B21A1

[buckybwhiplash]

I recently purchased a 91 prelude w/ a B21A1 and the head and the cams are basically in the trunk. The prior owner also was goofing around the bottom end and had a couple of pistons out. My questions are, what is the torque spec and sequence for the head? And what is the torque specs for the rods and mains? And last but not least, I noticed that the cam bearing/caps are numbered in what order do they go onto the head, and is there a timing spec for the cam position sensor? Or tell me where to find this info with in Prelude Driver. I downloaded the service manual and haven't found any the info that I need.

Thanks

[spiffyguido]

Hi BuckyB,

Welcome to PreludeDriver.com. We're happy you found us.

To bolt down the head, you should have 10 bolts. Two of them will be longer than the other 8. Spin all the bolts into place. The longer bolts are torqued first to 22 lb-ft. Then you work outwards in pairs from the center going to the left, then right, then left, then right. Tighten each bolt to 22 lb-ft, then repeat the sequence at 49 lb-ft. I encourage you to check out page 5-38 in the 1988 Service Manual in our manuals section. It has a picture of the steps.

Torque on the camshaft holders is 7 lb-ft (see page 5-34)

When you say cam position sensor, I assume you're referring to the CYL sensor. The teeth are grooved so that it can't be inserted in the wrong position, so you have nothing to worry about. The bolts for the CYL sensor are 9 lb-ft. See page 6-174 for more information.

Regarding the numbering, Honda always numbers their engines starting on the pulley side of the motor. So, in you case, number one is on the right side of the engine.

According to the Helms manual, the torque specs for the rods is 34 lb-ft and the torque spec for the mains is 51 lb-ft. When you torque in the mains be certain that you oil the threads and the bolt head.

I hope this helps.

[buckybwhiplash]

Thanks, the info is much appreciated.

BuckyB

[buckybwhiplash]

My last question is, if this motor is toast, living in the bay area its JDM central is it worth looking for another B21A1 or going with the B20A? I've looked around for another B21A1 and they seem to be quite scarce and some what spendy here. I've done some research into rebuilding and the expense out weighs the cars value (thats going with new cams, pistons, brgs and machine work on the block and head) or is there another source for hard parts that are a little more economical. I've read that a B20A can be used with using all of the exhisting fuel injection and sensors and the front motor mount is this true?

Thanks again
BuckyB

[spiffyguido]

As far as I've heard it's not that hard to get a B20A into the car as long as you get the right one.

There's a good writeup here: http://www.harleyc.com/prelude/writeUps ... fault.html

[buckybwhiplash]

I just finished the assembly of the B21 and it has a wrist pin/rod knock. I found a B20A5 and purchased it and was wondering as you mentioned that "you get the right one." Is that any B20A5 from another prelude or JDM only? And if what I have is correct with my purchase a domestic B20A, what intake manifold gasket do I use? I relize the gasket kit needs to the B20A but I noticed there are some differences between the two.

Thanks

[spiffyguido]

Let's start by clarifying the engines that we're talking about.

The B20A5 and B20A are different engines based on the same platform. The missing '5' from the B20A's name makes a difference.

The B20A5 is a motor that was available in many markets, including USDM. It's the engine that is found in the majority of 3rd generation Preludes. These engines are very plentiful. Mechanically they are a lot like a B21A1 with a few differences. The B21A1 is actually just a bored out B20A5. The B20A5 uses the same block as the B21A1, except that the B21A1 uses FRM sleeves and the B20A5 uses metal sleeves. The B20A5 is OBD0 where the B21A1 is OBD1. The B20A5 uses an intake manifold with secondary runners, while the B21A1 manifold has just one set of runners. Their exhaust manifolds are also different.

The B20A is a JDM engine that came in the 2nd and 3rd generation Preludes. The engine changed with the car, so a B20A from a 2nd gen Prelude is NOT the same as a B20A from a 3rd generation Prelude. These engines are similar in many ways to the B20A5/B21A1/B20A7/B20A3 series, but they also differ in many ways. The B20A is a favorite among 3rd generation Prelude enthusiasts because of its 160hp output, but you do need to make a few changes to get one into your car.

The B20A5, on the other hand will more or less drop right into your vehicle as it is similar to the B21A1 that you started with. All the manifolds from your B21A1 should fit on the B20A5 without issue, and viceversa. I've even heard of people putting B20A5 heads on B21A1 engines with little trouble. These engines are very similar.

So, to answer your question about the gasket, you can use whatever gasket that matches the manifold you'd like to use. The intake from the B21A1 flows better than the one from the B20A5, however, if you're swapping out the ECU and all for the B20A5 OBD0 system you'll have to use the B20A5 intake. Hope that helps.

[buckybwhiplash]

Sorry for the confusion about the B20A5 and the B20A. The engine I'm using is a B20A5 and am not switching to the ODB0 electronics. And I guess most of my questions will be answered when I get the engine out and look how they mate up to get the proper fitment of the gasket.

Thanks again.