coriolan

Yes , its easier to counterbore the head to accept a larger bolt size than reducing the bolt head to fit the smaller counterbore. That way you can use any off the shelve hardware. Going back to the stripped M3, it would be possible to Tap 5-40 SAE instead but that size of tap despite being standard is not easy to find outside of tool dealers. 5-40 is 1/8" OD (same as Cox .049/.051 prop shaft) and also a bit hard to find as cap screw!

Recycled Flyer

In what way is it easier to machine a non disposable part like the fins as compared to three small bolt heads? (Providing that you don't go too big on the bolt selection that is.)

It took my friend about 30 seconds each in a small Tiage Lathe and I got to keep all the anodising on the fins intact.

But I suppose that this hardly matters as either way works well enough.

earlwb

Well I have been thinking about machining down the heads of the screws too. I just haven't decided on whether I will do it or not.
I do have some of the oddball size taps and dies like 5/40, 4/48, etc. I was making some steam engines a number of years ago, and the plans were calling for odd size screws like that. I really wanted to find some small steel hex stock rods, but they only came in brass instead of steel. So it was something of a tedious hassle to make little scale bolts to use on a steam engine and its other accouterments.

gmedson

Hi - I use Boca Bearings, www.bocabearings.com. They have bearing sets listed for a very large number of engines. And if your engine is not listed, you can search by size and bearing type. Standard price for bearing sets (2 bearings) is $11.88. No tax or shipping charge. I am guessing that in case your engine is not listed, you can identify the set you need and make the transaction by phone and they will honor the $11.88 price. This is for their economy bearings, which seem to be good quality. That's all I have ever used. Finally, they get their orders out lightening quick. Two or three days and I've got 'em.

And, Earl - For steel hex stock, try Speedy Metals at www.speedymetals.com. I like leaded steel, 12L14, and their smallest hex stock is 3/16 in. They also have 4140 and 1018 steel, as well as brass, aluminum, and 303 stainless (which goes down to 1/8 in). Altogether, they have a really big selection of metal stock at very reasonable prices. No minimum order. No cutting charge because you can choose any of several lengths. Fast and pleasant service. They are great!

Regards. -Gary

earlwb

thank you very much for the info and heads up on steel hex stock rod.
I appreciate it,
With my version of Firefox, it claims the Speedy Metals Website has something odd going on with itand it may have been compromised. The shopping cart seems to not be the same URL address as the Website.
But anyway, now I know where I can get some hex rod from.

Oh yeah, I just discovered that Online metals has some 1/4" and 3/8" hex rod in 12L14 steel too. Cold drawn cold rolled steel hex
They must have just recently started stocking it. http://www.onlinemetals.com

Oh yeah, one other thought, does anyone know of a source for mehanite iron round rods to make pistons from?
I did see that there are regular iron rods for sale, but I didn't see any of the mehanite type, all I found were foundrys offering their services.

Thanks
Earl

earlwb

When I took my Marz 2.5 apart to redo the head screws, I discovered that the two screws I thought had stripped out, had in fact broke off. I didn't think that the cylinder and head were one piece at the time.

With my micro-drill press, I used a small drill bit to drill a hole into the two stubs (I used a center punch too), and then I tapped in a small hex driver and unscrewed the stubs. The threads look OK, but I'll chase them with a tap later. The screws were really too easy to drill into, which means they are made from a really soft steel alloy of some type. Definitely worth replacing the screws before you ever run one of these engines.

The bearings were horrid, rough and scratchy feeling and they were more or less new. One really needs to replace them too. When the engine was assembled the bearings didn't feel too bad. But once you have it apart some, you can really feel it then.
It looks like the front bearing is a OD=15mm, ID=6mm, width=5mm. The rear bearing is OD=19mm, ID=7mm, width=6mm.
Boca Bearings closest bearing matches look to be MR696-ZZ and MR607-ZZ.

Recycled Flyer

Whilst you have the crank out mate are you going to heat soak it to relieve some of the excess hardness?

earlwb

I have been trying to decide if I have a way to do it or not.
it does have a rather sharp corner where the shaft meets the crankweb, so I am not sure if any stress risers will develop right there or not.

Diesel Die-hard

The first thing that I always do with Russian engines is change the screws.Russian screw were all made by the Loadacrap Screw Company somewhere in Siberia! They're made of a material somewhat akin to Swiss cheese, and the threads are poorly cut. If you plan to run a Russian engine, lose 'em!

steve111

We were running a friend's Marz the other day - it went very nicely, but the exhaust was somewhat greyer and shinier than it should have been... We pulled it apart to try and work out where the metal was coming from, and discovered a wear band inside the case indicating rubbing of the big end. It doesn't seem to rub when the engine is cold, so perhaps it expands just enough when hot to come into contact. Interesting to see the same wear band inside the case in the photos here.

earlwb

Steve111 it is definitely worth checking out.
I took a closer look and the big end of the rod and it isn't touching anything. Just barely clears though. The tooling marks on the crankcase shows where they used a booring bar to cut a slightly deeper groove to clear the rod. I could assume they might be a tiny bit off with the boring bar and cut the inside groove a little too close to clear the rod though. it is worth checking out while one is cleaning out the engine, replacing screws etc. The way they machined the connecting rod, leaves sharp corners on the end and tip, so one could easily file a tiny bit off these edges to clear the crankcase if it is touching ever so slightly.

steve111

Hi Earl

This one really didn't look as if it had been machined that way deliberately - the colour difference was obvious, but the difference in depth was almost imperceptible when you felt it, probably less than 1/1000". You'd have to wonder why they'd bother machining out such a tiny amount; and also why they wouldn't just take off the corners of the big end instead!

Steve

Recycled Flyer

Steve, I have posted this pic before but here it is again anyway to show that the boring 'should' be evident in the casting around the con rod big end.

earlwb

Nice pic thanks for posting it recycled_flyer.

Anyway, i would just file a teensy bit off those pointy tips on the connecting rod, so it doesn't quite touch the crankcase,
Since these engines were made by just about anyone in Russia, it is hard to say who did what with them anymore.

steve111

Thanks Chris - very interesting. The relief for the big end in the casting of Col's engine is nothing like the amount shown in your drawing - in fact it is barely pereceptible to the touch, and makes me wonder if his engine somehow missed out on this particular bit of machining, leaving the big end to do the work itself. From what I have heard, this would by no means be out of the question. It's a shame that the quality control on these things is a bit hit-and-miss, as it's quite a clever design and potentially a very good runner when sorted.

Chris, have you flown your Marz Peacemaker yet?

Cheers
Steve

earlwb

I happen to have several different varieties or versions of the Marz 2.5cc engine. So as I replace the screws and or bearings, I'll check the groove inside to see how it looks or doesn't look.

Anyway, I'll be reassembling the one engine I took apart. I got all the parts now. I hadn't made a crankshaft before, so I thought that the more simple Marz 2.5cc crank would make for a good practice one to make. I was using the crankshaft as a reference as I machined out a spare crankshaft, in case I needed one. It took a long time to hog out the material from the steel bar on my little lathe, but I got most of it machined out. Plus I fitted the bearings and threaded it for the 6mm M6 prop nut. Later I'll make up a couple of fixtures to hold the crank so I can machine in the crankpin on the other end. Anyway it was a interesting first attempt. I had a hard time finding some cutting tools to cut the steel with, most all of my bits didn't want to shave any metal off of the steel rod.

original crank on top, newly carved one on the bottom.

earlwb

when I was going through my engines, I noticed there are several varieties or versions of the engine.
Ones with anodized cylinders and prop nuts, ones that are un-anodized, and then some with a large muffler or exhaust collector too. Some seem to have a different needle valve assembly on them as well.
After I replace the screws and bearings on the large muffler version, I am going to see if I can fit a little Thunder Tiger GP .10 size carb to it or not. If it works I'll maybe drill and tap the muffler for a fuel pressure fitting too. The big muffler does appear to have a bit of a gap on the bottom part where it goes around the crankcase, so I am not sure how well it seals if any.

earlwb

I had turned down the heads on a bunch of 3mm hex head screws Saturday. So I put together the first Marz engine that had the screws break on it. I also chased the threads with a 3mm M3 tap to ensure the threads were Ok and not buggered up too much. A couple of the old soft screws had bad threads on them.

While replacing the bearings on a second engine with the big muffler, I discovered they had only put in 10mm long head screws instead of 15mm screws. The screws were only held on by a few threads at that rate. So it was good to catch it. I also found that a Thunder Tiger GP10 RC carburetor fit just about perfectly into where the venturi insert went. So I fabricated a couple of 4mm carb clamps to hold the carb on in place of the control line needle valve. I didn't have time to test run the engine to see how it works with the RC carb.on it today though. it does increase the overall length more as well. So that may or may not detract from using it.

When I made the carb clamp screws, I built in a slight taper where the part next to the threads is slightly smaller in diameter, about 3.95mm and the opposite end is slightly larger in diameter at about 4.05mm. I threaded it for M4 4mm nuts. I am expecting the slight taper will cause the big end to be nice and snug and airtight, the smaller threaded end has the washer and nut to seal it up.

One final thought, it that I need to drill and tap the muffler for a pressure fitting. But I'll do that later before I try running the engine. But the muffler has such a large opening for the exhaust, so I have some doubts about how well it'll work though.

I am not the first to put a RC carb on the Marz engine. But I thought some others might benefit from the concept here. There are a number of variations of this engine out there. So I am not sure how well the carb might fit on a different engine, or if the needle valve hole stays the same at 4mm or not.

coriolan

Nice work on the crankshaft Earl, the TT-10 carb should be perfect size for the 2.5cc MARZ.

fiery

My Feedback: (2)

Nice! Looking forward to your running report.

earlwb

Thanks guys. I can't wait to try it out myself too.

Another forum member named Twin-stack posted this cool Marz 2.5 D engine review article from a number of years ago.

earlwb

I got some time and I test ran the Marz engine with the RC carb on it, The carb actually worked OK. I couldn't quite get it to idle too low as it wasn't using muffler pressure. But I'll rectify that later. I had run a few tanks through it, but I had some problems with the tommy bar coming loose, so I'll need to fabricate a lock bar or something for it. Then a head screw came loose, so I'll need to fix that too. The engine was unusually messy with lots of black gunk coming out. But the major culprit is that wierd large muffler ring, gets more loose when the engine heats up. So goop oozes out all over  the place.

I was running a Master Airscrew Pro Series 9x5 prop on the engine. A 10x4 prop might even have it idling better still. I ran Davis Diesel Development fuel with some extra castor oil added to it. I think for these Marz engines I don't need the extra oil. But what the heck, it only makes them a little more messy.

I think when I go to fix a third engine later, I will remove the small muffler ring and use it on this RC version engine instead of that large ring. But then maybe a little red high temp silicon gasket sealant applied neatly would do the trick. It does let the engine gain about a 1,000 RPMs over the small muffler rings.








I also ran several tanks of fuel through the other Marz 2.5cc engine that I had fixed previously.




It wasn't turning the same RPMs due to that small muffler ring causing it to lose about 1,000 rpms. It used the same prop and fuel that the RC converted engine used. The excess raw fuel was me spilling it when I was refilling the fuel tank, etc. But this engine didn't have all the excess black gunk coming out of it though.

Recycled Flyer

Earl, the amount of black oil looks disconcerting to me and if the difference between the two Marz's is the muffler then Iwould venture a guess that the black is caused by the alloy muffler ring rubbing somewhere.

And have you tried to dimple the contact end of the comp screw yet? I have heard that it takes away the likely 'point' contact of the vernier and replaces it with a 'ring' that is more resistent to turning.

Cheers.

earlwb

Not yet, I was saving my tinkering for the weekend coming up. The dimple idea is good, I was thinking of measuring the threads and drill and tap a locking bar out of some thin metal myself.

Yeah when the engine heats up the muffler gets loose. But the model diesel fuel tends to create a black goop too, more so if it is on the rich side, of which it was when I was first running the engine. So it is likely a combination of both the fuel and the muffler. I am thinking about replacing that big ring muffler with a little ring muffler. I can get more muffler pressure for the fuel tank so the engine should idle better then.

The other thought is something was rubbing on the inside. But when I had the engine apart, I didn't see anything wrong with it. it was fairly loose upon reassembly too, not like it had something rubbing on the inside.

Recycled Flyer

Or making the vernier screw out of a material that expands more than the cylinder jacket when heated - that way it will grip tight to its position all the better.

Locktite401 may know more about this and I believe that Steve Rothwell's diesels have just this feature. (I think that its brass but as to which type?)

Good luck.