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Rebuilding L20b and need pistons!


Draker

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16 hours ago, datzenmike said:

Standard L28 are 1mm over size for an L20B bore so 2mm for an over size L28 piston. Still if going over, go all the way.

If they will fit. 2mm is kinda large for an overbore on an L20B. Actually not kinda. You'd have to measure the cylinder wall thickness and move the bores around to get it that big.

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Draker, when I modified the L20B in my 720 I had taken the stock pistons (used) from an L18 I had and switched them for the stock pistons in my L20B. I did this with the engine block still in my 720. I pulled the head, raised the engine up and removed the oil pan and removed the piston/rods, swapped the L18 pistons onto the L20B rods and reassembled the engine so I didn't shop for aftermarket pistons.

 

I built an L18 using L28E N42 source pistons (10.9 cc dish) and had the flat portion of the piston top machined down so that it looked almost like a flat top and then used L24 9 mm bolt rods (133 mm) instead of the L18 (130.5 mm) rods. This bumped the c.r. and improved the rod/stroke ratio a bit at the same time. There is still a lot of piston material above the top ring land but this approach isn't ideal. The early L28E, '75-79 had the 10.9 cc dish and 8.3 c.r. iirc and then in '80-83 the F54 block L28E were flat top pistons with the quench chamber P79 head and larger cc chambers with 8.7 c.r. as I recall.

 

I mocked-up a long-rod L18 +1 mm bore using 86 mm Cadillac Cetera pistons but I don't remember if I crunched the numbers for an L20B using longer rods like Z22E or Z20E.

 

I'm presently working on putting together an L4 using a Z22 block (87 mm) with 0.5 mm overbore using Toyota 3ZV-E pistons (87.5 mm is std) and longer than L20B rods on an L20B crank (full counterweight) and a closed chamber head. If you look at the piston top configuration and the combustion chamber configuration you can see why I am exploring this.

The 3ZV-E is I-E, I-E whereas the L engines are E-I, I-E so I am fly-cutting the exhaust valve pockets of the 3ZV-E pistons to be symmetrical to the intake valve pockets.

Also, due to pin offset it requires using just Right-side pistons only. However, I found a source of 3ZV-E pistons with centered pin so I'm going that route for the initial engine. If it works out then I'll turn attention to putting together L6's using those same pistons and alternative rods.

 

The compression ht. of the 3ZV-E pistons is only 31.4 mm v. the L20B at 38.1 mm which allows me to use Z20E rods (152.5 mm).  38.1 - 31.4 = 6.7 mm. 145.9 + 6.7 = 152.6 mm.  Pressed 20.8 mm pin Nissan rod; pressed 21 mm 3ZV-E piston pin so dressing the small end of the rod to fit the Toyota pin will be easy. 

 

I'm crudely measuring the valve angle of the Nissan L cyl head to be about 15.5 degree and it is a good match to the valve relief pocket angle of the piston tops. I'm going to modify the exhaust valve reliefs to match the intake reliefs before I assemble rather than fly-cut notches with the pistons in the block and using the head for a jig. It will be much less busy work and not having to worry about aluminum shavings that way.

 

Note that in the photo there is no index notch or mark on the piston top designating which side of the piston faces the front of the engine. This is not the typical case because due to pin offset a set of these V6 pistons will be three R and three L pistons. To do what I am talking about would require R only pistons in the usual sets but this set I just found are zero offset so that any of the six pistons in the set are interchangeable. I have acquired enough R side pistons to do an L4 and some have nice thermal coating on the skirts and tops but I'd still have to cut the eyebrows to make them symmetrical and mess up the top coatings. I'm going to keep it simple on the first build and only a L4 to see how it works and then go from there.

 

 

 

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14 hours ago, RetroRocket said:

I-E, I-E whereas the L engines are E-I, I-E

E I E I O. And on his farm he had some pistons...

 

Please tell me you're going to massage down the sharp edges on those pistons. Any amount of deburring will help dramatically, but to improve flow, you should radius the hell out of those edges.

 

See the pics below. One stock piston, the other modified for flow.

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Yes I'll round-off the sharp edges. I will probably need to blend down the area in head chamber between the valves also because the quench pad on the piston is not heart-shaped but is instead a straight dam. What do you think about that mismatched area? I don't want to open up any more chamber volume than I have to but I think I will have to soften that overhanging edge on the head.

Everything that I'm doing could be accomplished with custom pistons. My objective is to use ingenuity rather than $$. The set of six pistons I'm working with was $120 shipped.

 

Had the L4 continued production I believe there would have been increases in displacement with an evolution to flat top pistons and closed chamber heads but deeper chambers and shorter valves just as in the L28 P79/P90 configuration rather than larger piston dish to mitigate compression ratio issues.

 

I'm going to modify one piston and a core peanut-chamber A87 head to see what dish volume/chamber volume I end up with and go from there. I should be in the ballpark but if I'm too low a c.r. then I'll probably use a Z22 crank instead of the L20B crank to gain stroke/displacement and increase c.r. while using a different length rod. But I'd like to end up with about 9.0 to 9.3 c.r. however I have to configure the dimensions. This will be a street engine without forced induction. These pistons aren't suitable for turbo although I might toy with the idea of low-pressure twin-scroll using a Mini Cooper turbo.

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Closed chamber head is 41cc or less if it was milled in the past.

 

Sharp edges on the head can be slightly less radii than the pistons as it's water cooled, the pistons from oil spray, if that. The piston is just the bottom of the combustion chamber, so material removed from either will lower compression.

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Thanks. I hadn't considered that the head would be less hot than the piston top. I think I'll do modelling clay on loose head and piston. Just crudely line up where the piston will sit in relation to the combustion chamber, as if the piston were in the engine, at squish the clay to see what it would look like inside the chamber (piston top and chamber roof relationship) when the piston is at TDC, valves closed. It would simulate the space and how close or clearance the sharp edges are and how critical it is to take metal off of the "cleft" part of the heart combustion chamber. Maybe I can just round the edges and leave it be.

 

 

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I'm wondering if I could generate the clay "imprint" disc and then try to use a glass container that has a rectangular shape to it about 4" x 1" x at least 4.5" tall. I could put liquid in it and fill it to about 4" and on the 1" side of the rectangular vessel mark the meniscus of the fluid level on the glass; then drop the clay imprint into the vessel and mark what level the meniscus displaced upward to. Then remove the clay piece, fill the empty vessel once more to the original meniscus marking and then use a graduated 30 cc syringe to refill the vessel up to the higher meniscus marking. Once it is reached, probably during the third syringe aliquot, I'd know the cc volume of dish + chamber. Then I'd just have the head gasket thickness/volume to calculate the real time c.r. I'd be at.

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There are ceramic coatings that you can apply a home. Combustion chambers and heads. It's like insulation and allows less heat to be absorbed so it's available to do more work. The heat given off by the radiator is all lost inefficient use of energy.

 

 

 

cc the head for free...

 

 

 

 Dish would be easy enough. A flattop with a dome also easy enough.  Not sure on a piston top that is a combination dome and dished valve relief.

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I know how to cc the head separately but I want to accomplish two things at the same time. One approach would be to use a grease seal between the loose piston and the head and lightly clamp them together; turn this assembly on its side and use a burette with an adapter using the spark plug hole to get the combined volume of the chamber and piston top at zero deck without head gasket volume and then the final assembled engine deck ht can be measured. Last step would be to select the head gasket thickness/volume to fine tune the c.r.

 

But I want to also see in 3D what the inside of the working chamber (head vol + piston top volume) will be. I was thinking modelling clay but a cast would be better. Using the same set up above, once the burette volume has been measured then remove it from the spark plug hole and blow the fluid out with compressed air to remove the residual fluid. Then pour in a material through the up turned spark plug hole that will solidify into a cast replica of the space. Plaster of Paris or some gel that will set up. Some variant of Jello would work. For a quick look it could be water and then stick the whole thing in our freezer, wait, disassemble promptly and photograph the ice casting. It would be more satisfying to have a mold that stays solid like plaster. Great Stuff would work but be messy. I'd use PAM spray or some other mold-release agent on the surfaces prior to casting.

 

In this photo three of the 3ZV-E pistons are side by side and I slid a wrist pin in place between the adjacent pin bores. The tops are the same relationship re I-E pocket locations and any of these pistons would need to have the exhaust pocket enlarger to mirror the intake pocket to use with an L series head due to E-I, I-E valve arrangement.

But note that due to different pin offsets the L piston would not work well in an inline engine because the pin offset would be double the desired amount and in the Wrong direction. The divot in the outer two pistons is reference to the front of the engine for installation on the V6 3ZV-E. The R side pistons would work fine in an inline but the L pistons are useless. The middle piston is zero (centered) pin offset and therefore there is no R, L, or indexing divot. The zero offset pistons are made to cut costs by manufacturing only one piston to be used L or R. The way to go is to use R pistons and machine all exh valve reliefs to match the int valve relief and have properly offset pins but buying two sets of six matched pistons to build three L4's or two L6's is a lot easier than searching for the odd broken set R pistons only.   

 

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All the pistons need to have the exhaust valve pockets machined to match the intake pockets. They have to be uniform in weight and volume and flow characteristics.

It would be tempting to just leave the pistons with the I and E pockets which correspond to the cylinders where they line up already but it wouldn't be uniform for all cylinders so all the piston tops need to look alike in all cylinders.

 

I've looked for other sources that would work with all of the piston valve pockets corresponding to the I-E and E-I order needed but haven't found any yet in bore sizes that could be useful.

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yeah, like you did I first thought wow this is great! Eureka! I have found it!! (Archimedes quote)       

and then, when I stumbled on a set of zero offset pistons that I hadn't known were available until now...I thought hey now maybe I can use the Left pistons on the L engine since there is no pin offset issue. Eureka!

sadly, that makes no difference in the valve pocket arrangement; that is not ever going to change so the former ex valve pockets have to be turned into a mirror-image of the existing intake valve pockets. Nothing needs to be done to any intake relief on any piston initially. Best to leave it as cast and measure the radial clearance and what the max valve size would be limit and go from there.

 

I'm developing the Real-Cyl (registered copyright) method of replicating to model three-dimensional engine combustion chamber configuration.  ha.

 

I've ruled out casting my model by using these casting materials: latex, cement, liquid urethane, jello, plaster of Paris hardware grade, water/ice transient state medium. I am comparing silicone, Perfect Cast craft-grade plaster of Paris, expanding foam urethane (great stuff categories), wax, and some others. Cost is going to drive my initial approach. That and how easy and most efficient the method is. (for example: wax and some grades of plastic can be melted down and recast into something else). Since I'll probably be modifying the configuration progressively and reimaging as a process. A downside of that medium is flammability when heating it up.

 

Off to JoAnn's Fabric and Craft Store to find some Perfect Cast.  [this is the type of material that people stick their hands in a bucket of the mix and save the castings] (why?)

 

At least I have a bunch of L pistons that I can use for pursuing this boondoggle. The tops are all the same R & L and it's just the top I am concerned with. Ultimately, I'll be lightly clamping the loose piston flush to the head with a large c-clamp with a sealing medium between them and mold-release agent coating on the surfaces that will be forming/casting the mold. There are additives that can change the viscosity for pouring.

 

If I don't ever have to work with expanding urethane foam again that's fine with me

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I bought an intake valve from RockAuto with a head diameter larger than the 42 mm Nissan valve (I just chose valve with the head diameter to match the piston intake relief from SBI valve catalog [I think it was about 1.730"], cross referenced the application (GMC big block exhaust valve) and ordered one valve. It was cheap. The stem diameter doesn't matter because I am making a tool out of it.

I'll glue 50 grit to the head, trim it even with the valve edge, and use it in a drill press with the piston in a jig at the correct angle to turn the exhaust pocket into a twin of the intake pocket. You can see the piston top centerpoint in the photo and the existing intake relief edge is just beyond it. The symmetrical relief I'll cut will overlap that center mark to duplicate the existing intake relief. I wish I had ordered two valves so I could make a 50 grit cut and also a finer git for the finished surface.

 

Note that it is a Left piston that I'll use as my first victim because Lefts are useless for use in the inline 4 and 6 cyl L-series. I have three L to experiment with from a set of six V6 pistons.

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I don't know yet the duration yet but I won't be using a big cam. 

Once I machine the piston relief I'll put mold-release on the piston top and the combustion chamber, position the piston centered on the cylinder and use a large C-clamp to set up proper alignment. Then with spark plug hole facing up, pour the volume of casting mix to reach the plug hole. Let it dry overnight and remove the C-clamp next day to see what my model of the actual, real-life full firing chamber looks like. the fluid casting mix volume is the dish + chamber = working volume. That volume is supplemented by the gasket ring thickness volume, and where the piston will sit in relationship to the deck, and the volume change due to that. Use the combined volume total to calculate c.r.

 

Once the chamber model is made the head can be installed (clay on the piston tops), engine rotated, then set at TDC compression, then head back off to see what valve clearances are, including radial clearance. 

 

Then I'll weigh the pistons and round off the sharp edges, and weigh again. Then even them up in weight.

It'll be a couple of weeks before I can get back to play with this concept.

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Only a real life mock up will determine how deep the valve reliefs need to be. Install one rod/piston, then the head with a gasket, and "setup" valve springs. Set the cam timing and run the valves down into the pistons with a dial indicator on the tip of the valve.

 

If you're not using a big cam, doubtful you'll need valve reliefs at all. The pistons I pictured were pop up domed pistons for 12-13:1 race motors with huge cams.

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The valve relief depth is already determined on these as-cast stock Toyota pistons and I can't make it any less. The exhaust pockets have to be modified to mirror the intakes that exist. This modification will make a bit more working chamber volume. I will accurately measure the working (actual) chamber volume (piston top + head cc).

 

Of course I will check/measure the working piston-to-valve clearance when I assemble. I anticipate it already being more than I require. If not, I will deepen the pockets. It is unlikely to need to do so with a stock or mild street cam. The point of making the casting is a volume measurement to calculate compression ratio (with the valves closed). This is not about building a race engine. It is about working combustion chamber configuration (which includes piston top/dish Volume) and beneficial quench table configuration. It allows for peanut-chamber head + larger displacement + quench configuration Without ending up with 12.5:1 c.r.   I only want 9.2:1 c.r. and not have to use a deep round-dished piston top, with no quench, to achieve that goal. So I am developing my own approach and entitled to do so. Independent thought.

 

I've used forged 11:1 pop-up pistons and big cam in another car/engine, my 1600cc SCCA H Production 510. That is completely different from what I am doing on this project.

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Core A87 head. It was rusty and looked about the same when I saved it from a scrap pile of cyl heads in a JY about 20 yrs ago. This will be used (#1cyl is TDC, both valves are closed) for my working chamber mold to make my model. Once clamped together with the loose piston, sealed, and coated with mold-release agent the head/piston mock-up will be oriented on the bench with the spark plug hole at the vertical apex and the casting material poured into the hole to cast the model. Once set, I'll disassemble and retrieve the cast model. I'll be able to measure the volume of it by submerging it in a graduated beaker of water (a clear plastic paint mixing cup) to see how much volume it displaces.

 

I'm just working with #1 cyl for my mold because both valves were closed when the engine was disassembled (and the parts apparently left outdoors to rust). I scuffed off the worst surface rust on those two valve heads and used Evapo-Rust Gel to dissolve the rust so that I could make a decent casting. I got bored waiting for the full action to take place on those two valve heads and so started painting the Evapo-Rust on the other valves even though they hadn't been even scuffed and half of them were open. I can go over what I learned about using the material (note the chop-sticks and gel bits) in general some other place than this thread. This thread is about selection of alternative pistons for a budget-build L4; not rust removal. To stop the rust removal reaction I flushed with water, dabbed dry, sprayed it with PAM to protect it from immediately oxidizing, and took it out to the garage (34* F). I did the rust removal process using an open oven rack and occasionally turning the oven to the lowest setting then off again. I wanted the head to stay at about 100* F while the agent was reacting.

 

TBC.

 

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I'm going to set it up with no gasket and calculate that volume in after the fact.

 

I'm going to use for a template to set up my molding apparatus a spare (used) Z24i head gasket referenced/indexed by head bolt holes. My 87.5 mm piston head will fit within the gasket made for 89 mm bore engine [Z or L does not matter for my purpose; the head bolt holes locate the same]. I'll stake them in place with tapered rubber corks. This should allow me to center my piston exactly as it will be in the assembled engine, using the fire ring of the Z24 gasket.

That step will be done at the same time I set up the indexing of my jig to the axial centerline of the crankshaft. This will be done with a slip-fit dowel rod passing through the piston pin holes of four pistons laid out on the head while on the bench. I'll use some mild sealing agent that will be sticky enough that the piston will remain centered when I remove the dowel and clamp the piston in place to the head. It is both centered in the bore and aligned with the crankshaft. I'll need to externally form a little dam where the cylinder wall will be in relation to the piston valve reliefs being exposed for the molding jig. I'll practice with clay or chewing gum or whatever works best. 

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