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Author Topic: h75 few modifications  (Read 1054 times)

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h75 few modifications
« on: January 15, 2023, 02:25:10 am »
Well as I was tearing the engine apart I noted a few things that bothered me, so below are the modifications I added. Some may help, some may hurt, all were things I thought were logical and had a good time doing.

Left boss to engage bearing inner race and otherwise faced the water pump pulley. this stops the rubbing of the OD race of the bearing from rubbing
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"hobbed" the water pump pulley groove and steel flywheel groove to improve traction on oring belt, just spun a 2-56 tap in the drill and let it lead itself across the diameter of the pulley for a few laps and it leaves a nice "thread" [ Guests cannot view attachments ]
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Noticed a lot of chatter on the crankshaft, so I polished each with thin strips of 1000 grit silicon carbide paper (cut strips with scissors to fit, spin crank to polish
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Added lube holes for both big and small ends of connecting rods. not sure if the small end will do much, but I figure it doesn't get much lube, and as I intend to fill the crank case to make it "splash" lubrication, this should help catch the rare drop of oil.
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Didn't take pics of it, but the gas tank on my h75 has been pretty loose in the clamps, so I folded up a plastic bag under the tank and now it is firmly mounted in the clamps.

that's all for now!



bonus pics, I broke a .007" drill bit when drilling the connecting rods, and spent the better part of 2 hours drilling around it and making super fine tweezers to dig it out.... shows me for pre-drilling haha, all 7 other holes were a breeze!
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Re: h75 few modifications
« Reply #1 on: January 15, 2023, 10:53:38 am »
Thank you so much for those excellent tips I cannot believe the chatter marks on that crankshaft rod journal that's ridiculous I think I better completely disassemble mine and inspect it before I ever run it!!!
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Re: h75 few modifications
« Reply #2 on: January 15, 2023, 06:40:34 pm »
Certainly worth checking out, I was quite surprised as well. 3/4 were rough, but only 1 that "needed" to be smoothed as I was already going to be cleaning the whole crankcase from sanding 1 I just went ahead and polished them all. Not sure if that will help or hurt things, but I figured smooth was better than not perfectly round.
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Re: h75 few modifications
« Reply #3 on: January 16, 2023, 10:28:40 am »
Yes, as much as I like the Chinese model engines, this shows that you should do at least a partial disassembly and inspection before running them.

I think it would be better if you put one lubricating hole on the bottom rod cap rather than two on the top side. This area gets the most force during the power stroke, and the two holes remove material where it is needed most. One hole on the bottom cap should provide plenty of lubrication in a splash system setup.

Thanks for posting your detailed modifications of the H75 engine.👍🏻
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Re: h75 few modifications
« Reply #4 on: January 16, 2023, 10:51:05 pm »
Ooooof yeah you are right, that would have been a good call not to put 2 holes there, eh what's done is done
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Re: h75 few modifications
« Reply #5 on: January 27, 2023, 07:30:21 pm »
Actually when it comes to high rpm the connecting rod is literally pulled apart from higher RPM. The forces are much higher than the explosion forces even with detonation the rod is literally pulled apart at higher RPM if you exceed the designed rpm speed limit.
 Piston speed is actually quite high changing directions at top dead center compared to bottom dead center it is relatively low speed changing directions.
 At the speeds these engines run one oil hole would be enough I believe whether it’s at the top or the bottom.
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Re: h75 few modifications
« Reply #6 on: January 30, 2023, 03:19:13 am »
I really liked how you've hobbed that pulley, can you walk me through the method you used to do that?
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Jim

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Re: h75 few modifications
« Reply #7 on: January 30, 2023, 02:24:26 pm »
You can kinda see the 2-56 tap in the groove in the images above, I just hold the tap in my drill, get it spinning, then drop it into the pulley groove. If you hold the drill still and don't let it pull forward, it will pull itself along in the pulley groove, and you can do as many laps of the pulley as you wish. Mine took about 3 laps on the steel pulley, 2 on the brass. I can't tell where the thread overlaps, but I assume depending on the diameter that it won't line up perfect. I have never noticed anything bad, but these are also 56 tpi, so it's pretty darn fine and probably just jumps to the next tooth! I have used this technique on many engines and accessories to increase traction, as I use the wilesco spring belts and they slide without something like this to grip. I can make a video if you think it would be helpful to see.
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Re: h75 few modifications
« Reply #8 on: January 30, 2023, 04:02:50 pm »
No hurry, but a video would be super appreciated.
The old saying a picture tells a thousand words..... a video must tell ten thousand words.

That would be such a handy skill to master.
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Jim

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Re: h75 few modifications
« Reply #9 on: January 30, 2023, 10:27:46 pm »
Not much to master, but certainly handy when belts are sliding! for a more professional result, a rig to hold the tap from moving relative to the pulley center would probably work quite nice. Similarly to a worm gear needs the worm to be restrained.  
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Re: h75 few modifications
« Reply #10 on: January 31, 2023, 04:03:33 pm »
Tom thank you very much for taking the time out to make video to show that, super appreciated.

How well does that work? Looks almost just as good as a Wilesco pulley.

Again.....thanks mate!
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Jim

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Re: h75 few modifications
« Reply #11 on: February 01, 2023, 11:37:15 pm »
It really works great, especially well with the metal spring belts like wilesco. I suspect Oring life is reduced, but with how little time these engines actually run it's worth knowing the cooling system is at least working
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Re: h75 few modifications
« Reply #12 on: February 01, 2023, 11:43:49 pm »
Appreciated the video Tom....thanks mate!
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Jim

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Re: h75 few modifications
« Reply #13 on: February 02, 2023, 07:29:20 am »
Quote from: Jim on January 30, 2023, 03:19:13 am
I really liked how you've hobbed that pulley, can you walk me through the method you used to do that?
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I agree, Jim. This is a great solution. Thanks, Tom!
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