L20B Rebuild Advice

[GRJones]

Hi I bought my 620 with a disassembled engine. Today I tore down my head as a first step today. One of my valves is cracked and a valve seal is broken. I for sure am taking the block to a machine shop and the head to check if it needs surfacing and boring. I would like to assemble back together myself. I was wondering what I should replace no matter what. All valves seem to be in spec. Camshaft is in good shape (Is it recommended to get new bearings for that) but I haven't measured crankshaft. Heard that needs to be polished.

 

Thanks

[datzenmike]

You'll have to replace the valve.

 

All valve seals must be replaced on a re-build.

 

Only the block will, or might need to be re-bored. If so, new over size pistons will be needed to fit it. Make sure you can get them first. A machine shop can measure the bore properly and determine if an over bore is needed. You can clean a re-use the old pistons if not damaged and use new cast iron rings. Chrome rings work best if properly over bored and the cylinders are pristine. For a quick hone job at home on the stock bore, cast iron rings are best. 

 

There are no cam bearings. The cam rides in the cast aluminum cam towers. 

 

You can polish the crankshaft main and rod bearing surfaces but not necessary. I wouldn't pay someone to do this but you can do it at home...

 

 

[GRJones]

Thanks Mike. Is it good practice to replace all valve guides and crankshaft bearings? 

[toshts]

Crank bearings, rod and main, are cheap and easy insurance especially if you've already got everything disassembled. Valve guides and seats can be reused as long as they're not damaged, your machinist will let you know though if you're getting a full valve job done there's a good chance they recommend new guides

[toshts]

Also I'd say to check if the timing chain has slack in it but with it disassembled that's a little trickier, I would check the chain guides and see if they're badly worn. If a chains not stretched it can be reused but also relatively cheap insurance. Double check the head bolts for warpage, they're not torqued to the point that they stretch typically but 3 of mine were bent with the last l20b head I did so I opted for arp head studs. Same with the rod bolts, heard mixed opinions on changing them but arp does sell better replacements 

[datzenmike]

2 hours ago, GRJones said:

Thanks Mike. Is it good practice to replace all valve guides and crankshaft bearings? 

 

I only would replace the guide(s) if needed. You may only have to install a new valve. A rough guide to follow is to push the valve up into the guide till seated. Then push sideways left, right and in parallel with the rocker arm where most wear occurs. Use a dial indicator to measure the deflection. The deflection limit is...

 

Intake and Exhaust....... 0.0008". This is not the clearance, just an indication the clearance has exceeded the wear limit of 0.0039" allowed. If under the limit skip the machine shop. 

 

If guides are replaced and reamed to size the valves have to be lapped. If seats are also replaced then they are ground for fit and then lapped. A shop can do this. If doing one, might as well do all.

 

 

Always replace the rod and main bearing shells. Cranks don't wear normally but they can be damaged. Damage will be obvious. Wear happens to the bearings so they are replaceable. 

 

[GRJones]

Hi again. Could this combination work? The exhaust is from a l16 and is the better one. No issues with clearance etc? I have the joined intake/exhaust from a 75 l20b

 

 

Edited November 30, 2025 by GRJones

[toshts]

23 minutes ago, GRJones said:

Hi again. Could this combination work? The exhaust is from a l16 and is the better one. No issues with clearance etc? I have the joined intake/exhaust from a 75 l20b

 

 

Yes, that exhaust manifold will bolt onto an l20b head. Might need to delete the emissions tubes but should work with both square port and round port heads. That's the highest flow factory exhaust manifold. The intake will work as well but it won't make much difference over a stock l20b nonmarried intake which you can find much cheaper

[datzenmike]

The biggest problem will be finding a 2 into 1 down pipe that matches the L16 style cast iron header.

 

There were two style of L16/18 cast iron headers made. Look at the underside. You want the one on the left below. It has the discrete 4 into 2 design. The one on the right just dumps into one with a dual output, the pipes are not separated like the one on the left.. 

 

 

 

This is my '76 710 that did have the co-joined manifolds on the U67 head. 

 

 

You will need to drill two 3/8" holes in the head to match that intake, then pipe this over to the thermostat and join to the thermostat by-pass pipe to make it functional. The other end of the by-pass connects to the lower radiator hose inlet to the timing cover.  

 

 

 

Above. You want the inlet on the right that has radiator hose, heater hose return and the other end of the thermostat by pass.

[GRJones]

Problem is I can't find a 78-79 intake but I bought that exhaust cheaply

[datzenmike]

An L16/18 intake will work but the runner diameters are smaller than the L20B ones. You could run it while looking for the correct one. Any..

 

'78-'79 620 truck or S10 200sx

'79-'80 A10 HL510 (Canada kept the L20B for '80 the US switched to the Z20)

'80 720 truck one year only.

[GRJones]

If I'm planning to go EFI could I use the l16 exhaust and bolt the separator plate to my 75 intake and use that? 

[toshts]

18 minutes ago, GRJones said:

If I'm planning to go EFI could I use the l16 exhaust and bolt the separator plate to my 75 intake and use that? 

Theoretically yes - there's some concerns on how solid the separator plate would be just bolted on and not sure of clearance but I think it'd clear fine. There's also the issue of heating, no coolant runners or exhaust runner to help warm it up so depending on the climate your in it could give you issues, maybe less issue with efi

[datzenmike]

 

 

 

 

I think it will clear by a mile. 

 

The carbureted 240/260 Z car engines had the water warmed/cooled intakes with the SU carburetors. This was dropped when the 280z with EFI came out so EFI should not need this. 

[GRJones]

Is there an easy solution for joining these into 1 or does it need to be fabricated at a muffler shop?

[datzenmike]

 

 

You are so lucky!!!!! I just happen to have an L16 flange which will make this much easier to fabricate. (thanks to Mklotz70) 

 

 

 

Takes a 1.5" INNER diameter pipe. Be sure the fabricator extends the pipes through the flange by 

 

3/16"?? Measure the depth on the manifold you have after cleaning it. This way, same as stock, fits snugger and less chance of a gasket leak.

 

Heat the manifold around the broken bolts yellow hot and, with luck, twist out with Vicegrips. This is what I did. Chase the threads with a tap. I used stainless steel studs to make them rust proof for the future.

 

 

[banzai510(hainz)]

people had flanges made to mate to the manifold shown but a shop still needs to make the y pipe part. there is places that have premade y tubes. 

 

somebody has the specs drawing for this to have made. Datzenmike and Mklotz have it for sure

 

Edited December 4, 2025 by banzai510(hainz)

[GRJones]

Well my intake is missing the bottom bolt hole. U67 doesn't have a bolt hole too. Guessing since it's attached to the exhaust it doesn't need the extra support. Just tap a hole into the head?

Edited December 6, 2025 by GRJones

[datzenmike]

There are no missing holes, they were never there. There are four bolts (though I'd replace with studs) along the bottom of the head. Each bolt has a cone washer and nut that bridges across both intake and exhaust flanges to hold them to the head. Check out the bottom of the exhaust manifold, it doesn't have any holes either.

 

If you took this intake off you would have seen this? In the picture below the intake fits down behind the loose bolt/washer.

 

[GRJones]

Huge relief. I did suspect that. Hard since I never saw it assembled. You're the best. Thanks Mike

 

*update*

I worked on my manifold today to remove all the accessory piping and I learned a lesson. When I was using a bolt extractor I cracked the cast iron. I am thinking I could carve out so the bolt hole is lower. It seems like there'd be enough meat left. Don't want the crack to spread. I ended up just drilling out the fittings since I tried for hours to save the threads.

Edited December 7, 2025 by GRJones

[GRJones]

I really wish I just cut off the pipe and filled the fittings with weld. The holes are out of round and I don't think I can tap them. Any advice on how to fill the holes. No experience welding cast iron. Extreme heat jb weld seems iffy. Brazing? Thanks

 

[datzenmike]

Just fill with weld. It's just a plug sealing a hole, not holding anything together. Put a short bolt in the hole and cover with weld.

 

 

[GRJones]

I got a cannon intake and was just wondering what are these 4 ports? #1 is blocked off

 

Thanks

 

 

 

[banzai510(hainz)]

take a photo of the otherside of intake.

1) is PVC valve

2) I dont know what you use it for

3)might be a water line. I got to see flage side to confirm

4) I assume hook to power brakes if you have it.

 

be honest hope you didnt spend alot of money on it. The stock intakes are fine and can just hog them out to get same performance,

 

this you dont need a weber carb adapter (weber fit right on there hole spacing is the same)and keep the car near the same height.

 

 

dont loose those wedge spape washers for the exhaust for the inner lower intake exhaust.

Edited February 20 by banzai510(hainz)

[datzenmike]

You need a vacuum port for the ATC (which you won't be needing with a Weber) and another for the charcoal canister. 

 

Runner lengths are really different.