Horizontal Air Cooled Engine

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Brian Rupnow

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The engine is almost fully assembled. I had trouble with the first cylinder head, so had to machine a second one. The engine is pretty well finished except for "tidy up" and cosmetic stuff. I'm not sure you can see it, but the camshaft gear ended up hanging below the baseplate by about 0.020", so I've had to epoxy a pair of filler plates on the bottom of the base to raise everything so the gear doesn't rub on the table top. This was an easy change to the engine plans, I just made the base out of 5/8" stock rather than 3/8" thick. I didn't want to remake the baseplate, so the filler plates will do for me---I have to tidy them up to match the contours of the engine base. The only remaining things to be made are the cast iron piston rings.
 

Brian Rupnow

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Does anyone have a source for 1/4"-32 Helicoils? I've managed to bugger up the threads in my sparkplug hole in the aluminum cylinder head. There is a considerable amount of time and work involved in making these heads, and I don't want to scrap this one. My options are #1--Find someone who manufactures a sparkplug with a 5/16" thread---(That would be my absolute best option)---Or find a source for 1/4"-32 Helicoils,----Or ---Make my own sparkplug with a 5/16" thread. I have made my own sparkplug before, but I don't really want to. Thank You----Brian
 

Brian Rupnow

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Something more--There are manufacturers of M8 metric sparkplugs, which are very close to 5/16". M8 is 0.315" diameter. 5/16" is 0.312" diameter. Perhaps my easiest fix here is to buy an M8 sparkplug and an M8 tap. They seem to be readily available on the internet.
 

Mike1

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Brian, Have you room to bore out your spark plug hole, and make a mild steel insert/bush bored and threaded to suit your original plug thread, and press fit into the head or fix with loctite, even JB Weld.

Mike
 

Brian Rupnow

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Mike--I have considered it, in fact, I came down to my shop intending to do just that. The cylinder head gets hot.--Hot enough to loosen the bond of Loctite, and even possibly J.B. Weld. It's Sunday here today, but tomorrow when the shops are open I intend to pursue the M8 sparkplug option.----Brian
 

dsage

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JB weld will be fine on it's own for any temperature your engine will achieve.
But, depending on how much room you have you can make the suggested bushing thread into the head instead of just a press fit.
That, along with JB weld will be fine. Been there, done that.

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Jasonb

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Brian, do you really have room to go upto an M8 plug, it looks like your existing plug hole comes quite close to the recess in the head and a larger one will leave very little if any area to seal against the raised edge of the cylinder.

Might be OK if you bore it out by plunging a 7mm milling cutter with the ctr slightly closer to the valve

If you do buy a tap make sure you get M8 x 1 which is finer pitch than the common metric coarse of 1.25mm pitch
 

Brian Rupnow

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Yes, I do have room for an m8 plug. I'm not sure of the thread specs are on the M8 marine sparkplug Amazon sells, but I will find out more tomorrow.
 

Brian Rupnow

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Having a very wimpy day here in the great white north. It has snowed here all day and I haven't been outside. I did a little tidy up work on the cylinder head and the rocker arms, and added a screw to the base to mount the condenser. Other than chasing down an 8mm sparkplug and making rings, the engine is completely "built". I have successfully made and ran cast iron rings in my last two or three i.c. engines, and now I will pull out all the notes I made with respect to 1" diameter pistons and see if there are any real changes to make rings for a 7/8" bore engine.
 

johwen

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Does anyone have a source for 1/4"-32 Helicoils? I've managed to bugger up the threads in my sparkplug hole in the aluminum cylinder head. There is a considerable amount of time and work involved in making these heads, and I don't want to scrap this one. My options are #1--Find someone who manufactures a sparkplug with a 5/16" thread---(That would be my absolute best option)---Or find a source for 1/4"-32 Helicoils,----Or ---Make my own sparkplug with a 5/16" thread. I have made my own sparkplug before, but I don't really want to. Thank You----Brian
Hi Brian,
Make an aluminium plug 5/16 -40tpi an Loctite in place and when set drill and tap for you 1/4 32 spark plug Cheers. John
 

aka9950202

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I discovered that the spark plug thread is not normal M8. It has an 1mm pitch.

Cheers,

Andrew in Melbourne
 

awake

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Brian, from what I am seeing, M8 appears to be a model number rather than a size; everything I am seeing describes that plug as having a 10mm diameter. Hopefully I have it wrong, and you will be good to go ...
 

Jasonb

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M8 is the thread dia just like a CM-6 has a M10 thead both 1.0mm pitch.

8mm sparkplug is liekly to have an "E" prefix which indicates M8 thread and 13mm A/F body with a 1mm pitch. C is the prefix for M10 threaded plugs eg CM-6

Typical details of an "8mm" plug, click the technical tab but ignore the way off imperial conversions in brackets


Plug code chart
 

awake

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I agree that M8 sounds like a thread diameter ... but when I search for "AC Delco M8 spark plug specification," I get links such as the following - which all consistently say that the diameter is 10mm, not 8:


Based on these links, I am thinking that M8 happens to be a misleading part number rather than an indication of size ... and that Brian is going to be disappointed with the actual size of the plug. :(
 

Jasonb

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M is used on premium and performance plugs, see the third chart in my link which as M10 x 1 thread. Standard would be E with M8 x 1 generally referred to as 8mm plugs

Maybe Brian can link to what he has bought
 

Brian Rupnow

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Brian--Thank you for that. I have almost wore out the internet looking for sparkplugs with a 5/16" thread, but haven't found any in the world. I have ordered what I thought was an 8mm sparkplug on line, but Sparky has cast doubt on that. My cheapest route by far is to machine another cylinder head, but I really don't want to do that. I just got word today about designing some light automation for a company in Toronto, so I'll probably just spend the money and buy the thread repair kit.---Brian
 

dsage

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I guess you've rejected the suggestions of making an insert (threaded or not) and JB weld (or loctited) in place??

Threaded and JB weleded worked for me on a much more detailed head that I definitely did not want to re-machine.
Just saying. Nothing to lose.
 

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