Surface gap plugs

[Plainsman]

1976 115 hp, modle 1150, serial # 4313752

I am using a surface gap plug, NGK BUHW, but want to try a gapped plug. I have seen where some folks are real happy with the surface gap plug. What plug should I use in it's place?

Thanks,
Eric

 

[Silvertip]

Re: Surface gap plugs

If you are happy with the surface gap plug and that's the plug that's recommended, why do you want to change to a conventional plug?

 

[Plainsman]

Re: Surface gap plugs

I'm not that happy with them. Seems the engine runs rougher and performace is not as good as it was with the conventional plug.

 

[Scaaty]

Re: Surface gap plugs

If the manual calls for SG L76V's use'em. Then do a tune up..quit blaming the plugs..there the LAST thing that will cause a motor to run rough if new and the Manuals call fer'em​

 

[Plainsman]

Re: Surface gap plugs

Thanks Scat

 

[Scaaty]

Re: Surface gap plugs

Theres a heck of lot more involved here. Just got done with my Merc Guru slamming a total rebuilt 135 for my Flatbottom build together. Carbs, pumps, mess of stuff that could use new or fixing. Love the old Tower of Power's, but they are finicky beast..​

 

[jimmbo]

Re: Surface gap plugs

Using regular type plugs in a engine speced for surface gap plugs can create all sorts of issuses. There might not be enough clearance between the piston deflector and the new plug. By placing the electrode further into the combustion chamber you are changing the point of ignition and the flame front of the combustion process, it's like advancing the timing a few degrees. The surface gap plugs can be good for many seasons. No deep creavases or ceramic tip to carbon up and foul. I know my 84 115 had the same NGKs it came with from the factory when I traded it 14 years later.

 

[Silvertip]

Re: Surface gap plugs

I suppose this will start another one of those never ending discussions but I guess that's why they call this a forum. Engine manufacturers don't design an engine around a surface gap plug. That said, the engine design they come up with may be tuned for what they feel is best all around performance with a specific surface gap plug. There certainly have been engine designs over the years that absolutely require SG plus for interference reasons mentioned. Now then -- I run a Merc 75 HP triple that calls for a surface gap plug but in which I have a much better starting, better idle, and equal WOT performance using a conventional plug and I can troll for hours with absolutely no problem. This is not the first time I've made this swap as I've owned several snowmobiles and boats over the years that were nothing but trouble with SG plugs. Those problems disappeared with standard plugs. This change was not made without serious investigation and I don't recommend anyone else do it unless they thoroughly understand what the ramifications are. Since plug reach is a critical issue I used a dial indicator to measure the distance from the piston crown at TDC to the plug gasket surface on the water cover. That dimension on this particular engine is 1.24 in. Again using a dial indicator I measured the plug protrusion (reach) from the crushed gasket to the plug tip on both plugs. The conventional plug measures 0.64 and the SG plug measures 0.45. That yields a piston crown to plug tip gap of 0.60 with the standard plug and 0.79 with the SG plug. There is absolutely no chance of piston to plug interference in this case. Next is heat range. I used the NGK application book to research heat ranges for a very large number of carbed engines and they all fall within one or two heat ranges of each other regardless of manufacturer (within the same HP ranges of course). Because I don't want someone sticking an incorrect plug in their engine I will not reveal what I'm running so if you insist on switching, make sure you know why you are making the switch. The long and short of this is that HOW you use your ending can have a bearing on what plug works best. Do I carry a fresh set of SG plugs? Yes! But they only go into the engine if I'm on a large body of water where I make 10 - 15 mile runs or more at WOT.

 

[jimmbo]

Re: Surface gap plugs

Slivertip. I'm glad it worked for you and that you did the math prior to doing the change, a lot of people don't. Oh well at least their engines wouldn't be damaged further once the piston bends the side electrode against the centre electrode, grounding it.
My dads 1980 35hp Evinrude came with surface gap plugs but I remember the owners manual also specced a conventional plug if the engine was to be used for lots of idling, trolling. In my engines, Merc and Evinrude I had good luck with the SGs, I would check them a couple of times a year, wiped them clean, put new gaskets on them. The Ficht was a different story. Your 75 is a looper with nice flat pistons, my old 115 I6 was a variation of a crossflow( direct charge) with a deflector on the piston. I never measured the clearance so I don't know whether it could use a regular plug. Never thought of it, but my engine ran fine on the SGs

 

[Plainsman]

Re: Surface gap plugs

Did they make SG plugs in 76? I know when I first got the motor that it did have the conventional plugs in it. So I am thinking that I would be fine using those once again, correct?

 

[Silvertip]

Re: Surface gap plugs

You bet they made surface gap plugs in 1976. I bought a new 1975 Rupp snowmobile and it ran SG plugs.

 

[Texasmark]

Re: Surface gap plugs

I'm running conventional plugs in my '02 3 cyl 90 hp looper per Silvers recommendation/comments....same plug he runs in his 75. Plenty of room between the plug and the piston top. I picked up 300 rpm's with mine over surface gap. Engine starts faster (cold and hot) and seems to idle smoother.

I read up on them on the NGK www site and they explained what the surface gap was designed for. I do neither (sustained WOT and/or sustained trolling).

Even though the spark arrives in 40 microseconds and fires before it can bleed off across the insulator, I don't like the short circuit you get on surface gaps even after a couple of minutes of operation.

Course millions of hours are put on them annualy so they must work.

Mark

 

[Silvertip]

Re: Surface gap plugs

My experience and main gripe with SG plugs has been at slow speeds. Walleye fisherpeople do a lot of slow speed. Any hint of a track across the gap on these plugs and you lose a cylinder and they will not clear out like a conventional plug. Starting, especially cold, is much improved with standard plugs. That's my story and I'm stickin' to it.

 

[Laddies]

Re: Surface gap plugs

Eric, the first outboards that I ran into with surface gap spark plugs were 1962 McCulloch engines they ran 100 to 1 oil mx with there own oil. Champion designed the J19V plug specifially for that use as it allowed a higher HP per cu. in. without detonation and engine damage. Mercury in 1966 saw the advantage of the surface gap plug and combined it with CD ignition. I have never found a engine that was designed for surface gap plugs that was properly tuned to run better with regular plugs. There are a number of people out there that might recommend a regular plug to you, but not among profession mechanics--Bob

 

[Plainsman]

Re: Surface gap plugs

Thanks for all the input guys!! It is greatly appreciated.

 

[OldMercsRule]

Re: Surface gap plugs

Wow I feel like a sponge here. Eric: as you know: I run a lot of old inlines. My Mom had a 1967 500 that had Champion SG plugs from the factory, I had a 1967 650 that had the same plug as the 500, (I think; but I'm getting old and memory fades of events in the late 1960s). My 1968 1250 came with Champion SGs and my two 1969 1000s have 'em too. All three of my 115s have 'em too, (1978 1150, 1979 115 and a 1981 115). I have learned that NGK plugs run better then the Champs. I don't usually idle my inlines for long periods as I have a 6hp Johnny that I troll with if I need to troll. All my motors presently on my boats, (the 1250 is dead but ran great for many many hours always started and idled fine), run start and idle well, (I guess two of 'em are still not boat tested yet as I picked 'em up this winter). I found that the SG Champs would develope hairline cracks in the porcelain, (I sand blast 'em from time to time after a change out to clean 'em up and the cracks show up if I have my specticles on). As I eleminate the cracked Champs, (I always keep the old ones to sandblast), I am moving to the NGKs that also develope these same cracks but not as rapidly as the Champs seem to do. Once you dial in the proper plug Please PM me, as I am curious. Thanks JR

 

[Scaaty]

Re: Surface gap plugs

OK, I'm chiming here here too...run some non SG plugs and keep us posted...Jimmbo brought up some interesting info (ALL good info in this thread!)...I'd like to see a follow up. Just got done with a total rebuild on a 72 (I'm paying attention this time Jimmbo!) 115hp...​

 

[Texasmark]

Re: Surface gap plugs

I had an early '80's 4 cyl 85 and an '89 115 tower and ran the factory plugs in them with no problems. No reason to look for something better. On the tower I ran 2 sets of plugs over 7 years (1+6)......not too bad, and WOT was 6 grand.
This current '02 3 cyl 90 hp just runs better with them. Like Silver says, it initially starts better, starts up after fishing easier, and if I load them up (like I did the SG) idling too much while launching and parking my truck, they clean out real fast.

Course I have since learned to shut off the engine and park the truck. Then the initial punch out from the no wake zone is good and solid.

Now I will give you this. I have a drive on trailer. The boat sometimes needs a nudge, after you're on the trailer, to get up close to the bow chock. I do think I heard a tad of pre ignition the last time I was out, with the standard plugs I am currently running, that I never heard before with the SG. Course the engine is under some serious strain when doing this so I'm not too concerned about why it did it and it supports the theory that SG's tend not to do that. I'll just have to modify my loading procedure as I like the rest of their good points.

Mark

 

[Clams Canino]

Re: Surface gap plugs

I'll make this real simple. If you run a regular gapped plug in direct charge inlines - you WILL kill it. Detonation will get you. Call me when you need pistons.
Nuff said.

-W

 

[Scaaty]

Re: Surface gap plugs

I'll make this real simple. If you run a regular gapped plug in direct charge inlines - you WILL kill it. Detonation will get you. Call me when you need pistons.
Nuff said.

-W

Thats what I would think too..that hot tip of a gapped plug can't be good either... (by the way, I'm here for info, and would never think of anything but SG in my Merc Inline 6 just put together crank up). And although Champs in the Merc now, I have found out my old 88 Force 125 runs a lot better/longer with NGK's and cheap Walmart oil. I agree with CC...heck, I'll change plugs all day if I have to...holed a piston on my old XS150, and its a little more work to change a piston!​