Gonna put another B20A in my other lude. It's a 90 b21 engine but I need to know what connectors or components are OBD1 THAT I need to save.

B21a1 to B20a5/A Swap - Write up - Honda Prelude Forum : Honda Prelude Forums
^^ this should get ya nice n confident.

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God's Not Dead

Most people who do this swap keep the b21 manifolds and just use the b20 block and head. There is no issue with the wiring/connectors using this method. The only real issue with this method is the b21 intake manifold runners don't align perfectly with the b20 head ports. You might be able to port match these, but you'll want to be careful doing that. It's easy to remove too much material and really mess up the air flow even more than it would be if you left it alone.

That's a decent write-up you posted Mike. I wish Sean would go in and remove the conflicting information in it though. It's a little confusing the way it's written IMO.

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1991 Prelude Si ALB 4WS
2017 Ram Rebel

That write up was pretty good. I noticed that it said to get a different water tube if you're going to manual. That's a little overboard. I just cut a few inches off with my saw zall and only took a minute and cost nothing.Gotta swap it cause after a week after changing oil it turns black plus it goes through a quart a week. Gas went down in price but oil hasn't caught up yet.

Cutting the water pipe is a solution for that, but make sure you have a good hose clamp on there. Since you cut the pipe there's no longer a flare on the end. So in theory the hose could slip off the end of the pipe. Remember that the cooling system is under pressure at operating temps.

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1991 Prelude Si ALB 4WS
2017 Ram Rebel

I wonder if the intake gasket is the same? If not, I guess I'll use a b20.For some reason there was a mention of difference.

Yes, there is a difference in the intake manifold gaskets between the b20 and the b21. In my first post I mentioned a difference in the b21 intake manifold runners and the intake ports on the b20 head. I'm not sure which gasket would work best for this.

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1991 Prelude Si ALB 4WS
2017 Ram Rebel

Maybe I'll go to the parts store and order both to see what the difference is.All I know is the b20 intake gasket will fit the head.

That would be the best way to see the difference in them.

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1991 Prelude Si ALB 4WS
2017 Ram Rebel