Originally Posted by Truckracer

Considering the compressible nature of base gaskets, machining tolerances, etc, I would consider a .002" difference between cylinders to be quite good. Certainly nothing that would account for performance loss.

Ok here is the issue: When I torqued these bolts, they were wet with blue Loctite. I know for a fact that I may have stretched the bolts too tight, and therefore may have compressed the gaskets differently on the two pistons.

Contrary to what I thought, more compression is not necessarily better. This is why I want to compare with engine #2. I will do tests with no gasket and then with the gasket before final torque. I will do the tests with putty since I will remove the jugs after each test.

Clearly, this may not be the cause of my missing RPM, but since we are into this topic, I might as well explore the issue.

I have new and used base gaskets here from a DA100. On average, the used gaskets were compressed .006" and the amount of compression was not uniform across the whole face of the gasket, with a variation up to .002" at one point. The used gaskets came from an engine that had never been disturbed from DA factory assembly. While my gaskets are not from a 150, this does show the gaskets compress quite a bit during assembly. I still doubt this would account for much of a performance difference though.

Your engine was a drone engine. Does DA maintain the same level of machining accuracy on these engines as the ones sold for RC use? Just curious. This could account for a performance difference. It sure would be nice if you could get your hands on a virgin 150 and compare it to the one you rebuilt.

I use to be a very active pylon racer, QM, Q500, F-1, QM40. On occasion I would get an engine that just lacked the performance of my good or great engines. No matter what I did to these lesser performing engines, I just could never make them run like the good engines. This included replacing just about every part on the engine including the case, measuring anything that could be measured by the methods I had available, etc. and still they would lack performance. I finally gave up and quit wasting my time on engines that started out life as sub-par engines. Not suggesting that you abandon your engine, just suggesting that perhaps yours is one of those that is just a bit below normal and there is nothing wrong with that. I have multiples of several different gas engines. They all have their own unique personalities and performance levels. Same when I was racing trucks. Every engine was unique from every other one even though some were built with the same parts configuration, cam, etc. The blocks and heads were prepared by the same person on the same equipment, yet they were different. Just the nature of the beast I guess.

Originally Posted by Truckracer

Your engine was a drone engine. Does DA maintain the same level of machining accuracy on these engines as the ones sold for RC use? Just curious. This could account for a performance difference. It sure would be nice if you could get your hands on a virgin 150 and compare it to the one you rebuilt.

From what I see, DA has the same quality level on these engines as the sold ones to the public. The only difference is the cosmetic finish since they will fly but once.

The quality of machining is fantastic!!!!! I was unsure if I may have had a bent rod I did not recognize or that i improperly torqued the bolts which I did. From what I see, the rods are good.

I may pull the trigger on a new engine, but it would be a waste since I do not fly this size planes.

Engine #2 is ready for assembly. I am awaiting some bolts from DA . As you all recall , we had a dummy engine that we experimented with in the assembly process. I will use this engine to do squish tests. I will also put an indicator on the head and measure the gasket compression during torquing .

Engine 2 has a new crank since the runout on the original one was around .0015 inches, at the top of the allowable specification. The new crank also has a different key. The new design no longer uses a Woodruff key but a dowell. If you blow up the picture you will see this.

I partially assembled the dummy engine and performed a squish test utilizing the plastigage material. DA informed me that the minimum squish for this engine is .4mm or slightly less than .016. I got arout .0175 using my measurement. This is not as accurate due to the fact that we are blind and can't see the actual position of the gage material, but it is a good indicator non the less. It is clear the the final squish will be dependent on the initial thickness of the gasket and the amount we compress the gasket during bolt torquing.

da150 Squish no gasket (2 min 22 sec)

The nice thing about the plastigage is that it comes with a scale on the package so you do not need a micrometer or caliper. So what we see is .017" about what I measured with the calipers. This is why the package is color coded. You must use the yellow rod on the yellow package only.

I placed an old gasket measuring .034 in between the head and crank case. The engine was placed in a vise anchored to the metal work table. I torqued the bolts evenly to 10 in-lbs. The indicator was set to zero. The bolts were tightened in 10 inch -lbs increments to 65 inch lbs. The gasket compressed .003 inches, and .001 to 10 in-lbs.

Assembled Squish = Squish without gasket+ Gasket Thickness-Gasket compression

Squish=.017+.034-.004=.047

The squish measurement was .0465....Close enough for government work.

Therefore how even the thickness of the gasket and how accurate you torque will make a big difference on squish.

I probably over tightened on the torque on engine 1 thus making the difference. This gasket should measure .032 pre installation. The old gasket I used had some sealer on the surface.

Truckracer did mention that gasket material varied in thickness. The majority of the gasket material I measured from DA was .031 to .032 which is pretty good.

I will pay close attention to this measurement when assembling engine 2. This may not be the cause of the missing RPM but we will see.

Question: What is the normal ring gap for DA rings. I know people talk about tight fitting rings. Is ,005 space ok?

It will be interesting to see how #2 runs.

Originally Posted by RTK

It will be interesting to see how #2 runs.

If they run differently, I doubt it will be because of a slight difference in torquing cylinder retaining bolts. ChemE, I completely respect what your are doing with these engines and your learning from them but I doubt that a modest difference between squish clearances will account for any major performance differences. Changing base gaskets or different gasket compression thickness differences in base gaskets can also slightly change port timing as well as squish / compression ratio but for the most part, it would take major differences to cause any serious differences in performance. Our engines are just not that critical on mechanical setups that would affect performance that much. A degree or two in timing would probably create a greater performance difference in an otherwise mechanically sound engine. If our engines were tuned and designed to perform on a ragged edge of performance, I would think otherwise.

I agree with Truckracer.



From my limited experience, the carb tuning will have a bigger effect on the power output than the small differences in squish and compression etc.



If you run the numbers these engines run at quite low piston speeds compared to race engines, they aren't running all that hard in the interests of a long life.


I am looking forward to see if you can get the expected power from the first engine..

Truckracer. I agree with you 100%. The little issues I am looking at would not change things much. Timing will work just as well.

My issue is that I have only taken a few engines apart. I am trying to come up with a process that is reproducible. What the squish clearance actual number is, is not important. We are not reengineering.
On engine 1, I slapped the pistons together without looking at any of the clearances. I paid more attention to the crank case.

Just opened the gasket kit and am measuring .004 difference in head gasket thickness. So you were correct in the variation in gasket thickness.

Engine 2 case assembly is complete, with the bearings oiled and pistons assembled.

I measured the gap in between the old ring and it was .008 while inside the jug. Does anyone know what this gap should be? This engine ran 5850 RPM with this ring.

I will assemble the heads , reed block and then do a leak test in a few days when the silicone dries up.



See post 144, page 10 for assembly video and page 8 post 119. I followed the same steps. Latest assembly procedure is REV 5. Please be aware in post 119, I did not use loc-tite. This was a trial assembly!

New rings from Bjorn Baal, depending on actual wear on the cylinders can measure as thin as .001". So, the tighter the ring gap, the better. As everything expands as it all gets warm. So, my opinion is that .008" is loose, but a good loose. I have no idea what the gap is on a DA when fresh and new.

Originally Posted by Bunky F. Knuckle

New rings from Bjorn Baal, depending on actual wear on the cylinders can measure as thin as .001". So, the tighter the ring gap, the better. As everything expands as it all gets warm. So, my opinion is that .008" is loose, but a good loose. I have no idea what the gap is on a DA when fresh and new.

Both new rings and old measure the same, There is no wear on the cylinder at all. This engine does not even have one hour on it.

Engine 1 Engine 2
Left Cylinder Head Bore (mm) 48.98 48.96
Right Cylinder Head Bore (mm) 48.97 49.00
Left Ring (mm) 2.11 2.12
Right Ring (mm) 2.10 2.10

I appreciate brother Knuckles mention of the Bowman/ Bjorn rings. Though tighter clearance, I am reticent in any change from OEM equipment. In this thread, I am more interested in bringing the equipment back to OEM specifications. The reason for my dislike of advancing the timing, is the potential reduction of engine component life, similarly increasing the compression may do the same.

The goal right now is to achieve the OEM nameplate RPM. If I do everything as I should, this number should be achieved. This is why I am interested in what the specification is so I can cross it off a check list and not to re-engineer and replace parts.

Engine 2 is complete, less the hub. A leak down test was conducted and 2 leaks were found. See pictures.

I cut my own gaskets to have better control of the thickness. I duplicated one of the gaskets in my kit that measured .028/.029. I had some gasket material in the shop and fabricated my own. Squish test measured .042/.043. Both pistons had same squish. This should be eliminated as an issue.

One item of notice, several removed gaskets that i held for retain measured .034 to .036. This was due to gasket sealer being used. I did not use gasket sealer at all.

I will start engine in the weekend and compare to NO 1

Just got in a new prop so I would not have to change props in between engines.

I set the ignition to top dead center. The censor is positioned in the middle of the adjustment as in engine 1.

A 10mm screw was drilled to allow a starrett depth gauge to penetrate and sit flat. Top dead center was determined by spinning the hub counter clock wise and measuring the depth. When the depth read a minimum, that is top dead center. The mark on the censor was then adjusted to the magnet whose leading edge was covered with a piece of tape.