GoNavy

My Feedback: (15)

I hope others have solved two issues presented by the older MVVS 77 with the 6mm diameter of the prop shaft.

There are two problems, the first in fitting a spinner, and the second in fitting a prop.

Spinner Backplate Fit:
From the thrust washer, there is a short, about 1/8inch in length, 9mm diameter shoulder or collar. None of the standard spinner backplates (DuBro, Goldberg, Great Planes) have an adapter for 9mm. The largest diameter adapter is too small. The hole in the backplate without an an adapter is too large.

Has anyone addressed the spinner backplate fit issue, and how did you solve it? I suppose Dave Brown or Tru Turn could make a custom backplate with a 9mm hole, or perhaps they have 9mm adapters.

Prop Shaft Hole Fit:
The rest of the shaft, beyond the shoulder, is 6mm in diameter, which is less than 1/4 inch, the diameter of the hole of all props.

If I wasn't going to use a spinner, I could ream a short portion of the the prop shaft hole to 9mm, but I will use a spinner and the backplate will cover the 9mm shoulder.

I presume I need some metal adapter or sleeve, to place over the shaft and increase the diameter to 1/4 inch (or more) so I can use standard props. Is this how others have solved the problem of fitting the prop? Do you know of a source for such an adapter? Is there some other solution?

earlwb

That is a good question. I am responding to bump this up to the top for you.

I would assume that you need to make or find someone with a lathe to make it for you, some spacers as needed. The plastic props that come with spacers might happen to have something that would work too. I have made spacers for some small engines before that had a similar problem. So it isn't a big deal to get them made. Tru Turn might have some spacer adapters for their spinners. You could contact them and ask about it.

Hobbsy

My Feedback: (102)

Earl, I did some digging at Tru-Turn and they list two MVVS .77's and list the adapter for both as 7mm. Somethings amiss. I'll ask Dar.

Hobbsy

My Feedback: (102)

I contacted Dar Zeelon and he said some early engines had a 6mm stud screwed into the nose of the crank and several people broke them off tightening them. Later ones had 7mm threads and even later had 8mm. I don't know where to go with it now.

GoNavy

My Feedback: (15)

K & S/Special Shapes in Chicago have telescoping brass tubing 6-7,7-8mm, and I found a 6-8mm bushing of unknown length at Hobby Express.
MVVS factory rep said it would cost $300 to manufacture a replacement 8mm prop shaft. (Are crankshafts heat treated after machining?)
Since I have already cut the fiberglass cowl to fit the 77, and purchased a muffler for it, I guess I will go ahead and hope for the best. Once the prop shaft breaks, it is probably history unless someone has a spare.

earlwb

Well then one can find someone in their area with a lathe and made some spacers or bushings to fit.
One could use the lathe to make a new prop stud while they are at it too. One end to fit the crankshaft and the other end to match the propellers without reaming them out.

I have used in a pinch some masking tape cut into narrow strips. I wrap it around the crankshaft making it more thick until the propeller or spinner even just fits onto it nicely. But I never tried it with a glow engine though, just electrics. But once you have the spinner and prop centered, then the prop nut is tightened up, the spinner and prop nut aren't going to move any and will stay centered.

GoNavy

My Feedback: (15)

There is a local machinist who was had a contract to make something for the space shuttles.
But, I thought the metal used to make crankshafts had to be heat treated, and that the heat treating was done after all the machining, such as thread cutting, was complete. Isn't heat treating a specialty on its own?

earlwb

They normally don't heat treat the threaded end. Usually it is the crankpin that gets the hardening. the rest of the crankshaft doesn't normally need hardening. But some of the steel alloys used can be quite hard though.

You may have a stud insert in the front that is threaded. If it is you can get one made as a 6mm on one end and a 1/4 inch on the other end or some other size as desired.

GoNavy

My Feedback: (15)

Well if that's the case I will call Peterson Machine and Tool Company of Westboro, Wi, and see what he has to say.

spaceworm

Here is a page from an online stud source. They should also be available at an auto engine rebuild parts supplier. My Zenoah G23 has a prop stud threaded into the crank and yours may also be fitted that way. Search online for "metric studs"

"Home :: Metric Studs :: Metric Step Studs 6-8-10mm [h=1]Metric Step Studs 6-8-10mm[/h]
Metric step down studs

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[TD="class: productListing-data"][h=3]S6X8X23 - Metric Step Stud[/h]Metric Step Stud M6x1.00x8x1.25x23, M6x8x23 Diameter: 8mm to 6mm Pitch: Coarse Thread Overall Length: 23mm Left Thread Length: 11mm Middle Shank...
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Click For Pricing[/TD]
[TD="class: productListing-data"][h=3]S6X8X31 - Metric Step Stud[/h]Metric Step Stud M6x1.00x8x1.25x31, M6x8x31 Diameter: 8mm to 6mm Pitch: Coarse Thread Overall Length: 31mm Left Thread Length: 12mm Middle Shank...
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[TD="class: productListing-data, align: center"]
Click For Pricing[/TD]
[TD="class: productListing-data"][h=3]S8X10X35 - Metric Step Stud[/h]Metric Step Stud M8x1.25x10x1.50x35, M8x10x35 Diameter: 10mm to 8mm Pitch: Coarse Thread Overall Length: 35mm Left Thread Length: 15mm Middle Shank...
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[TR="class: productListing-even"]
[TD="class: productListing-data, align: center"]
Click For Pricing[/TD]
[TD="class: productListing-data"][h=3]S8X10X38 - Metric Step Stud[/h]Metric Step Stud M8x1.25x10x1.50x38, M8x10x38 Diameter: 10mm to 8mm Pitch: Coarse Thread Overall Length: 38mm Left Thread Length: 15mm Middle Shank...
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[TR="class: productListing-odd"]
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Click For Pricing[/TD]
[TD="class: productListing-data"][h=3]S8X10X46 - Metric step stud[/h]Metric Step Stud M8x1.25x10x1.50x46, M8x10x46 Diameter: 10mm to 8mm Pitch: Coarse Thread Overall Length: 46mm Left Thread Length: 16mm Middle Shank...
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Sincerely, Richard

GoNavy

My Feedback: (15)

Thank you very much for the tip... your approach would let me avoid a number of issues.

Does anyone know how I would go about removing the prop shaft stud from the main crankshaft on the MVVS77? Is this a job that someone whose metal skills are limited to Metal Shop 101 in high school 55 years ago can safely tackle? Do I have to remove the crankshaft + prop stud assembly from the engine?

spaceworm

I would feed small diameter rope or cord into the exhaust port with the piston down. Get as much in there as you can, but leave a tail out for removal. The line acts to block the piston from moving. Put a double nut on the end of the prop shaft and tighten them together so that they act like a bolt head on the prop shaft. Try to end up with the flats lined up on the two nuts. Then use a wrench or socket that spans both nuts and turn the shaft out. Tighten the new threaded prop shaft/stud/adapter with double nuts as before with the rope still in place. To remove the double nuts, put a thin wrench on one and loosen the pair with another wrench. When all done screwing around, remove the rope. Worked for me a number of times. Good luck.

Sincerely, Richard

earlwb

Also, do double check and make sure it is a threaded stud in the end of the crankshaft first though. It should be fairly obvious when you look closely at where the stud meets up with the end of the crankshaft.

GoNavy

My Feedback: (15)

I wonder if, what I have referred to as a "shoulder", the silver colored metal in front of the trust washer, is in fact the end of the crankshaft. If so, then Spaceworm's technique should allow removal of the existing stud without opening the case. I have sent a pm to a fellow who seems to know these engines well, to make sure I do this right.

earlwb

I normally do not put a stud into the crankshaft so that it bottoms out. I leave it a thread or two farther out. Then it is easy to get off later when needed.

Here is a video that shows what Spaceworm was telling you. There are stud removers but they tend to damage the threads though.
https://www.youtube.com/watch?v=66ATKL4G04M

GoNavy

My Feedback: (15)

Can you tell me if I have a threaded stud, by looking at the photo in post #14?

earlwb

Well it is hard to tell for sure from the picture as it is a bit out of focus. But it does look like it. You should be able to see where the threaded stud screws into the end of the crankshaft. If it was not threaded, you will not see a joint there. It would be neatly tapered into the threads instead.

GoNavy

My Feedback: (15)

MVVS factory rep in Czech Republic confirms that the prop stud is 6mm right hand thread into the crankshaft.
I have applied the Spaceworm Method but so far stud won't budge. Next step will be to ice the stud.

downunder

To loosen a tight stud (and they should really only be finger tight at most) use the double nut method as spaceworm said but then remove the carb and slip something reasonably soft like a toothbrush handle down through the crankshaft port to lock it in place.

spaceworm

In my initial comment, I had suggested stuffing soft cord through the exhaust port to "fill" the combustion chamber and keep it from turning over. Any method will work. I have used a piston stop for motorcycle engines for my larger gasoline engines, but prefer a softer lock than the hard pin of the piston stop for smaller engines. "Reasonably soft" works for me. Also, insure you are turning the stud CCW to loosen.

If the double nut slips and won't allow the stud to be turned, you can weld a nut to the end of the existing stud and use it like were a bolt head to remove the stud. .

Sincerely, Richard

1QwkSport2.5r

The picture isn't in focus, but what I see is a threaded crankshaft and not a stud. Usually a stud will be threaded the whole length with no flat shoulders. Though I'm not real familiar with MVVS, it doesn't appear to me that the OPs .77 has a stud.

earlwb

Yes I would have preferred to see a better picture of the crankshaft end myself.

Hobbsy

My Feedback: (102)

Earl and Tim, the threaded stud in my .25 has cold rolled very precise threads with about .375" unthreaded between the the threads that screw into the crank and the threads that hold the prop and nut and washer. The stud screws in silky smooth, no looseness at all. It's Blue-Loc Tited in there now.

Is it my imagination or do I see the remnants of Red Loc-Tite around the stud in his picture?

1QwkSport2.5r

If we had a clearer picture, it might be easier to see. I have my doubts as to it having a studded crank, but I'm curious to find out the results either way. I think it may be necessary to pull the crank out of the engine to heat it up to get the stud out (if it's a stud) without breaking it.

earlwb

I can agree. I took a look see at the MVVS website site and the parts drawing showed a 1 piece crank. So thus my doubts. I was wondering if they could have made some with a one piece crank or not.