stunthanger.com
Engine basics => Engine set up tips => Topic started by: Robin_Holden on July 26, 2022, 03:26:43 AM
-
Morning folks from North Yorkshire , England.
Took out a Magnum 36 the other day to install into one of my profiles.
It’s a converted ex R/C motor , a nylon Venturi , blocked boost port , with a single head shim.
Always kept boxed , clean and well oiled , whilst in dry storage.
When I tried to turn it over it ‘jammed’ , didn’t want to turn over.
Took off the cylinder head . Turned over perfectly.
No bearing issues.
Put the head back on , and bingo , didn’t want to turn over.
Any suggestions fellas ? Any contributions much appreciated.
Robin.
-
Did it jam like a bind or like it was blocked? The first thing would be to check head clearance.
Motorman 8)
-
Was the plug installed?
How tight were the head screws?
-
Did it jam like a bind or like it was blocked? The first thing would be to check head clearance.
Motorman 8)
It was blocked.
I’ll check the clearance as you suggest.
Much obliged ,
Robin.
-
Was the plug installed?
How tight were the head screws?
They were as normal , not over tightened.
The plug was installed.
Robin.
-
I have a Magnum 36 (square head) converted from RC with a delrin venturi but without blocked boost port. Its a great runner and very powerful for its displacement. I have never encountered the problem you describe.
Will it turn over with the plug removed?
How is the boost port blocked? Any chance it is interfering with piston movement?
Are you sure the head is installed front-to-rear correctly, ie not reversed?
-
Did it jam like a bind or like it was blocked? The first thing would be to check head clearance.
Agreed, and it may be that this issue is caused by another common modification - "dropping the liner" (usually to try to reduce the exhaust duration) without adding the necessary additional head shims could give it insufficient or negative head clearance. I am not sure about the Magnum, but many engines have liner shims to set the liner height, if those were omitted (on purpose or accidentally) you would likely have a clearance issue.
I can check my Magnum engines to see if there is anything tricky about it in this regard. Maybe tomorrow night, I have just about had it after 3 straight days of driving and a 2-hour traffic jam at the NV-CA border today, courtesy of CALTRANS.
I also note, yet again, that *the engine runs fine as it comes* for stunt, and modifications are unnecessary, or in this case, detrimental.
Brett
-
I also note, yet again, that *the engine runs fine as it comes* for stunt, and modifications are unnecessary, or in this case, detrimental.
My Magnum 36 is bone stock except it has a venturi instead of a carb, and it runs great. I have a few motors that have been "improved" and they run -- not so great.
Which is why seeing
... blocked boost port ...
makes me worry that you may never find joy with this engine.
-
Try just holding the head on in the correct position and turning the prop. You should be able to hold the engine with the loose head in one hand, and turn the prop with the other. If you can feel the head move when the piston hits TDC, then for whatever reason, you have negative piston-head clearance and you need to fix it.
-
When I tried to turn it over it ‘jammed’ , didn’t want to turn over.
Took off the cylinder head . Turned over perfectly.
On further consideration, I am suprised to hear that, I would have expected the piston to bind up in the liner near top dead center, and push the liner up, not just pass over TDC with no effort. This is intentional, they have a tight fit and a lot of taper and usually squeak over TDC even with a glow plug in. If it turns over freely without pushing the liner up, I wonder what the fit is like.
Actually, are you sure that the "jammed" engine is not just binding up at TDC due to taper and being cold? Or is it actually hitting the head?
Brett
-
+1 on what Brett said. Magnum 36 came through with various piston fits. You might have gotten a real tight one.
-
I've seen one Magnum XLS .36 that had two head gaskets from the factory. With one removed, the piston hit the head. Didn't make it a bad engine at all...maybe better than most. I suspect the counterbore for the cylinder flange was too deep, but it wasn't mine and it ran just fine with two head gaskets.
The first of multiple Magnums I've had (all are PRC produced) said to break-in with at least 20% castor oil. More recently, the instructions said to use fuel with at least 20% oil, with no mention of castor. I think that's because you probably can't get any fuel with 20% castor oil.
Regarding cylinder "pinch", I haven't seen a Magnum with anything similar to an ABC K&B or Rossi .15, but way more pinch than any of the ABN OS engines. D>K Steve