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Old 04-12-2017, 10:13 PM   #1
R3LLI1
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Question 1966 Volvo B18 Max Bore?

Hello All,

I'm currently working on rebuilding a B18 for my 122s and I was wondering if someone knew how far it can be bored out to? I've heard you can make a B20 a 2.5L but I don't know if it applies the same for the B18. Thanks for reading all comments welcome.
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Old 04-12-2017, 10:37 PM   #2
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The 2.5l b20s were bored and stroked using b21 pistons and a crank that has a longer stroke. A b18 might be able to get to 2.1 liters with b20 pistons and a penta stroker crank.

I'd say that b20 pistons are probably the furthest to go... it's a .120 overbore.
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Old 04-12-2017, 10:48 PM   #3
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It's actually a .1875" overbore and they tend to run on the hot side so a good working cooling system is a must.
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Old 04-12-2017, 10:59 PM   #4
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I've heard mentions of OHV Penta cranks that offer a longer stroke than the automotive cranks, but I can't find any evidence of such pieces. OHC engines, but not the old pushrod engines.

I'd sure be happy to learn about such things, though. Any sources you know of?

Thanks,

Cameron

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Originally Posted by cwdodson88 View Post
The 2.5l b20s were bored and stroked using b21 pistons and a crank that has a longer stroke. A b18 might be able to get to 2.1 liters with b20 pistons and a penta stroker crank.

I'd say that b20 pistons are probably the furthest to go... it's a .120 overbore.
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Old 04-13-2017, 12:08 AM   #5
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I've heard of people using the rear thrust penta 90-92mm (stock is 86mm) stroke crank from the ohc motors in b20s using either aftermarket pistons, or scarier, milling the pistons... I've never had the opportunity to see what the innards look like, so just hear say, but the numbers are right.

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Old 04-13-2017, 04:24 PM   #6
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Quote:
Originally Posted by hiperfauto View Post
It's actually a .1875" overbore and they tend to run on the hot side so a good working cooling system is a must.
hiperfauto is correct about "they tend to run on the hot side."

Have done it once in the past and noticed the same hot running issue and was considering doing it again on another B18, but changed my mind.

Got my automotive engine engineering books out over the weekend, and read about cylinder wall thickness once again - most production cars use about .200" thick cylinder walls. Basically what happens when you bore the cylinder walls out .093" a side (.187" on the diameter) which in this case is a very LARGE overbore, the heat from combustion will travel thru the remaining cylinder wall about twice as fast as before. This will cause overheating with the small 122s radiator under certain conditions.

The other factor you should also know about is during combustion with a 9.5:1 CR and a 3.50" or 88.9MM B20 bore size in a B18 block the pressure is about 800 PSI which will balloon the thinner walls as the piston descends, resulting in compression loss and excessive blow by into the crankcase.

Will sonic test a B18 block when I can find the time to do it.
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Old 04-13-2017, 08:40 PM   #7
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I'm confused by most of this talk...

B18 engine was from inception designed with the intention of eventually boring to B20 spec without significant dimensional modifications.
Are you saying that a B18 bored to B20 spec (3.313 to 3.500) has resultant thinner cylinder walls than an untouched B20?
Not in my experience...
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Old 04-14-2017, 12:14 AM   #8
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Some castings had flaws, same deal as the b20s going to b21 pistons. But I can say that mine looks thinner than the b20, or the casts were slightly different.
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Old 04-14-2017, 08:01 AM   #9
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Why don't you start with a B20 block? Are they hard to find these days? Then you could bore it .030 or .040 with no concerns & use all your internals (except pistons).
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Old 04-14-2017, 05:19 PM   #10
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It seems to vary from block to block. Some B18's will go to be 2.2 while others are near the limit on 2.0 but never had an issue boring one to take B20 pistons
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Old 04-14-2017, 06:03 PM   #11
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The answer:
Yes you can displace it to 2.5, but behold the heat of it and cracked block.
I strongly recommend stay within 2.0-.2
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Old 04-19-2017, 09:59 AM   #12
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Thanks for all the responses it has given me lots of information to think about!
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Old 04-19-2017, 10:15 AM   #13
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To the best of my knowledge, the internal castings are different on a B18, they didn't have an extra 3mm of metal there just hanging around through a decade of production waiting for the B20.

I've done a B20 piston rebuild on a B18 before, there was nothing to note in terms of issues. No overheating, no ballooned cylinder walls, only thing was convincing the machine shop it was OK to take that much metal out of a block.

I've done 2 B20 blocks up to B21 piston sizes. Same results as with the B18: no drama of any sort. The 2.1L B20 keeps cool with no issues in my PV with its small B16 radiator. Only issue was the same - reassuring the machine shop it was OK to take that much metal out of a block.

I have heard of rare cases where the internal voids were slightly out of position (core shift) and it caused problems in a large overbore like this. Might want to have the wall thickness checked sonically before boring it, if they're not fairly even you might not want to do it.
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Old 04-20-2017, 11:17 AM   #14
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Quote:
Originally Posted by JohnMc View Post
To the best of my knowledge, the internal castings are different on a B18, they didn't have an extra 3mm of metal there just hanging around through a decade of production waiting for the B20.

I have heard of rare cases where the internal voids were slightly out of position (core shift) and it caused problems in a large overbore like this. Might want to have the wall thickness checked sonically before boring it, if they're not fairly even you might not want to do it.
John, As you mentioned, shifting of the water jacket cores does occur during the casting process in some blocks resulting in thin areas. I sonic tested a B18 block bored to B20 size with a core shift, and in the thinnest area of a bore the iron was only .050" thick, and it was .180" thick on the other side. This engine was run many miles but when carefully measured w/a dial bore gage, and it was out of round by close to .004" front to back in the bore where were the shifting had occurred, but not side to side where a cylinder bore normally wears.

Also checked four other B18 blocks and one of the four had a core shift. Neither of this block or the one mentioned above could have been bored successfully to B21 size and would be very thin in spots at a B20 bore size.

Blocks with a core shift will probably work OK when driven normally at a B20 size, but with the possibility of some blow by and a loss of some compression, but with a high-HP normally aspirated engine or with a blower or turbo it will balloon the thin areas on the power stroke and probably crack or split the cylinder wall in time as seen below.


Split cylinder wall.

Have bored a lot of cylinder blocks over the years and, you can actually hear the difference in sound when the cutter passes over a thin spot. In these areas, the cutter will actually force the thin areas of a cylinder wall outward as the cutter passes by resulting in an out of round bore - a rigid cylinder hone will also do the same thing and it is noticeable during the honing operation.

Another alternative is to offset bore the affected cylinders if the head gasket and clearance between the piston wrist pin boss and the rod will allow for this, or one or both can be trimmed by a small amount to get the required clearance. If this last approach is taken all of the pistons or rods need to be balanced. If both are trimmed everything needs to be balanced.

This image below shows a cylinder wall that was to thin and cracked and started leaking shortly after it was first run. Worst case scenario if large enough chunk of the wall breaks out it might hydraulic and ruin everything.

So the best thing to do before boring a B18 to a B20, is a JohnMc mentioned earlier, is to have the block sonic tested. If it does not pass the test find another block and try again. High performance and racing engine shops should be able to check a block for you for under $100.


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