Piston Experiment

[vanapplebomb]

Time to experiment with a set of used pistons/cylinders and use my current project as a Guiana Pig. I decided to go out on a limb and test out a couple things that I normally would't have thought to do.

1st: Piston coatings. No, not break in coatings or your run of the mill dry film lubricant coatings. Ceramic coatings that build up piston skirts for use in slightly oversized cylinders.

I was taking to my local engine machinist the other day about my project and he told me that if I wanted to avoid the Chinese cylinders and have like new piston skirt clearance (provided the ring end gap is still within spec, which it is), he can bring them to a buddy of his in Indiana and have them coated with a ceramic that builds up the skirts a few thousands of an inch. The ceramic film is tougher than the aluminum the piston is made out of and supposedly holds up fantastic. Says he uses it all the time on vintage motors where new parts are either lesser quality or unavailable. After the hard ceramic film is applied, a dry film teflon based lube is used to coat the skirts, which is nothing new. Tried and tested. Last engine he did with them he added 0.003in of ceramic film to the skirts. Pretty cool. My pistons are fine, but the cylinders have been honed at least twice before. The NOS rings I have still have good end gap in them, the skirt clearance really is the only issue on a couple cylinders that were more worn than others.

To me this makes perfect sense because when it is all said and done it will cost me less $$ than a new 93mm AA Chinese pistons/cylinders, and be of better quality. He highly recommended taking them to his buddies shop to get coated rather than buying new AA pistons/cylinders. Should be interesting. We will see what we end up with. For those who want to see something similar, look up Swain Tech piston skirt coatings. He said it is the same idea, just a smaller shop that he has a personal connection to.

Some T4 pistons are getting harder to find. THINK 90mm DOME TOPS!! Factory 90mm dome tops were kick a$$, but are now harder to find and more expensive as only small runs are produced from time to time by AA with less than ideal quality control. Eventually those parts will be discontinued, gone, no more. If this works, it would make saving those old factory pistons and cylinders we normally throw away worth doing!


2nd: Shaving pistons. Yup, crazy as it sounds. Mind you, these are factory dished pistons, so the thinner part of the crown near the center of the piston will not be touched. This would be a no go for domed or flat tops. 93mm Mahle pistons have 10mm of solid meat above the top ring land. I will be trimming 1mm off the top edge.

So, why shave the piston instead of adding a base shim? Compression ratio. As they sit now, they have a near zero deck hight. Just a couple thousands between the face of the piston and the head. Trimming the piston reduces the dish volume. The way I figure, it would put me very very close to 7.8-7.9:1. Once I'm done I will cc the dish I will have a definitive number, but that estimate should be darn right close.

So what is the catch? Basically it comes down to a lot of speculation. Not enough people have tried it to know for sure, 100%. Although it sounds sketchy, when you take a look at the construction of the dished pistons, it starts to make sense. Hey, if it is good enough for STF member Piledriver, that is more than good enough for me. That guy knows a heck of a lot more about building T4 engines than I do, that is for sure, and I trust his experience and opinion.
 


So what will be the result? Who knows. Got to experiment to find out sometimes. Everyone has to build at least one experimental motor in their lifetime. Heck, if I'm unhappy with it I will just get the German cylinders bored out and run Keith Black 96mm pistons. 


Time for an adventure

[81pumagtc]

Dirk:

I have some experience with both of your ideas. Adding a few thousandths to the skirt should be fine. I had lots of hours on a SAAB race engine with ceramic piston tops, and they were expensive, but worked really well.

Adding a tiny amount to the skirt should be fine. I personally would do as little material as possible, because ceramics do crack if they are not on a very stable surface, and I am wondering if a piston skirt is a truly stable surface. Thin ceramics would help to make this point mute.

As for machining material off of the top of the piston, it has been used successfully for quite a while. I had a Moto Morini 3 1/2 motorcycle (350 cc's). What was interesting about these bikes were the fact that the actual combustion chamber was located in the piston face...a cup that formed the combustion chamber was part of the piston. The body of the piston would go right up to the head, and the valves would actually be inside these cups. I actually purchased racing pistons that were a stupid high compression, something like 15 or 16 to 1 that could never actually be used, and then you would machine material out of the piston to give you whatever compression ratio you desired. I then balanced the pistons to the con rods.

The beauty of this system was that you could play around with the compression ratio without doing head work. Motorcycle cylinders are generally not really suitable for base gaskets because the crankcase will often flex a bit.

What you are thinking of is less radical than what the Morini did, and will work fine.

Have fun, and keep us posted on the results!


Dave Riedle

[vanapplebomb]

Dave, that was my first concern with the ceramic coated skits. I thought the expansion rates would be different enough to cause some issues with separation. Apparently the stuff adhears well and expands with the piston well enough to make it a non issue. Coating was designed specifically for building up piston skirts.

From what I'm told, the poly lubricant top layer is pretty tough as well. Not like breakin coatings that wear off the skirts a few minutes after start up. They rebuilt some marine engines that used the skirt coatings and after a lot of hours, the Pistons came out looking good with hardly any of the poly lube film worn through. The base ceramic coating wasn't fazed. Sorta make me wonder what type of ceramic is used, prep work, and how it is applied and cures so it adhears well. Very interesting stuff. Materials fascinate me.

[MadMike]

Dun't take much to fascinate an 'Engineer'
Dirk get that thing fired up, ,,,,,already Then add a Turbo

[vw1s]

Soooo, what happens to this harder than aluminum coating if it does begin to flake off? Does it end up in the oil? What will it do to bearings if a flake makes it that far?

[vanapplebomb]

Thats a really good question Pat. Really not sure to be honest. Experimentation and time will tell I suppose. For now I guess I will trust the word of those who use them on a regular basis. Bosch oil filter I have is 99% efficient at 20 microns, which actually is pretty good. The oil filter boss on the engine does have a ball/spring pressure relief bypass for when the filter starts plugging up which could allow uncleaned oil to get to the bearings, etc. However, I should think that if it holds up good in industrial stuff like marine engines that run a hard life, it should be fine in a light weight buggy. I have heard of people using it on 6 cylinder 911s with good results, and those live a pretty hard life. Like I say, I don't know. Guess we will find out. I like to cut open oil filters after running them to see what kind of crap is getting into the oil. If any chips come off they should show up in the strainer or filter.


Mike, hold your horses. Hahaha. I know, I want to get it done and running soon too, but I got to save money for some big things I need coming up soonish, like a ring and a house just to name a couple. For now I just set a little cash aside at a time to play with and I get done what I can when I get the time.

By the way, about the turbo...There is a Dutch guy, Wally, on STF's Type4um that pushes 400hp. If I remember right, it was just a mild cam for torque, stock rods, stock flywheel, stock head castings, stock 66mm crank, 103mm pistons (for 2200cc), intercooler, and 20 + PSI from the turbo. Ran it that way around town and at track for a couple years until he upgraded some parts and took it to over 450hp. Europeans get pretty serious about their Type 4 stuff. So there you go Mike, next project for you, a 2.2 turbocharged T4 daily driver/slalom track racer!






And yes, most engineers are easily fascinated by simple things.

[vanapplebomb]

Right, first thing I did was measure the height of the piston in four places and scribe the height on the insides of the piston where I measured. The tip of the skirts were all 80.01 a 80.03 mm from the crown and the bottom edge of the sides were 61.24 - 61.26mm. I used these sanity measurements so I could confirm the cuts were made parrellel to the original face. Example of before and after crown to skirt measurements. Same piston, just opposite sides of skirt (note slight scuffing on one side), but you get the idea:





Side by side comparison of shaved piston at right vs stock at left:




Side view of dished face with 1mm shaved off the top gives a better perspective of the resulting smaller dish. I will cc it coming up shortly, but I expect ~30% decrease in size:

[vanapplebomb]

Forgot to mention that I also scribed the arrows pointing towards the flywheel on the back side of the Pistons. Don't want to get that backwards.

[vanapplebomb]

Pistons got dropped off at the shop to get their skirts ceramic coated. I left them the cylinders as well so they can tell me the final piston to wall clearances. Hopefully I'll find out in a day or two what kind of a time frame I will be looking at since they have to get taken down to Indiana, get coated, and sent back, and possibly hone a jug or two if they need to get opened up slightly in the case that some shirts end up on the tight side. I'm super curious how this will turn out. Never done anything like it before.

[gokart]

Maybe I don't see your whole plan, but just shaving your pistons actually lowers the overall compression ratio. Was your compression ratio too high before you shaved them?

[vanapplebomb]

Sorry if I wasn't clear.

First a little back ground. The type 4 engines from the factory usually had a 0.008-0.010 inch shim benieth the barrels, and then a 0.030 inch thick aluminum gasket between the barrels and cylinder heads. Although the head gaskets were hardly ever an issue on the 1.7 and 1.8 engines, following a series of 2.0 engine failures, the factory recommended removing the head gaskets. There were two problems that appeared most significantly on 2.0 engines. The big issue, and the reason the factory sent out a bulletin recommending the removal of the gasket, was that the 2.0 94mm pistons were seizing in their bores shortly after start up because they heated up so much faster than the cylinders. The head gasket is a pressed together three layer wafer, and it slowed the heat transfer from the hot heads to the cooler cylinders. By removing the gasket, the heads which warm up fast-ish, would help heat the cylinders up faster so things would expand in a more synchronized manner. Also in the bulletin was the rod notching piston oil cooling mod to reduce piston temps. The other problem that crept up on 2.0 engines especially was the gasket burning and causing a bad head leak. Although only specified for 2.0 engines, it has become pretty standard practice to pitch the head gaskets on all type 4 engines and lap the cylinders to the heads.

With no head gaskets or cylinder base shims, the deck hight was only a couple thou on my engine, so I had to increase the deck to keep the piston crowns from smashing into the cylinder heads. Two practical options to achieve the desired deck hight. Use a barrel spacer, or trim the pistons back a bit.

United States spec 1.8 engines had a wussy compression ratio compared to the 1.7 engines or Euro spec 1.8 engines for emissions related issues. But, due to the lack of overlap on intake/exhaust, the engines respond very well to compression ratio changes, even if all else is stock, so it makes the most sense to use the option that gives the best compression..

Shaving the pistons reduces the size of the dish in the top of the piston. so, if the same bore, stroke, and deck, the shaved piston will have a higher compression ratio compared to a shimmed barrel set up because all that air is being squeezed into a smaller place. Make sense now?

Pistons are coated. Just need bores checked to verify piston to wall clearance and hone to open up the clearance if nescesary.

[gokart]

Ok, you took 0.040" (1.0 mm) of shims and gaskets out of the engine, and shaved 1 mm off the piston, resulting in a higher compression ratio by reducing the dish. Nice work.

[vanapplebomb]

Yup, that's the idea. With the shims in place the compression ratio would have been at 7.3ish:1. With the shaved Pistons I am expecting to approach the 8:1 mark. When I get everything back this week I will cc it and find out for certain where I am at.

Thanks by the way. Should be interesting to see how this all comes together.

[vanapplebomb]

I measured the piston dish and head chamber volume, and I was bang on at 8:1. In my opinion, that is a respectable improvement given the alternative. I measure dish and chambervolumet by weight. 1cc happens to conveniently be 1gram. Triple beam balances are handy for making fine measurements. Put the tub of water on to beam, weight it, use eye dropper to move water from tub to head or piston, and weigh tub again. Difference of weight in grams is the volume in cc. To eliminate some of the guess work, trim a blank clear CD protector. Most blank CD packs come with one clear CD that covers the top of the pack to protect other CDs from scratches. Then lightly oil the perimeter, and fill water from hole in center. When bubbles are gone, your done. I dyed my water green to make it easier to see.

Once I had the compression ratio figured, I weighed all the
pistons, pins, clips, and rings. Rings and clips were all bang on. Pistons and pins varied slightly. I put the heaviest pins with the lightest pistons, and everything worked out to within 1.1 grams. That is far better than I actually need, so I left it alone and didn't do any more balancing.

Pistons fit nice and snug in their jugs now. I'm really happy with the way things came out. Coaters added 0.0035 - 0.0045 inches to the skirts, and then a ptfe lube top coat. Looks good. The piston pin bores were honed for a nice fit. Ring side clearance is great and ring gap is still within spec despite cylinders being slightly oversized by a few thousandths of an inch.

Everything got cleaned with soap and water to remove any water based coolant/lube from honing. Then everything got hosed down with WD40 and wiped clean with rags and sprayed with acetone (brake clean) to remove any oily residue and trapped grit. Took about three repetitions to get a spotless rag after wiping off the parts. rings were particularly bad.

All rings a cast iron. Oil control ring is two pieces. The coil spring nests inside the cast ring. I put the ends of the coil spring opposite the gap in the cast outer portion. Then I installed with gap facing up at 12:00. The other two rings I installed at 4:00 and 8:00. The top and bottom faces of each ring were very lightly moistened with oil. I used a single drop on a lint free rag and wiped all the rings with it. Just enough to make the full cast rings shine, but not enough to run. I don't like the idea of having lots of oil in the compression rings

Pistons were installed "V8" style...well, that is what I call it anyways. I lather up the cylinder walls with oil, compress the piston rings, and insert the pisto from the top of the cylinder. The oil control ring scrapes excess oil off the walls and the other two rings get all the oil they need. Once all the pistons were in, I put a thin bead of Permatex Ultra Copper on the cylinder bases, slid the assembly over the studs, pushed the piston pins through and inserted the circlips. With Type 4 engines, you should instal #4 before #3 because of the oil cooler boss. Slide the #4 piston pin in from the flywheel side. Nuts and pieces of tuning were used to hold cylinders in place while turning over the engin. Once one side was done I torqued down the heads and moved to the other side.

Once both sides were done, I installed the oil cooler, oil filter, distributor, and breather chimney to keep dust out. I also have lint free rags stuck in the lifter bores and spark plug holes to keep dust at bay. Before I do any more, I need to get three pieces of cooling tin cleaned up and installed. Then I can instal the lifters, pushrods/tubes, and the rockers/rocker shafts.