Which engine to build?

[Easygoingjc]

So the l20b in my 75 620 longbed has low compression accross the board. 50-75psi. Before pulling the head to see the cylinders should I even waste my time with this engine. 

 

Here I pose a question to the old datsun gurus?

I want a reliable in city cruiser.

Should I rebuild this engine or is there inherant problems with them.

I do have the l16 from my other 73 620.

I also have another possible engine from a 81 720 with I think is a 22z engine, is that right? It has two plugs per hole. 

 

Which one should I go with? 

 

Thanks in advance 

Jordan.

 

 

[datzenmike]

First eliminate a faulty set of readings....

 

Set the valve lash and try another set of readings on a warm engine. Even if all the rockers will wiggle slightly when the cam lobes are pointing up will do. Overly tight valve lash will cause low readings. 50-75 is so low it's hardly believable.

 

Test your compression gauge on another vehicle with known good compression.

 

 

 

The cause of very low compression is either in the block or the head so if possible find out which because a new or reconditioned head is a lot easier and cheaper than a rebuild. It could be a bit of both but if it's the head that's a lot easier to fix. I've run an L16 head on my L20B while the L20B head was being fixed.

 

The L series engine is unbelievably reliable and tough. The only inherent problem with them, is an owner that doesn't look after it. Keep it filled with oil and water and never over heat it and it will last forever.

 

 

If expecting to run a Z22 engine you would also need the transmission that goes with it. I had a Z24 in my 620.

 

[Easygoingjc]

I could prolly buy the whole 720 truck for around 800 bucks. Just going through options. I also heard somwhere that the z engines were prone to head gasket failures at the back cylinder, or maybe that was the z24.

 

I was thinking the same thing on my compression tester. May go get a renta tool to double confirm my tests. I was thinking a low compression to be around 100psi.

 

I wonder if the truck not running for a decade could just have stuck rings?

 

 

[datzenmike]

100 is quite low but if all cylinders are even and not burning oil it will run pretty well. I wouldn't want 120 or even 130.

 

There may be a good reason it's been sitting for 10 years. Just worn out and burning oil. 

 

Squirt some oil in the spark plug holes an re-test. If it jumps up noticeably the rings are not sealing. If not much change, the valves and seats may be pitted and not sealing 

[Stoffregen Motorsports]

The way I approach any major engine decisions doesn't seem to be overly popular here on Ratsun, but as someone who built Datsun engines professionally for over a decade, I hope that you can weed through the BS and make your own judgement of the weight of my opinion.

 

I do understand the desire to keep things tidy, but the only real way to gauge the health of an engine is by a physical visual inspection. Compression tests and leakdown tests are not foolproof and rely heavily on the user knowing the tricks of each. Pulling the head to inspect the bores and the oil pan to get a visual on the bearings is the very least one should do to get a baseline bill of health. I say baseline because you still can't know for sure without removing the pistons to inspect the rings.

 

You don't want to remove the head or the pan? I get it. For most people, this work is tedious. So the next best thing is to roll the dice. I use a gambling term on purpose because if you roll and come up aces (apologies for the mixed metaphors), you win big. What I mean here is - so you spend some time to get the engine running, enough to get a good feel for the condition of the engine. 75% of the work is work that you would need to do anyway in an engine replacement, so if you find that the engine is not healthy, what have you really lost? Maybe a couple gaskets and about six or seven hours labor. But what if you get it running to find that it runs great? That's the gamble. Sometimes you win.

 

This kind of work is easy for me, because I am set up for it and have been doing it forever, so excuse me if I simplify things a bit. There are tricks to bringing life back to an engine and some of them may seem like snake oil, but some tricks do work, and can save lots of time and money.

 

Before we get going though, one question - what is your ultimate goal?

Edited September 15, 2019 by Stoffregen Motorsports

[Easygoingjc]

7 hours ago, Stoffregen Motorsports said:

Before we get going though, one question - what is your ultimate goal?

I had just bought 2 620 field finds. The 75 is a long bed. This truck is complete and will be used as a truck for dump runs, hardware store hauls etc. Basicly a weekend truck. Just want it to run and start reliable not going to drive it hard.

 

Once the 75 is running I can play and tinker with the 73 which is not complete right now. It will be the fun truck hopefully.

[Stoffregen Motorsports]

If that's your motivation, I would just get it running, put some hours on it (assuming it runs well) to loosen it up. Possibly dump a quart of ATF in the engine oil, which can help loosen stuck rings, due to its high detergent content.

 

If you are certain that there is a problem, then dig in and do a physical inspection. Doing a compression or leakdown test on an engine that has been sitting even for a little while can be totally misleading. Pulling the head on an L4 takes only about an hour, so if it does run poorly, or smokes or uses oil/water, maybe do both - a compression test to get a base line, and also remove the head to get real info.

[distributorguy]

If your compression really is that low across the board, its likely one of two things:

1.  corrosion on the valve seats and holding the rings stuck to the pistons so they don't seal, and rust on the valve seats

2.  the timing chain jumped or was installed wrong and the cam timing is all wrong - off by about 2 teeth at the cam sprocket.  

 

A leak-down test is done with the valves on the specific cylinder your testing closed, so it'll let you know if the valve seats or rings are leaking (by hearing bypassing air.)  That's prove or disprove #1.  

A visual inspection using your Datsun shop manual can confirm #2.  

 

#3 is the next step.  You can buy a bore scope for under $30 on Ebay or Amazon that uses your smartphone as the display.  Take a look in the cylinders before pulling the head for any deep gouges in the cylinder walls.  Scuffing can be acceptable, but gouges are problematic and will be obvious if they exist.  

 

[Tom1200]

So in 2014 I had an A12 in my car that had been in the car for something like 60 race weekends. The last 3 laps of it's last race motor dropped 100 rpm ever lap. 

 

When I got it home it had 80psi across the board. While the valves weren't bad the rings were shot and the bores weren't great. If it were a street car I'd have slapped some rings in it (possibly some bearings) and called it a day.

 

A basic leak down test will give you an idea of what's going on. You can do a basic hillbilly rebuild (as noted above) or do a basic rebuild. You could probably get a piston set from Rockauto for around $125 and your local machine shop likely charges $150-$175 range to  bore the block. Add in the cost of bearings, gaskets and possibly a valve job and you may be all in for $600.

 

I'm of the same mindset as Stoffregen; take it apart inspect it, replace what needs replacing and leave the rest.

 

[thisismatt]

L20b to 22Z would be a poor swap choice IMO.  From what I see, the 22Z makes less power and you have all the hassle of modifications for no gain.

[datzenmike]

With an L20B and a Z22 you can swap parts around and make an LZ22 or 'stroker engine. It would be basically an L22B.  Z22 block and everything in it with L20B head and manifolds, oil pan, all the timing components forward of the steel block. It's a 10% larger L20B with 9.84 compression.

[Easygoingjc]

Thanks for all the replies. Picked up another compression tester and now compression is sitting around 150psi across the board. Rebuilding the carb at the moment. Had it fire but the accelerator pump had a massive leak. Once running I will see if it burns oil and how it runs. 

[Easygoingjc]

15 hours ago, datzenmike said:

With an L20B and a Z22 you can swap parts around and make an LZ22 or 'stroker engine. 

I was curious if you could make kind of a mutt from a combo of parts? Just curious though.

 

Doesn't seem to be the engine for reliability. Rather a fun engine for the other truck.

 

I am not opposed to any rebuilds, nor ignorant when it comes to any mechanical work. Just was wanted to know what engine you experienced datsun/nissan guys would build for reliability, also a nice n/a option to play with for another truck once I have a reliable one.

 

 

Edited September 20, 2019 by Easygoingjc

[datzenmike]

L series for reliability and simplicity. Converting a Z22 with an L head would be just as reliable and larger displacement. Although I haven't installed it yet, I converted a Z24 with an L head. Maybe next summer.

[banzai510(hainz)]

dude just put the L20 in there stop overthinking this. If you new just do the L20

a Z series head swap there is some gasket issues and your not there yet

[Easygoingjc]

New to Datsun and their quirks, not new to engines or rebuilds, that is why I was asking. 

 

Didn't want to waste time or money on a engine lemon. I am sticking with the l20 now that everyone has confirmed them to be reliable. Once running again will tell if rebuild is in order.

 

Thanks for the other engine info as well. I do have two trucks, so maybe in time the second one can become a true ratsun.

[datzenmike]

To be fair the Z series are just larger L series but with different head. Just as reliable but head design limits performance additions.

[Easygoingjc]

What is the second set of plugs for? 80's Emissions?  Has anyone used the distributor of the z engine on L engines for HEI. Matchbox distributors seem rare around here. 

Edited September 20, 2019 by Easygoingjc

[datzenmike]

Two ignition points do several things. It burns the fuel / air in a shorter time allowing less ignition advance (usually 3 to 5 degrees vs. 10-12 degrees for a single plug) so less chance of pre ignition. There is also less time for oxides of nitrogen to form.

 

Pro stock, funny cars all use them because they work.

 

I see no reason why you can't run a Z series 8 plug distributor in a four plug L series. You would have 4 unused terminals on the distributor but so what?

[DanielC]

"You would have 4 unused terminals on the distributor but so what? "

Do not use the second coil in this setup.