Cranks and Pistons

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February-16th-2003, 04:16 PM

snart

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Cranks and Pistons

Whats the diff in the cranks of the BPs??? I know the SOHC is cast and the DOHC is forged (right?) Didn't size change at some point (27mm?).

Thanks...

ALSO... Its been said SOHC pistons will lower comp ratio in a DOHC. Why is this?? IS the SOHC head diff cc's????

February-16th-2003, 11:09 PM

snart

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guess this one isnt so useless???

February-16th-2003, 11:17 PM

TheMAN

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http://www.protegeclub.com/forum/sho...070#post148070

February-16th-2003, 11:27 PM

snart

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Thanks!!!

February-16th-2003, 11:30 PM

snart

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Is any version better than the other??

February-17th-2003, 12:40 PM

midnightblue97

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I don't know if this was answered in that thread link that edwin posted, but I think the answer you're looking for is in 1990 there was a bad problem with the Keyway failing on the Miata crank, so in 91 they made the crank snout bigger to remedy this. It cost my brother 3 engines last year. On the solo Miata site I forget exactly, but it said that in 1990 they were 19 or 21mm (I forget) and in 1991, they changed it to I think 27 mm.
If thats not what you were looking for then just call me an idiot .

February-23rd-2003, 01:34 PM

snart

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Quote:

Originally posted by midnightblue97
I don't know if this was answered in that thread link that edwin posted, but I think the answer you're looking for is in 1990 there was a bad problem with the Keyway failing on the Miata crank, so in 91 they made the crank snout bigger to remedy this. It cost my brother 3 engines last year. On the solo Miata site I forget exactly, but it said that in 1990 they were 19 or 21mm (I forget) and in 1991, they changed it to I think 27 mm.
If thats not what you were looking for then just call me an idiot .

Thats actually quite helpful! I knew they went to 27 at some point, and that one was better than the other. I didnt know the Pro had crankwalk issues??!! Very interesting.... but I guess they fixed it...

February-23rd-2003, 11:20 PM

Big Mike

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Quote:

Originally posted by PseudoRealityX

Its the keyway that held the pulley on IIRC. It basically would eat itself apart, Davard might be able to answer in more depth...or maybe Mike if he ever checks the damn board.

Ok you harassed me into it there Jesse. From what I know, all BPs use the same basic design forged crank which is good for quite a bit of hp. The sohc have different pistons and heads, but are the same otherwise, right down to the MBSP <main bearing support plate> later models were cheapened out and the MBSP wasnt present, you can find these motors because they have a stamped steel oil pan rather than a finned aluminum pan. The snout size is a change only to the front main and out area, the crank was lengthened and raised in diameter because of alot of past issues, the main issues are 1. the nose snaps clean off of the engine due to vibration and 2. the keyway where the woodruff key sits wears away due to either improper torque or installation of the woodruff key itself...These issues were leftover from the B6 family designs and were changed on ALL b series mazdas in or around 92...Thats all I can think of at the moment...