Preliminary 89mm Big Bore 140mm Long Rod 79mm Stroke L28 engine

[Distraction240z]

Hello everyone

I have a 1971 240z that is pretty original, the engine is getting tired and I am looking to rebuild a L28E form a 1983 280zx that I have found locally for $1500 Canadian.  I am looking for your opinion on the simplest rebuild with the most bang for my buck that will.

1)      Run on 87 octane fuel.

2)      Still use my original SU Carb.

3)      Be easy and reliable, not looking for big power anything over 150 at the wheels is great.

4)      Cost no more than $6000 of my Canadian pebbles.

I think the safe option for a slightly better rebuild than stock will be the use of the Z22S Pistons and L24 Con Rod.  The compression height of the Z22S pistons is 35.5mm with a bore of 88mm.  The Datsun L series engine have a block deck height of 207.85mm. 

Or Nissan FJ20 140mm Connecting rods, and Xr500 89mm pistons.  About 1500$ for connecting rods, pistons, rings, everything but bearings.  From my research there are two small problems.  An extra 1.25mm in deck height (209=(79/2)+29.5+140) and the small end width of the Honda piston 20mm vs the 20.5mm of the FJ20 con rod.  I think I can solve the piston issue with a thicker head gasket or taking some material off of the top of the pistons, which one is better?  I also don’t think it would be a big problem to remove 0.5mm from the con rod small end width to fit the Honda Piston.

 If you haven’t guessed I want to run FJ20 Con rods and XR500 pistons, but am concerned with over boring to 89mm.  If you have some old school fixes, tips, advice or if my information is wrong, please share with the class. 

measurements are mm	FJ20 Con Rod	L24 Con Rod	Datsun Spirit Con Rod
Center to Center	140	133	139.5
Big End Dia	53	53	53
Small End Dia	21 or 22	21	21
Big End Width	26.8	26.8	26.8
Small End Width	20.5	23.9	23.9
Price USD Per Cylinder EST	58.17	111.33	204.17
	XL500	Z22S	Datsun Spirit Piston
Bore	89	88	89
Compression Height	29.5	35.5	29
Wrist Pin Dia	21	21	21
Small End Width	20	Standard	Standard L24
Piston Height	66
Price USD Per Cylinder EST	80.45	100	237.5
Rod to Stroke Ratio	1.772151899	1.683544304	1.765822785
Compression Height	209	208	208
Price Per Cylinder EST	138.62	211.33	441.67

Edited December 31, 2022 by Distraction240z

[1600dave]

Based on your stated requirements, I'll suggest stock L28 rods, L28 flat top pistons (oversized if you desire) which will then give you more room in the budget for stuff that will make a much bigger improvement.like some head work and a mild cam. Also, Z22 pistons are dished so not the best choice

 

Edited December 31, 2022 by 1600dave

[gav240z]

Agree with David.

My recommendations are.

4.1 ratio LSD diff.

L28 with flat top pistons / N42 head for 10:1 comp.

SM needles for SUs.(rebuild the SUs - Z-therapy do excellent work)

Since you're in North America contact Steve Bonk for recommendations on camshaft specs.

Nice set of headers to suit (if don't have already).

E12 Electronic Distributor (Hitachi) off a 280z/280zx. Have it re-graphed to suit your cam / engine set up. A good ignition shop will be able to do this. Around 16 degrees initial advance and a further 16 in mechanical/vacuum total is probably a good combo. But get an experienced shop to set it up.

You'll easily spend $6k if you're rebuilding the motor and doing above. But you'll hit 150+HP and have a really good set up in an early Z.

I'll find an excellent thread to review also in a minute and link to it.

If budget allows I'd highly recommend going Triples..

 

 

[gav240z]

Have a read of this.

 

 

[Distraction240z]

I have read that post and a few others and i am split between working on my L24 or going out a buying a L28 and modifying that.  I really want to retain the stock look with the SU carbs.  If i can find a L28 for a reasonable price I want to build the long rod bottom end that I can then build off of down the road.  This year i want to build the bottom end and get it in the car running and on a dyno with my cleaned up SU carbs and a head that i port myself.  If i get to 200hp then i have achieved success on my own.  If i get 150hp i will be happy, but after a year or two send the carbs to Z therapy, or send the head out to get rebuild, new cam, port and polish.  

Right now i am looking for advice / info to build the bottom end.

1)  Does anyone have experience running XR500 pistons or FJ20 Rods, what did they do and how did they do it?

2) Do i need to cut valve reliefs in the piston or will turning them down to 28.5mm compression height work?

3) Is the top land thick enough for this, or go with a thicker head gasket?

4) Will the standard XR500 piston rings work?

5) Is it really that hard to bore out the F54 block to 89mm and still have a cylinder wall thickness of 3mm (0.120")?

6) Is it hard to remove 0.25mm from each side of the small end of the con rod to make them fit into the XR500 Pistons?

These are the pistons and rods i want to purchase.

https://www.cmsnl.com/honda-xl500s-1980-a-usa_model1145/piston-kit-std-non_06131429003p/

https://www.maxpeedingrods.com/product/steel-engine-connecting-rods-conrods-for-nissan-fj20-21mm-pin.html?search=fj20

 

The car currently has the Subaru STI R180 Diff with CV joints, 1983 280ZX turbo Trans, a 123 ignition distributer, a lumpy cam in the E88 head of unknown specs.  I did all of this over the year by myself even though I have mostly stuck to small engines like dirt bikes, snowmobiles, and am looking for a new challenge.  I also need to install new engine mounts as I have a tone of rear wheal hop, any recommendation? 

Edited January 8 by Distraction240z

[1600dave]

1. XR500 pistons were popular here in Australia in rally 4 cylinder engines back in the 80's. I knew a few guys that ran them, have no idea what they used for rods or how they made them work though.

2. Depends on the thickness of the piston crown for both options. Depending on your setup (valve sizes, cam lift, etc) you may need both. Ideally you'd mock it up and check valve to piston clearance. I run L28 flat tops in an L18 and would have had just enough clearance with a 72 degree cam, but I put valve reliefs in the pistons anyway in case I moved to a higher lift cam at any stage.

3. See point 2, depends on specs for piston

4. Should do

5. Ultrasonic tester is cheap from China. Mine set me back $60 or so, and they were Australian $$ so not many USD at all. Or strip it down and take it to a machine shop to get measured up. I'd do this first, before buying pistons (or anything really....). I've seen people say they needed to go thru a stack of blocks to find one that would go that big. 

6. Can't see why it would be. Will .25mm be enough (depending on whether its a press fit pin or fully floating) to allow for any misalignment / movement. Of course, depends on the rod itself and how easy it would be to remove metal. I can have a look if you want, have a set of Maxpeeding FJ20 rods in the shed. Keep in mind for your budgeting that you'll almost certainly need to buy 2 sets of FJ20 rods, doubt you'll be able to buy just 2 to make a set of 6. Plus I'm not sure where you get FJ20 rods with 21mm small end ?

I still reckon its a lot of messing around for not a lot of benefit given your engine goals when L28 rods and flat tops would do the trick. I understand what you're trying to achieve, I love tinkering and messing round, just trying to be devil's advocate 

Edited January 9 by 1600dave