We break our engines so you don't have to. (blew up another damn motor) - Miata Turbo Forum - Boost cars, acquire cats.

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Old 05-09-2011, 01:39 AM   #1
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Default We break our engines so you don't have to. (blew up another damn motor)

Broke this back in March but we've been so busy in the shop that I haven't had a chance to throw these photos up yet.

All of this damage was caused by the same failure - number 1 left-hand main bolt failure.

#1/#2 main caps split - #1 was broken in the motor, #2 broke on disassembly



Broken OEM trigger wheel (cracked between the sharpie marks)



Broken OEM oil pump housing (cracked in 3+ places)





OEM 2002 MBSP, cracked significantly in at least three places





Got a lot of good info from this motor, though.

1. 0.004" is too big for a Supertech motor. The skirts looked great but there was too much ringland wear for a motor this old (~20 hours). The replacement block is getting narrowed down to ~.003".



2. The lower limit for stock main bolts is 300-350whp. They aren't expensive enough to skip over in builds, especially since they do seem to fail even with an MBSP in place. They will become standard on all our built motors.

3. The MBSP is strong, but not that strong. It was strong enough to keep the two broken main caps intact, which kept the rest of the motor intact, but it wasn't strong enough to keep enough stress off of the bolts themselves.

The replacement motor will be another OEM block, same pistons, rods, but new rod bolts, ARP main studs, fresh BE oil pump, etc. The old block still appears to be OK, but it will get magnaflux'd and go into storage for later use (high compression, longrod, billet main caps).
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Old 05-09-2011, 01:44 AM   #2
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Carnage. Why must stock parts be declared unfit for my goals after I get my motor back together?
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Old 05-09-2011, 02:03 AM   #3
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now i understand your oil pump (out of) specs.
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Old 05-09-2011, 02:31 AM   #4
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That's some pretty impressive carnage. I can't recall ever having seen a main cap break in these engines.

Of course, now that we have a documented case of it, someone needs to create and market an expensive set of billet main caps, so that everyone else can avoid having this happen to their engines.

Last edited by Joe Perez; 05-09-2011 at 03:34 AM. Reason: schpelling
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Old 05-09-2011, 03:32 AM   #5
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That's some pretty impressive carnage. I can't recall ever having seen a mail cap break in these engines.

Of course, now that we have a documented case of it, someone needs to create and market an expensive set of billet main caps, so that everyone else can avoid having this happen to their engines.
You mean these?

http://www.flyinmiata.com/index.php?...umber=04-66201

Glad I put the ARP main studs in mine!

--Ian
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Old 05-09-2011, 03:34 AM   #6
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Quote:
Originally Posted by Joe Perez View Post
That's some pretty impressive carnage. I can't recall ever having seen a mail cap break in these engines.

Of course, now that we have a documented case of it, someone needs to create and market an expensive set of billet main caps, so that everyone else can avoid having this happen to their engines.
Pro-gram billet steel main caps
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Old 05-09-2011, 11:11 AM   #7
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Quote:
Originally Posted by Joe Perez View Post
That's some pretty impressive carnage. I can't recall ever having seen a main cap break in these engines.

Of course, now that we have a documented case of it, someone needs to create and market an expensive set of billet main caps, so that everyone else can avoid having this happen to their engines.


You are so spot on!

I really want to see the oil pump gears. Truthfully, there is nothing on the nose of that crank that should have survived regardless of what unobtanium we made them out of.
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Old 05-09-2011, 11:28 AM   #8
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Any details as to what lead up to this? I went ARP mains in mine so I should be ok but this is still shocking.
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Old 05-09-2011, 11:34 AM   #9
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Expensive paperweights. What preceded engine failure? I'm surprised you didn't have a set of ARP bolts in there already.
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Old 05-09-2011, 11:46 AM   #10
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I know what is going in my motor. ARP's.
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Old 05-09-2011, 01:42 PM   #11
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I don't really think billet main caps are really necessary. The main caps here broke as secondary reaction to the bolt breaking.
Billet caps are like $500 plus whatever the cost of line boring would be for all of them. Maybe for all out race motors you might think about it but I think ARP mains would be good enough for most.
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Old 05-09-2011, 01:44 PM   #12
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I don't really think billet main caps are really necessary. The main caps here broke as secondary reaction to the bolt breaking.
Billet caps are like $500 plus whatever the cost of line boring would be for all of them. Maybe for all out race motors you might think about it but I think ARP mains would be good enough for most.
This isn't an all out race motor?
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Old 05-09-2011, 02:05 PM   #13
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Expensive paperweights. What preceded engine failure? I'm surprised you didn't have a set of ARP bolts in there already.
No warning. Smooth running, then a little smoke in the cabin from an oil leak (FWD dipstick cover plate was cracked off the oil pump), then an intermittent knocking noise in the bottom end (chunk of maincap rattling against the crankshaft). I had never seen a main bolt failure so I hadn't bothered to put them in before this.

The billet caps will be reserved for the next motor. This one needed to stay fairly stock-ish (compression/rod length) so the turbo comparison testing we have planned for later this year was still relevant. The next motor will get a bunch of goodies, and this one will be the spare.
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Old 05-09-2011, 02:25 PM   #14
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The sadist in me wants to see a pic of the crank if you have one.

I think you don't hear about broken main bolts because most people go to ARP. They're like $100. I have a set that's literally been through the digestive track of a rottweiler, and they came out like new (yes it was a disguising way to get my $100 back, no I didn't handle that part of it myself).
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Old 05-09-2011, 02:52 PM   #15
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Wow I don’t think anybody had known the limits of the stock main bolts before. This is the first one I have ever herd of failing.

Glad to see your finding the limits and not me this time.

You have to modify the MBSP a bit to clear the ARP studs when you go to those.

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Old 05-09-2011, 03:17 PM   #16
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What car is that out of? I would have figured you would have the black cars motor more fortified.
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Old 05-09-2011, 11:04 PM   #17
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The sadist in me wants to see a pic of the crank if you have one.
Doesn't look damaged at all, but I'm sure magnaflux would tell another story.
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Old 05-09-2011, 11:46 PM   #18
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ATI Super Damper?
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Old 05-10-2011, 12:31 AM   #19
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5x the cost of the studs and I don't know if it would be as effective.
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Old 05-10-2011, 04:16 AM   #20
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I should've been more clear. I was actually wondering if you were running one but apparently not. Between the two I would definitely choose the studs, especially for a race motor where ultimate strength is more important than longevity (e.g. tri-metal vs. aluminum bearings). My build is progressing slowly but surely, I have the ARP main studs and am contemplating the damper. I feel like high power and high rpms will cause fatigue and all sorts of mystery demons to come out without the damper though.
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