I have a B20F (8-bolt crank) and I was thinking about rebuilding it using B21 pistons.

I can get pistons/rods from a '81 B21F. Can it be done?

Paul

It's very easy to do but there is a huge problem, they are incredibly heavy. The absolute worst place that you can add weight to a car is the pistons because you have to spend a lot of energy bringing the pistons to a stop & then starting them again every time they get to TDC. A 6-bolt bottom end with 3.54" pistons will beat an 8-bolt bottom end with 92mm pistons every time. The bigger bore is no help at all & the special headgasket needed is just a big PITA.



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Three 164's, Two 144's, One 142 & a partridge in a pear tree.

6 bolt and 8 bolt rods are not interchangable I measured myself. 8 bolt and mrods are as I measureded myself. I ended up using mrods because the 8 bolt motor I used to rebuild looked in good condition but one rod had a slight bend to it, Weak rod someone told me bad detonation can bend the 8 bolt rods. I had my mrods lightened and balanced and they work great,.
Eric

tHE BEST ( so I have been told) rods for this conversion are the Mrods from B21 motors and others. The stock 8 bolt rods are very week and heavey, unlike the stock 6 bolt rods that are stronger and lighter, But you could also get custom h beam rods.
JERIC
ps I used the mrods in my B20 motor very happy with them.

"Mrods from B21 motors and others"

What years did the B21 come with the Mrods? The B21 pistons and rods would come from an '81 engine. I have a '73 B20F block (6-bolt crank) and '74/'75 B20F block (I'm told it's a '75 block - '74/'75 both had the 8-bolt crank?) that I can mix and match from.

Paul

BTW- I also have a '70 B20E head (from an 1800) and the '74/75 B20F head. I want to use the B20F head because it should have hardened valve seats in it. How can you check for hardened valve seats?

M rods were used '80 - '83, so you should have those available. They are somewhat heavy, but they are MUCH stronger than the 8-bolt B20 rods.

'74 - '75 heads do have exhaust seat inserts. Don't use the E head, or you'll have way to high compression with the larger displacement.

You can use any B20 block, as long as you use the later crank.

Hi Phil,

Do you know if it's possible to use-rods with B-20 pistons (rather than the b-21 pistons)?

I need to track down a set of rods for the '75 engine that will be going into my 122. (One of the rods that was in the engine had a slight bend in it.) I'd be using the 8 bolt crank.

I'm just wondering if M-rods should be considered, or if I should look for something else.
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http://dylans122.blogspot.com

One of the rods that was in the engine had a slight bend in it

Sounds like it ingested water or fed 40psi of boost to it, that's the only way they bend. If you want to keep it simple, find a 6-bolt bottom end. The lighter piston & rods will save fuel & give you a couple of free HP.

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Three 164's, Two 144's, One 142 & a partridge in a pear tree.

Phil-

Thanks for the info.

Doesn't look like the F head I have is from a '74/75 engine (don't see any seat inserts). It does have an "F" stamped inbetween the injector ports? I was given the engine/head (head wasn't attached to the block) and was told both was from a '75 240. So the block is a '74/'75, and the head has to be a '72/'73.

So is the earlier F head worth using?

I understand:

-the stock B20E head breathes better, but needs more octane. (can I use the E head with a thicker head gasket?)
-the early B20F don't breath great, and don't have alot of inner material to port
- the later B20F don't breath great, but has a different inner casting to do a better port job on.

The E heads have better exhaust ports stock, but the F heads are better after porting. All the F head castings are the same, '72 - '75.
In the U.S. market:

If the center head bolt on the manifold side sits on its own little round island, it's an E head. If the island bridges forward around the #2 injector hole, it's an F head.

If it's an F and has threaded holes for aux air valve and coolant temp sensor on the flat area forward of the manifold, it's a D-jet F and does not have hard seats from the factory. If there are no holes in that area (except some have a single 3/8" stud there), it's a K-jet F and does have the seats.

Never, ever use a fatter head gasket than you have to. It will lower compression, but it also destroyed the squish and quench, resulting in slower combustion. You then have to run more timing advance to get any sort of performance, and then you're back into pinging. It actually does more harm than good.

Bottom line: use one of the F heads, deck the block to bring the hard down closer to the pistons and compensate for the fat big-bore gasket thickness, then shave the head however much more is necessary to product the compression ratio desired. If you have to have exhaust seats fitted, it only adds about $60 to the cost of rebuilding the head (at least at the shops I use).

I've heard that there are two different types of F heads. One is the early type, not as good for porting. Then there is a later style, which is better for porting.

The early style has the middle bolt island connected via a single isthmus to an adjacent injector pad. The later style is connected on both sides by two isthmuses.

Late style F casting:

--
'63 PV544 rat rod, '93 Classic #1141 245 (now w/16V turbo)

Of the dozens of B20 heads I have seen, yours is the only one with the double bridge next to the center head bolt boss. I think that's a rather rare casting.

The earlier F castings have plenty of metal for street type porting using stock valve size. So if that's what you have by all means use it.

The metal in the exhaust bowl does get thin when you cut it for hard seats. I recently cut apart a head fitted with hard seats and the top of all exhaust seats had cracking into the water jacket. I assume that fitting hard seats to a B20 head must be done carefully and probably precludes the use of bigger exhaust valves.

Charlie

The picture is of my R-sport head. I'm not sure if they used a special casting for them, but I have seen similar looking late F head castings.

Head's a bit grungy, but the valves very nearly touch - you can barely fit a fingernail in between them, but it does have hardened seats squeezed in.


I also noticed that it has a more pronounced hump in floor of the exhaust port, as compared to my early F head. Makes the exhaust port a little less of an abrupt turn.

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'63 PV544 rat rod, '93 Classic #1141 245 (now w/16V turbo)

Here is an E head

Here is an earlier "F" head with a similar exhaust port floor, same as yours. It had a bid of work done to the roof of the port.

I've never seen that casting either, including late Swedish heads. Could it be one of the R-Sport castings?

We've put 38.5mm exhaust valves with hard seats in a fair number of F heads without problems. I've never cut one of those up, though, but the coolant stays on the side it belongs on.

I have two of that style B20F head in my garage. Both came from 1975 240 series. I also have two 1974 B20F heads from 1974 140s, and they have the single raised portion at the same head bolt.

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john

Just asking a silly question.

Would that increase the CR or decrease it? Because of the dish top of the piston.

Paul

Dished pistons would decrease compression.

IIRC the B21FT piston dishes are about 12cc.
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'63 PV544 rat rod, '93 Classic #1141 245 (now w/16V turbo)

Unless you're planning on running some sort of forced induction, or you're planning on revving it really high (double valve springs, rather extreme cam) then M rods are probably overkill. They're heavy, and not that much stronger.

Just look around the exhaust valves. If there is an inset metal ring in the cast iron head, it has hardened seats. If the cast metal runs right up to the valve, there aren't hardened seats. If you have hardened seats put in, might be worth a little extra money having slightly larger exhaust valves installed, along with some bowl work. The exhaust ports seem intentionally pinched off in F heads for some reason. I suspect some sort of emissions reason, perhaps a passive EGR?
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'63 PV544 rat rod, '93 Classic #1141 245 (now w/16V turbo)

Yup, you get the odd block with core shift where the liners go too thin, but generally it's fine. KG Trimning do the appropriate head gasket.

What is the maximum it can be bored out to - I have some new 93mm (+40 thou) B21 pistons and a 1974 8 bolt B20E engine? The engine was Kjet injected.

What are the best rods to use, the existing ones or others?

I have the right head gasket for this bore.

Most blocks will go out that far, but not all. I would take it to a machine shop and have it sonic tested to see how well the bores are centered in the block casting, and how thin the cylinder walls would be after boring it to 93mm. You should be okay with .100" wall thickness for a medium-performance street engine.

The '74 - '75 rods have a weak area. If you plan on the engine seeing high revs at times, I would use '80 - '83 B21/23 rods, the ones with a casting mark of "M" -- they will work with the crank you have.

I would use '80 - '83 B21/23 rods, the ones with a casting mark of "M"

Casting mark? I hope not! :)

What's the difference between the M rods & the first 8-bolt rods? Did they get rid of the horrible bolts?

--
Three 164's, Two 144's, One 142 & a partridge in a pear tree.

Doh! Forging mark, of course...

I don't have either type to compare right now, but I remember the B20 8-bolt rods being weak inboard of the bolt heads, and the M rods are not.

I think given how cheap new aftermarket rods & custom pistons are, I'd keep the crank & throw away the 1 ton rods & pistons. It doesn't matter if they are from a late B20, B21 or B23 GT, the weight makes them the ultimate in anti-performance pieces.


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Three 164's, Two 144's, One 142 & a partridge in a pear tree.