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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.
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Old 08-29-2019, 08:08 PM   #5
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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.
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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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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Old 04-24-2020, 10:44 PM   #14
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I'm gonna bump this thread as I'm getting serious about either building a B20 for the 145 or going B230FT.
If I stay b20 I'm not going turbo.
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Old 03-07-2022, 07:36 AM   #15
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Well I've been inspired to big bore my B20B by seeing the Amazon belonging to R32rennsport on IG.

I was thrilled in to thinking my '74 144DL with only 77.xxxkms was had an 8 bolt crankshaft.

So 2 weeks ago (finally) took drivetrain out and...well...it's a 6bolt...

I had ordered a set of B21A pistons which came with rods as well as the big bore head gasket.

Now I'm kinda thinking of what to choose... rebuild as is with a different cam and maybe twin body carbs, bike carbs (oldskool ITB) or try and source an 8 bolt crankshaft and rods.

I'd like your opinions please... I've been reading a endless posts and contributions...
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Old 03-07-2022, 11:04 AM   #16
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I always get blocks sonic tested before boring for a ~2.1. I've had 8 bolt blocks that didn't have adequate wall thickness, and I've had 6 bolt blocks that did. I really think that the only way to know whether your block will accept the bore is to sonic test.

So far, I've only come across 2 or 3 blocks, out of 20 or so, that didn't pass muster.

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Old 03-07-2022, 01:30 PM   #17
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I dont really see a reason not to stay with a 6 bolt crank. If the block is good at 92mm, then you're good to go. 6 bolt and 8 bolt are both a forged cranks, so doesnt matter. Rods, sure 8 bolt rods are thicker at the big end shoulder, and run a larger wrist pin. There are manufacturers out there that will make you a rod, or even a rod and piston setup that will fit that crank.
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Old 03-07-2022, 02:13 PM   #18
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I don't mind using the 6 bolt crank.

But not to keen on paying huge money for custom rods or pistons.

The concept of Big Bore B20 kits is basically B21A pistons on late B20 rods and an oversize headgasket.

My only concern is running wrist pins without bushing on 6 bolt type rods
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Old 03-07-2022, 02:21 PM   #19
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And the concern is.... what exactly?

The wrist pin is used in an interference fit on the rod small end - it's pressed in (with the rod warmed up and the pin chilled). Then the pin is held firmly by the rod, and the piston rotates on it. So it's not fully floating with circlips holding it in as in normal Volvo style.

If that doesn't sound good to you, then either find an 8 bolt crank, or get some custom pistons made, or get some custom rods made. Or stick to B20 sized pistons.
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Old 03-07-2022, 04:28 PM   #20
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I don't mind using the 6 bolt crank.

But not to keen on paying huge money for custom rods or pistons.

The concept of Big Bore B20 kits is basically B21A pistons on late B20 rods and an oversize headgasket.

My only concern is running wrist pins without bushing on 6 bolt type rods
22PSI on 22mm pins and they took it just fine. Stock rods, stock crank, stock pistons... For a minute there I was having instances of overboosting where I saw 30psi at 7k.

If you're using a stock crank... a 6 bolt crank is a rear thrust with a 54mm rod journal. Same as an 8 bolt, same as a b21. So take your piston and rod assembly from the b21, clamp a bearing in it, and measure. If you dont have the tools, drop off at the machine shop and have them check.
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Old 03-07-2022, 04:52 PM   #21
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Rod journal diameter and width is not the same on 6 and 8 bolt cranks. 6 bolt crank rod journals are slightly larger (54.102 mm vs. 54.0 mm) and slightly wider (32.05 mm vs. 30.05 mm) than 8 bolt crank rod journals. The main journals are the same.
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Old 03-07-2022, 05:11 PM   #22
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Rod journal diameter and width is not the same on 6 and 8 bolt cranks. 6 bolt crank rod journals are slightly larger (54.102 mm vs. 54.0 mm) and slightly wider (32.05 mm vs. 30.05 mm) than 8 bolt crank rod journals. The main journals are the same.
Hmm, interesting... well, then press that pin in the small end and ditch the bushing.
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Old 03-07-2022, 05:44 PM   #23
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That would be the easiest way out of his predicament. Another option is to locate an 8 bolt crank and rods as it will bolt right in to his block. Early B21 cranks have a removable seal sleeve and are identical to an 8 bolt B20 crank. Later B21/B23 cranks have an integral seal sleeve and won't work.

Another option is forged pistons. Sure they cost more but we have these made in 3.6" (91.44 mm) diameter so that a stock head gasket can be used which saves ~$100. They're also a lot more forgiving than a cast piston. Displacement is a little smaller than IPD's kit (2101 cc vs. 2130 cc).

http://hiperformanceautoservice.com/...roducts_id=456
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Old 03-07-2022, 05:48 PM   #24
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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.
FWIW, that's what I ended up doing with the PV motor. It ended up being an almost identical failure on the piston, in almost the same exact spot on the piston, on the same cylinder. Weird. I used one of the 3 pistons from the previous set, had the machine shop install it on the rod since the interference fit isn't something I think I can do at home, and then balanced it myself at home to within a gram (big end and piston end separately) to the others. Spins nice and smooth, I'm back to abusing the PV like I stole it.
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Old 03-08-2022, 12:14 PM   #25
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There is a 3rd option. But might take a little bit more maths, machining, and a finger crossing.

Have the 8 bolt rods opened up by .004" (.102mm) bringing them to 6 bolt spec. Then measure, maths, and fashion some shims on the piston side to give you a piston guided setup.
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