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I am running a 1967 100 hp engine......................

B

Badbear 45/70

Member
I am running a 1967 100 hp engine with the 1966 dist and coil and need to know what is the best plugs to run. I have fiberglass reeds in the motor and 0.020 cut off the heads and will be adding a bubble back exhaust this winter with a porting job and yes, I have everything I need to do this improvement. I am presently running Champion L82C plugs and they seem to work OK but with the next upgrade I wonder if there is a better plug to run.
 
Cutting heads does not add compression to these engines...make sure plugs don't hit piston top now especially at WOT as rod/piston stretch !!!!
 
I bored one of these blocks out by 0.060" to 3 -7/16"-----Fitted and installed standard pistons from a 1970 model 115 HP ( a lot of FITTING and work ).------Motor ran over 6000 RPM with the highest pitch 2 blade bronze prop I could find.------Still have a couple of those 2 blade bronze props for that 100 HP gearcase.
 
Neat stuff. As you may know, monitor plug color and adjust accordingly. You may have to drop a bit colder. Are you running the highest octane in non oxy fuel? What is your oil type and mix ratio?
 
timguy said:
Neat stuff. As you may know, monitor plug color and adjust accordingly. You may have to drop a bit colder. Are you running the highest octane in non oxy fuel? What is your oil type and mix ratio?
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I run unleaded premium with a 25 to one oil mix with TCW III oil and have never had an oil problem with one of these older engines.
 
faztbullet said:
Cutting heads does not add compression to these engines...make sure plugs don't hit piston top now especially at WOT as rod/piston stretch !!!!
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It is impossible to not cut the heads and not raise the compression ratio. I alway check for plug clearance before I ever start the engine. I was a certified Johnny/rude mechanic for years I know my stuff.
 
Yeah, I was about to dispute faztbullet's statement too, but then I got to wondering just how much would the compression increase by shaving the heads 0.020". I'm thinking not much inrease, but haven't attempted to figure it out. I suppose you have to first figure the area of the circle and multiply X 0.020 to come up with the cu/in difference and go from there (????). Makes my head hurt.
 
Certified OMC mechanic for years you say.-----Now asking what sparkplugs to run in this motor.---Sorry had to say it.
 
On a crossflow you reduce the chamber size to raise compression,shaving head will not do it. Shaving head on looper WILL raise compression..about 10PSI per .020

low compression head
images



high compression head
images
 
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faztbullet said:
On a crossflow you reduce the chamber size to raise compression,shaving head will not do it. Shaving head on looper WILL raise compression..about 10PSI per .020

low compression head
images



high compression head
images
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And when you reduce the area above the piston then the compression goes up which is just what I did and the compression numbers climbed about 10% and that means the compression is higher regardless of which engine type you have.
 
And when you reduce the area above the piston then the compression goes up which is just what I did and the compression numbers climbed about 10%​
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Usually its around 5% increase and as you reduce the area you increase the chance of piston hitting the plug, especially if its as gapped plug and the ridge around plug. Seen it too many times as others tried to hop up engines.
 
faztbullet said:
Usually its around 5% increase and as you reduce the area you increase the chance of piston hitting the plug, especially if its as gapped plug and the ridge around plug. Seen it too many times as others tried to hop up engines.
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I have never had a piston hit a plug but I was racing Mercs with their surface gapped plugs but I am smart enough to know to check for piston clearance on plugs.
 
racerone said:
I bored one of these blocks out by 0.060" to 3 -7/16"-----Fitted and installed standard pistons from a 1970 model 115 HP ( a lot of FITTING and work ).------Motor ran over 6000 RPM with the highest pitch 2 blade bronze prop I could find.------Still have a couple of those 2 blade bronze props for that 100 HP gearcase.
Click to expand...

You say you have a couple of props laying around? What pitch are they? If you are interested I would buy they or at least some of them.
 
Racer.....you ruffled "Badbear's" hair with that comment on post #10. BB is exploring waters "outside" basic OMC training. I really don't blame him for asking our opinion on plugs. You, Racer, have modified this motor yourself and have experience to advise. BB needs some props now. You are perhaps our most experienced man now in OMC.
 
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