Carl]

Thanks for the info on repair time.  I will keep that in mind if things go south in the future.

 

I am wondering what the effect of the crankshaft being out of time with the camshafts has on power.  Anyone have any idea how much the power would be reduced if the crankshaft is 12 degrees retarded compared to the camshafts?  I assume that the plugs are firing at the same point relative to the camshafts, but the crankshaft will just be retarded relative to the camshafts due to the worn keyway and key.  Am I understanding that correctly?  

 

If I am asking stupid, annoying questions at this point, feel free to tell me to go away.  I have that effect on people some times.

[dstevens]

This is a major problem and needs to be fixed if you are going to run the car.  Not fixing it is not an option.  You could do the Loctite fix to hold you over in the meantime.  That's better than not doing anything but not as good as fixing right.  Not fixing it or even using the shade tree fix is only delaying the inevitable.  It's going to be much more painful fixing it later than it is now.

[davew]

Based on the photos you have posted, do not bother putting it back together. That crank is now a paper weight/boat anchor.

This is a rcecar that will be running steadily at 7000 rpm. Not a high school beater. Fix it right even if it means missing a race or 2

 

Dave

[Steve Scheifler]

Heck, my high school beater (PL510) lived at 7000+, but yeah, don’t risk catastrophic failure with a crude fix.

You may not find it easy to locate a good short nose crank, so if you switch to a later one be sure to get all the pieces out to the big bolt including pulley and an oil pump.

[Carl]

I found a short nose crank and it will be going in soon. 

 

After all I have read trying to understand the problem, it seems that even an install with a good crank would benefit from loctite on the shaft, keyway and bolt threads. That way some of the load holding the lower gear in place is distributed from the bolt head to the "adhesive" bond between the crankshaft and gear.  Does that make sense?  Or can anyone offer a reason not to do it?

 

Thanks for everyone's help.  

 

Carl

[dstevens]

I found a short nose crank and it will be going in soon. 

 

After all I have read trying to understand the problem, it seems that even an install with a good crank would benefit from loctite on the shaft, keyway and bolt threads. That way some of the load holding the lower gear in place is distributed from the bolt head to the "adhesive" bond between the crankshaft and gear.  Does that make sense?  Or can anyone offer a reason not to do it?

 

Thanks for everyone's help.  

 

Carl

As long as you install it properly there is no reason to Loctite the crank.  It won't do anything.   Many of the guys on this thread are pro level race teams so if that solution proved usable they would have done it.  Have the crank replaced the right way using the right parts as per spec and you'll be fine.  Absorb the knowledge, don't over think it and get back to racing.