A piston stop you can build for about $2. Find TDC, Timing.


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Baraboo, WI, USA
While doing an install of the Hoos Racing Vape ignition charging system I needed to find TDC on a rotor with no key, a start from scratch situation.
So came up with this TDC finder that any shade tree mechanic can make in an hour with probably ZERO cost.
Parts needed
1 old spark plug, as used on the XS650 I had an NGK other brands should work.
1 3/8" machine bolt 3 1/2" long it needs to be the type with a shank not one with full length threads. A carriage bolt I tried had a slight smaller shank which didn't work as well. Bolt needs to be straight if you are digging through the old bolt bin to find one.
3/8" nut to match the bolt
Tools needed;
3/8" drill bit and a drill
Some way to cut; an angle or bench grinder dremel tool even could do the whole project.
I have a lathe, so I used that. :shrug:
First up is getting the core out of the spark plug. Note how you have to cut part way into the hex to get the crimped metal fully released. Could be done with a grinder, angle grinder or dremel cut off wheel. In the lathe is nice and neat. Grind off the side electrode, then drive the core out with a drift against the center electrode. Now drill through the plug with the 3/8" bit, start from the hex end of the plug. it's not much more than a skim cut so could prolly be done just fine holding the plug with a vice grips, using a hand drill, a vice and drill press would be better, I did it in the lathe. lightly chamfer, sand or file the ends of the hole so the bolt will side smoothly up and down. Cut off the bolt head so that you have about 2 1/2" of shank before the threads start. Round the shank end evenly, round end is probably the most critical thing here. Carefully chuck the threaded end it in a drill and rotate while rounding the end on your grinder. If your bolt is rusty or marred, smooth the shank with fine sandpaper, lightly grease the bolt. run a nut all the way to the end of the thread. You NEED something on the end of the bolt so it cannot slide through the spark plug guide into the cylinder!

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that's it you have a TDC finder.
Note; some pics show one I made with the bolt head on the shank. But cutting the head off the bolt lets you chuck the shank in a drill to make a consistent rounded end to contact the piston top and improve results.

Using it: You have set your valves, so you know the deal; remove both spark plugs, remove at least the RH set of valve covers. Watch a piston come to TDC through the plug hole. Conveniently the valve springs pushing on the cam "almost" center the crank on TDC. You need the LH piston to be on compression, so the RH valves will be the pair that move as you rotate the crank back forth across TDC. You do NOT want valve heads getting near the bolt shank of your TDC finder! That's why I like this no stop design compared to the typical solid stop TDC finder. That type makes you rotate crank completely around to get both sides of TDC marked on your rotor. contacting a valve head is too easy to do while fully rotating the crank and that can get ugly.
How accurate do I need to be? 1/4" on the outside of this 4.4" rotor = about 6 degrees of timing. So I'm thinking find TDC to 1/16"! Closer is better.
Taking a break, need to get some more pics, change how I do this a bit. Expect further editing.

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to be continued.....
Key words
TDC top dead center piston stop hoe built tool timing time ignition rotor key keyway setting set points electronic vape pamco boyer marsden coil coils spark


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Dial indicator on the end of the shank? That will get ya REALLY close
The goal here is something a guy with very basic tools can do and get acceptable results.
Actually I was messing with setting up a dial indicator yesterday,not trivial to get one set up good and solid!
I'd try getting a dial indicator as vertical as possible, and using the slimmest extension possible. That sort of rules out a typical 2 stroke ignition timing gauge with a concentric holder with spark plug thread.
A dial gauge bracket held by the point cover screws or the inner point housing threads would be my choice
Yes, similar, but not the same. Gary's isn't threaded, the "pin" floats up and down. Technically, Gary's isn't a "stop", more of just a TDC indicator.
Similar to yours Gary. Difference is I used the TIG to weld a nut on it.

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I had one like yours jim, the spark plug ID is nearly 3/8" NC thread ready ran a tap through and bingo. What I don't like about the stop type TDC checker is that you need to run the crank around nearly a full revolution to bring it up to the stop on both sides. coming up after BDC you run the chance of a valve opening against the plunger. With an indicator type you only rotate through about 20 degrees either side of compression TDC, the valves never open.
So far in spite of my best efforts at honing and smoothing, bolt and bore the angled force from the piston tends to jam the bolt in the bore, it won't just slide up but you can easily pull the bolt up, pass TDC and let it go back in. I think it is more than good enough and meets my goal of being simple and cheap to make. There's more to come setting up timing marks for the Vape rotor.

Dial indicator on the end of the shank? That will get ya REALLY close
You can clamp the magnetic base to the frame backbone above the engine.
I messed with it for quite a while, with the plunger stiction issues using a dial indicator is a tough nut to crack. Like arctic says above, the piston's angled relationship to the plug hole is the problem. But both shank/bore fit and lack of bearing material make a freely sliding pin hard to do. At least with my limited machinists "skills" LOL. I feel the manual set method is accurate enough, I ran it 4 or 5 times with +- 1/32" variation. Will set up and scribe the Vape advertised and factory spec timing and advance marks on the rotor so timing can confirmed with a timing light.
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So let me get this straight, the only thing holding the bolt from falling in is the head on the bolt cause it's not threaded inside the spark plugs?? I tried making one like Jim but I don't have a welder (yet... ) So I used JB weld and was having concerns about valve clearance.
So let me get this straight, the only thing holding the bolt from falling in is the head on the bolt cause it's not threaded inside the spark plugs??.
Correct and yes you need to avoid the valve opening part of the rotation. It IS possible to damage parts with enthusiastic crank rotation, with that bolt sticking into the cylinder.
On the COMPRESSION stroke you can safely rotate the crank, say 90 degrees before and after TDC with no valve opening or interference with the stop bolt. Just like valve checking, watch the valves and clearance to be sure you are on TDC of the compression stroke not the other TDC when the exhaust valve is closing and intake opening.
This was the whole point of my sliding stop; to avoid having to rotate the crank through positions where valves open.