Sparkplug Question

[bob]

Briggs and Stratton 252707, 11HP. Sparkplug hole is stripped. Had been repaired with helical coil but not enough thread left for spark plug to reach. I repaired again with helical coil or something similiar but what i noticed was the sparkplug does not protrude through the hole. in fact it is about 1/8" up inside the hole from the bottom. I just always assumed the electrode was below the bottom of the hole or below the bottom of cylinder head. Please explain. Thanks

[bgsengine]

With the correct spark plug in place, most L-Heads will have the ground electrode roughly about level with the spark plug hole opening - typically they'll be a 3/8 inch "reach" (thread length) , although K series Kohler and Magnum cast iron single cylinder from 10 to 16 HP typically used an H-10 champion , which had a 7/16" reach with an L-Head you'll generally have turbulence and cross-flow (because valves are to the side) so they'll have the spark start at a higher point in a more stable area, flame would spread out more evenly across the top of piston (You may note that Tecumseh plugs were a bit off-set and located more between the valves and cylinder, and they weren't quite as "recessed" up in the plug hole.. so by the time the still-moving air/fuel charge ignited , the flame would be spreading out more evenly across the piston, since it had moved slightly from where the spark ignited it) At least, that's my theory.. I never saw any engineering reports that explained the reasoning otherwise..

[bob]

Briggs 25270 calls for Rj19LM sparkplug which has 3/8" reach but this spark plug hole in the cylinder head is like 1/2". That puts the the electrode well up inside. Very confusing. A Champion H10 has a 7/16" reach. I will have to see were it ends up

[bgsengine]

If you used an actual Heli-Coil insert, then the thickness of the head should be standard.. however if you put in one of those solid (non-wire) inserts with the flange on it, then yes your head would be too thick then (unless you counter-sunk the plug hole to take the replacement insert) So, the question is, does the spark plug gasket seat on the head where it is supposed to (As in, original head surface) if it does, then the plug has the proper reach, even if it does seem to be recessed in the head. One trick I saw used with those flanged inserts was, they removed the spark plug gasket (since the thickness of the flanged insert was about the same)

Those flanged inserts are easier to install than the true heli-coil inserts (which require the installer tool) However, they are far less quality (Due to thermal expansion properties difference between the insert and the aluminum head, they are notorious for blowing out of the head under heavy load/high heat conditions) But, for best results, better off replacing the head entirely with a good used one (often find them listed on eBay and Etsy, etc)

[KE4AVB]

That a good question on what he used. There is at least different inserts that can be used. A standard Heli-coil, a shouldered Time-sert, and a threaded insert. Both the Heli-coil and thread insert keeps the original plug install height but the shouldered Time-sert style increases the install height. The threaded insert appears to be the only one that can be used with a taper spark plug seat though the Heli-coil should able too. The Time-sert version appears to be for only gasket type plugs.

During my research found that Heli-coil branded products can be a standard Heli-coil or a threaded insert for spark plug hole repair. Got to pay attention as to which you are buying. A standard Heli-coil can be removed but the threaded insert can't be so it is an one shot deal with the threaded insert.

Just my observations during researching this type of thread repair.

[bob]

It is just a cheapy thread insert with the flange. But what I actually did is come up from the bottom so the flange was on the piston side and I just ground it off even with bottom of sparkplug hole. I did what Bg's mention and took off washer from sparkplug and looks like a better fit. Also tried the H10 sparkplug which has a longer reach and it looks better too. I am going to try these things later on in year and see what happens. Just went through a hernia surgery so I am on light duty for awhile. Thanks for help

Blair

[hanz63]

It still has to be better than the kid who stripped out the threads in his mom's mower who used JB weld to hold the plug in. That plug wouldn't fire for nothin'. At least he was trying to help!

[38racing]

Hopefully the heat range of the h10 is ok.

[bgsengine]

Hopefully the heat range of the h10 is ok.

It is pretty close - before J19LM came along, they used a J8 (or a CJ8 where clearance was needed - the "shorty" plug) then the J8C when champion added the copper cores, and eventually J17LM and J19LM H10 isn't too far off in the heat range (and for a cold-blooded engine, maybe a bit better)