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Old 08-29-2019, 10:55 AM   #1
vwbusman66
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Default 6 bolt VS 8 bolt B20

This coming summer, I am planning on doing a pseudo rebuild on a b20 in prep for mild turbo use.

Which one is stronger in stock form?
From what I've read, the 6 bolt has larger rod bearings, but the 8 bolt will more easily accept B21 pistons and M rods (B21 right?).

Let's ignore differences in the heads/valvetrain as I'm flexible on that.
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Old 08-29-2019, 01:11 PM   #2
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Can't use the B21 pistons without custom rods on the 6 bolt crank.
Other than that, and the fact that 8 bolt's went metric, can't help out on the strength of one over the other.
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Old 08-29-2019, 01:15 PM   #3
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6-bolt rods will handily accept 24mm B21 wristpins - just remove the bronze bushing, and take a *tiny* bit of metal out, and install the pins in an interference fit. And pitch the circlips.

I had a machine shop do that with a set of B18 rods, and later on with a set of B20E rods.

No problems with the rods or wristpins, although stock cast pistons have been a different story.
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Old 08-29-2019, 08:05 PM   #4
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Well there you go. I was always told you need the 8 bolt rods for B21 pistons.
Learn something new every day!
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Old 08-29-2019, 08:08 PM   #5
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Quote:
Originally Posted by JohnMc View Post
6-bolt rods will handily accept 24mm B21 wristpins - just remove the bronze bushing, and take a *tiny* bit of metal out, and install the pins in an interference fit. And pitch the circlips.

I had a machine shop do that with a set of B18 rods, and later on with a set of B20E rods.

No problems with the rods or wristpins, although stock cast pistons have been a different story.
Can you/I/anyone discard that bushing because 6 bolt cranks are piston steered, or will the pin rotate in the piston instead of the rod?

Also, how does a 6 bolt block take the ~3mm overbore of b21 pistons?
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Old 08-29-2019, 08:52 PM   #6
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In OEM configuration - the B20 pistons are crank steered (AFAIK), and the wristpin freely floats on both the piston and small rod end (held in place by the clips). With the pin pressed into the rod small end, obviously it's held in place there, and all the movement takes place at the piston, and the clips aren't needed. So it's not entirely original usage. I'm not sure if there are any durability issues related with press fit pins. I know some OEM's do it that way, just not Volvo.

From the most distant hearsay and third hand anecdotes, the 6 bolt blocks are somewhat less prone to core shift than the later B20 blocks are. And core shift is really the issue when doing a 3mm overbore. The blocks are *generally* thick enough for it to not be an issue for normally aspirated thrashing. I'm not sure about boosted. Since you're getting your HP by cramming more air in, you might be better off keeping those cylinder walls 3mm thicker and using B20 sized pistons.
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Old 08-29-2019, 11:59 PM   #7
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That's my current dilemma as well John. My 74 block is already at 92mm. Do I run it if there's wall thickness or sleeve it and lose a little displacement. Need pistons either way, just gotta figure out the plan.
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Old 08-30-2019, 08:28 AM   #8
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Maybe have a machine shop use a sonic tester on it and see if there's any core shift making it particularly thin in spots?
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Old 08-30-2019, 09:29 AM   #9
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Yup, once it's torn down that's the plan.
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Old 08-30-2019, 09:43 AM   #10
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Quote:
Originally Posted by JohnMc View Post
Maybe have a machine shop use a sonic tester on it and see if there's any core shift making it particularly thin in spots?
What is core shift? Slight off center/fore/aft movement of the bores when you hog them out?

Edit: Yep. Well, sorta.
https://mooregoodink.com/what-is-cor...t-detrimental/
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Old 08-30-2019, 09:57 AM   #11
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Misalignment of the casting 'blanks' that create the water jacket spaces inside the block. In original bore sizes, they are designed to have plenty of metal on all sides, to minimize the chance that the water jacket will be cast so far off 'nominal' that it will cause an issue.

But when you take 3mm more out? Might be within danger distance of the water jacket.

I think Volvo did a better job than many, back in the day. I've had several blocks bored out (one B18 to B20 size, 2 B20 blocks to B21 size) and the machine shops were always skeptical, had to be absolved of any liability if the block was junk afterward. But I've never had any issues that I know of* with any of those engines.

* The PV has broken a piston in the past - a big flake came off and compression dropped enough to be noticeable, pulled apart before any further damage occurred. And about 8K miles later, after a new set of B21A pistons and B20E rods, it's got low compression again, another piston issue (leakdown into the block), but I haven't torn it down yet. I don't think it's related to the block though, just revving stock cast pistons to 7500 with some regularity)
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Old 08-30-2019, 10:05 AM   #12
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Quote:
Originally Posted by JohnMc View Post
Misalignment of the casting 'blanks' that create the water jacket spaces inside the block. In original bore sizes, they are designed to have plenty of metal on all sides, to minimize the chance that the water jacket will be cast so far off 'nominal' that it will cause an issue.

But when you take 3mm more out? Might be within danger distance of the water jacket.

I think Volvo did a better job than many, back in the day. I've had several blocks bored out (one B18 to B20 size, 2 B20 blocks to B21 size) and the machine shops were always skeptical, had to be absolved of any liability if the block was junk afterward. But I've never had any issues that I know of* with any of those engines.

* The PV has broken a piston in the past - a big flake came off and compression dropped enough to be noticeable, pulled apart before any further damage occurred. And about 8K miles later, after a new set of B21A pistons and B20E rods, it's got low compression again, another piston issue (leakdown into the block), but I haven't torn it down yet. I don't think it's related to the block though, just revving stock cast pistons to 7500 with some regularity)
If you stay b20, what are you going to do about pistons? Aftermarket forged?
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Old 08-30-2019, 10:11 AM   #13
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I honestly don't know. The PV dropped compression right after I pulled the wagon apart, and I'm not organized enough to have two cars apart at the same time. So it's just been sitting, and I'm not entirely sure what's wrong with it.

Last time I replaced all 4 pistons with a new set, I have 3 'good' pistons sitting around. I have a half a mind to fix the PV quick, dirty, and cheap by just pulling the motor apart, swapping a piston (having them all balanced), and putting it back together.

And another thought about throwing some bling into the bottom end with some longer rods and shorter, lighter forged pistons.

And a third thought about sticking the 16V turbo from the wagon in it.

I might just do a combo of 1 & 3, do a quick cheap fix on the B20 for now, drive it around until (and if) it has another problem, and the put the DOHC motor in. One issue is that neither of the manifolds on the 16V would fit in the PV at all.
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