Main bolt failures on the BP - Miata Turbo Forum - Boost cars, acquire cats.

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Old 08-19-2007, 03:03 PM   #1
Lex
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Default Main bolt failures on the BP

I am in the process of assembling a built BP motor with all the goodies to handle big boost. However, for the main bolts, if I were to use ARP main studs, I'd have to drill the main bearing support plate because the studs interfere with it being longer than the OEM main bolts.

My question is, has anyone heard of the stock main cap bolts ever failing? Is it worth going through the trouble of putting in the main cap ARP studs?

My power goals for this new motor are ~350whp.
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Old 08-19-2007, 03:10 PM   #2
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They are not needed. A member on m.net is making over 500whp with the stock main bolts and caps on a 1.8. The main bolts don't really see alot of load. Also, with arp mains you have to get an align hone because of the high torque the bolts require.
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Old 08-19-2007, 03:41 PM   #3
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The mains see more load then the rods.
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Old 08-19-2007, 03:46 PM   #4
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could you explain that view please.
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Old 08-19-2007, 03:48 PM   #5
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There is no question that the mains see load and it is difficult to tell exactly how much. However, if it isn't more than they can handle in the 350whp range daily, I am not concerned.

I am building this motor as a daily driver and I am not going to strive for anything more than the power I mentioned. I just want to ensure the mains are reliable because I know everything else in the motor is.

I personally have never heard of a main failure being around BPs for the last 3+ years.
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Old 08-19-2007, 03:57 PM   #6
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I was going to go with arp mains on my build and considered billet caps but I hear alot of people running big hp on the stock main caps so maybe I'll save the 100s and not buy the billets. I'd just hate to have a main failure. If you look at the end view of a crank, with a rod at 90deg, and picture the forces transfering thru the crankshaft you can see the fulcrum point being at the main bearing.
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Old 08-19-2007, 04:36 PM   #7
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Name:  drawing_1.jpg
Views: 24
Size:  26.9 KB

It's a cad drawing (not to scale) but visualize the forces at work. The power stroke forces down at point A and, due to the rotational mass/potential of the crankshaft, you could say the force is "restrained" at point B. The only direction left for the force to travel is counterclockwise as indicated by the arrow C. The greater the load on the crankshaft the more forces transfer at point B. Like the cranks on a bicycle... All your pedal force is transfered at the bottom of the frame in the crank bearings. It's transfered to a chain but the point which the transfer takes place is the same.

I'm not a physics instructor or my explination would be more complete. =)
I gave it my best shot.
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Old 08-20-2007, 03:12 AM   #8
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anyone have any more input on the subject of main failures?
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Old 08-21-2007, 12:52 AM   #9
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bump?
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Old 08-23-2007, 01:19 AM   #10
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I'd like to hear about the oil pump failures as well. What are you going with for the pump? I keep hearing about these billet gears for it.
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Old 08-23-2007, 04:48 AM   #11
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I'm going with a new OEM pump. I feel safe going with it and don't plan on spinning the engine too much past the stock redline.
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Old 08-23-2007, 02:40 PM   #12
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So it's the rpm that kills it?
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Old 08-24-2007, 12:38 AM   #13
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RPM and acceleration probably. Perhaps even load on the crank.
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Old 08-24-2007, 03:09 AM   #14
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Well 400hp is alot of load. Maybe I should get those billet gears too. There expencive though.
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