L20b teardown w pics (and some questions)

[rjgilligan]

A few years back, the chain on my 79 620 jumped and left me stranded on 95 north outside of Philly during rush hour. Heard a series of large bangs and the engine had barely enough compression to get me to the shoulder. I assumed there was most likely damage to the valves. By some miracle, a few weeks later an L16 with an A87 head popped up on craigslist right in the area for $250 so went up to Yardley and strapped it down in the van. 

 

I stripped down the old engine on a stand, but despite more than normal oil residue buildup I can't visualize any issue with this l20b head. Everything LOOKS good. Is it possible that the chain didn't jump enough to bend valves? Or can you just not visualize this sort of thing?

 

 

 

 

Regardless, I had this other engine, so I pulled the stripped l20b off the stand, put on the l16 and pulled the head.

 

 

 

 

Cool. I knew the guy was racing it since it had an oil cooler hanging off the side of it, but this is what I ended up with.

 

I'm gonna put this peanut head on the L20b, but can I get away with putting these flattops in the L20b or will I run into issues that can't be remedied? I don't mind running the truck with better gas or tuning carbs or whatever. This is my fun vehicle, I'm not about to throw dual side-drafts on it or anything, but it's not like I haven't already spent some time/money replacing nearly everything on it and repainted it turquoise.

 

 

[datzenmike]

I don't think it's physically possible for the chain to 'jump a tooth', there just isn't enough slack. In addition, there is the timing chain tensioner that controls what slack there is. In addition to that one tooth is only 9 degrees of cam position change, not enough to affect much as at TDC on the exhaust stroke, the exhaust is almost closed and the intake just barely off the seat opening.

 

What you describe could be a blown head gasket.

 

 

 

Identify what you have. Look on the block just behind the dip stick handle. The engine size is stamped there.

 

Now measure the diameter of those flattop pistons. An L16 has an 83mm bore, the L20B is 85mm. This will tell you if they will work.

 

Not convinced from the picture that the A87 head is a closed chamber 'peanut' head. Clean it up and look. The combustion chamber is not round and does not extend over to the cylinder wall. A closed chamber head has areas that are separated from the piston top only by the width of the head gasket and look like this....

 

[kelowg]

Ck the piston and rings, run l series lean and u burn pistons/rings to shit. Broken ring lands and/or rings.

[banzai510(hainz)]

first where you get flat top L20 pistons????????

 

I think flattops on a L20 would be too much for a daily driver. If you race and have 98 octane your fine. eastcoarst might have 94 if I remember right

 

 

flattops on L16 is fine.

 

 

olddatsuns.com  read the Datsun Tech section

[rjgilligan]

Hey Mike - 

Thanks for your input! This A87 is the peanut head, the picture is bad and it still has the gasket still on it, but it's definitely not a round chamber. 

 

Its good to hear you say that about the chain. I never was convinced the chain had jumped, but I couldn't decipher what else it could be. Now that I'm thinking about it, there were a series of bangs, maybe 10-12 that were evenly spaced and got quieter as they went on. Then, no compression. I was in 4th and had to merge quickly, but never downshifted when it happened.  If the chain COULD jump, the bangs probably wouldn't have been evenly spaced.

 

However, I can't find any issue with the head gasket that came off. What makes you think the head gasket went? And what could have caused those bangs?

 

The lower block of my engine is an L20B. The "race" engine that has the flattops in it is an L16, so unless they bored out the L16 to exactly match my engine, which I doubt, there's probably no sticking those flattops in my L20b. I'll go out and measure later for the hell of it.

 

kelowg - I'll probably end up pulling the pistons next weekend and will be able to see what, if anything, happened on the bottom end. There's no metal in the oil pan and no scoring on the cylinder walls, though.

[rjgilligan]

hainz - 

Sorry,  I wasn't clear. The L16 that I'm taking the peanut from of has the flattops. The L20b is the engine that was in my truck when it blew.  I was wondering if I could get those flattops INTO my L20b, but it's looking like they are a different bore. Will check later.

[distributorguy]

No, there are many reasons the L16 pistons won't work - both bore and pin height are all wrong.  

[banzai510(hainz)]

well you better read up L16 piston will not fit . Bore size L16 is 83 mm stock and L18/20 is 85mm stock.

 

To find out of cahin jump is EZ just rotate the motor to TDC clock wise on crank dial up to Zero degree on and then ck the cam timming marks

 

olddatsuns.com in the Datsun tech has my youtube videos I made. watch them and you can figure this out.

 

get a L series timming chain wedge also fro Amazon, shit maybe 2

[datzenmike]

No, there are many reasons the L16 pistons won't work - both bore and pin height are all wrong.  

 

Sorry but ALL, L series pistons have the same pin height of 38.1mm

 

If you can find a set of 280zx pistons and over bore your L20B to 86mm they will work... but you compression with a closed chamber head would be 10.87. Open chamber head (like a U67) would be 10.11

[distributorguy]

Sorry but ALL, L series pistons have the same pin height of 38.1mm

 

 

 

Not mine.  

[banzai510(hainz)]

well i think basic L series pin height are the same unless special pistons made for swapped out cranks

[Anonymous Waffle]

Deleted - I was wrong, remembered wrong.