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Old 08-22-2020, 10:10 PM   #21
estout81
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Default Re: AutoLite 216's in Offy heads

Chamfer the plug hole.
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Old 08-22-2020, 10:42 PM   #22
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Default Re: AutoLite 216's in Offy heads

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Chamfer the plug hole.
Are you kidding? Gary "GOSFAST" who is a professional engine builder has already given you the answer. Hell, I'm just an amatuer, and even I knew better. Autolite 216's (7/16" reach) in a a spark plug hole almost 3/4" deep? "Chamfering" the spark plug hole? These are aluminum heads; The manufacturers designed them with a longer reach because aluminum is not as strong as cast iron. What are you guys thinking?
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Old 08-23-2020, 09:52 AM   #23
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Default Re: AutoLite 216's in Offy heads

I will be putting in a set of AutoLite 425's this afternoon. 1/2 inch reach. What do you think the minimum clearance should be between the exhaust valve and the spark plug grounding lug.
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Old 08-23-2020, 10:10 AM   #24
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Default Re: AutoLite 216's in Offy heads

Tubman,
Have you notified Briggs & Stratton of their design error?
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Old 08-23-2020, 10:55 AM   #25
Tim Ayers
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Default Re: AutoLite 216's in Offy heads

Am I missing something here? Why not get the proper reach plugs instead using what's on hand?

Rock Auto sells most of their plugs for under $2 a piece (NGK, Bosch, Autolite, etc.).
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Old 08-23-2020, 11:15 AM   #26
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Default Re: AutoLite 216's in Offy heads

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Am I missing something here? Why not get the proper reach plugs instead using what's on hand?

Rock Auto sells most of their plugs for under $2 a piece (NGK, Bosch, Autolite, etc.).
That is what I am trying to figure out. I think the 375 Offy heads have less clearance than the 400's and 425's
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Old 08-23-2020, 11:15 AM   #27
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Default Re: AutoLite 216's in Offy heads

There are many ways to skin this cat - heck, I'd just pick one. You can run longer plugs with 'shims' (I'd prefer them to be made of copper), you can chamfer/change the bottom portion of the head chamber-side (chamfer/blend), you can machine steel or aluminum inserts and machine the original threaded holes to insert them - these can do TWO things:

1) Gives you a specific length and quality of thread as far as wearing out over time. Many of us have experienced that sickening feeling when you tighten a plug and the old aluminum threads give way . . . boy does that suck.

It usually happens when you are in a hurry to get someplace, are far away from home and your tools and/or have no time to now pull the heads and "fix" them . . . . that 'Murphy' is a rotten Devil!

2) You can make the threaded inserts to support 3/4" reach plugs - which are by far the most common and give you a ton of different heat ranges, suppliers, etc..

Keep in mind that folks like Edelbrock retooled most of their patterns (if not all) and machining operations to produce heads that take 3/4" reach plugs . . . a very smart thing to do! This is one reason I use Edelbrock heads on many modern builds.
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Old 08-23-2020, 11:26 AM   #28
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Default Re: AutoLite 216's in Offy heads

I had the same problem with a slight miss with the same bore/stroke engine. I found the low speed jet to be a little lean. Standard size I believe was .029ths. I drilled it out to .031ths. Runs Fine. Hope this helps, wasn't spark plug related.
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Old 08-23-2020, 12:24 PM   #29
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Default Re: AutoLite 216's in Offy heads

Autolite 216 plugs have a 7/16 in. reach, A8 heat range and non res. 425's have a 1/2 in. reach, B9 heat range and are resistor. 4092's are 1/2 in. reach, A9 heat range and non res.
My Offy 400 heads had about 5/8 in. of threads. I used the Autolite 4092 plugs and chamfered the combustion side to remove the excess head threads. For clearance of plug to valve, anything over 1/16 in. should be plenty. I did this several years ago and many miles and the engine runs fine.
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Old 08-23-2020, 04:08 PM   #30
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Default Re: AutoLite 216's in Offy heads

For those curious about the shins McMaster Carr has the 14mm copper shins .052” thick relatively cheap.
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Old 08-23-2020, 04:16 PM   #31
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Default Re: AutoLite 216's in Offy heads

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Tubman,
Have you notified Briggs & Stratton of their design error?
Since we are talking high performance heads here, I think you just made my point.
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Old 08-23-2020, 04:54 PM   #32
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Default Re: AutoLite 216's in Offy heads

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Originally Posted by estout81 View Post
Autolite 216 plugs have a 7/16 in. reach, A8 heat range and non res. 425's have a 1/2 in. reach, B9 heat range and are resistor. 4092's are 1/2 in. reach, A9 heat range and non res.
My Offy 400 heads had about 5/8 in. of threads. I used the Autolite 4092 plugs and chamfered the combustion side to remove the excess head threads. For clearance of plug to valve, anything over 1/16 in. should be plenty. I did this several years ago and many miles and the engine runs fine.
Did a search on auto lite sited and could find nothing that tells me what A9 and B9 are
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Old 08-23-2020, 05:44 PM   #33
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Default Re: AutoLite 216's in Offy heads

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Since we are talking high performance heads here, I think you just made my point.
Wait, now I'm even more confused. Tubman, are you saying chaffering the head decreases performance?
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Old 08-23-2020, 06:15 PM   #34
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Default Re: AutoLite 216's in Offy heads

I'm saying that the heads were designed specifically to have a longer reach because aluminum is not as strong as cast iron and requires more thread length to provide the proper amount of support for the spark plug threads. Chamfering the plug holes reduces the designed thread length and weakens the holding power of the threads in the head. I also believe that thoughtlessly modifying well-engineered designs is not a good idea. I am also saying that anyone that looks to Briggs and Stratton for tips and tricks to upgrade the Ricardo combustion chamber design to improve performance is barking up the wrong tree.

Just about every set of used heads I have ever seen had at least one stripped spark plug hole if not several. Those threads are there for a reason, and I don't think eliminating any is a good idea. Even if enough threads are removed to be visually pleasing , the plug is still shrouded to some degree.

Bottom line is that I don't think it is a good idea, especially when people with the experience and knowledge required to build performance engines have already provided a better alternative.
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Old 08-23-2020, 06:27 PM   #35
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Default Re: AutoLite 216's in Offy heads

OK, got it. Thank you for clarifying. Not sure why I'm having such a hard time following the progression of this thread.

I am pulling out something you said though. the Ricardo combustion chamber

I thought only Harley and some very rare Model A heads were able to utilize Ricardo's chamber design. Something about head bolts being in the way with the a V8. I could be totally off base on this.
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Old 08-23-2020, 06:41 PM   #36
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Default Re: AutoLite 216's in Offy heads

Here's what the internet said :

A "Ricardo" type cylinder head had the spark plug and the main combustion chamber placed over the valves of the ell type head engine. The volume of this chamber was considerably larger than the small clearance space provided above the piston and the transition throat that connected this space to the main combustion chamber. As the piston approached the top of its compression stroke, the fresh air/fuel charge was squeezed, or squished, through the ever narrowing transition throat. The rush of the increasingly dense air/fuel charge through this transition throat created considerable turbulence in the combustion chamber. When the spark plug ignited the turbulent mixture, the flame front spread fast resulting in quicker, smoother, more complete burn of the fuel, reducing spark plug fouling and lowering the exhaust temperature. This resulted in more, smoother, power, better fuel economy and was less susceptible to detonation than the slower burn of a relatively stagnant mixture of a main combustion chamber above the piston in previous cylinder head designs.

This describes a flathead Ford combustion chamber to me, and explains why it is so important to have minimum clearance over the piston top and the head, which most of us try to achieve.
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Old 08-23-2020, 07:17 PM   #37
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Default Re: AutoLite 216's in Offy heads

Thank you, Tubman.

What am I thinking of then? I know Harley used this specific chamber design in their racing flatheads. I thought it was designed by Sir Richard Ricardo. I think Frank Otto's book has a chapter on it. Let me look it up.
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Old 08-23-2020, 07:23 PM   #38
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Default Re: AutoLite 216's in Offy heads

It was Sir Harry Ricardo. I believe the Harley racing flathead was the final and best application of his theory and worth emulating. The same can't be said for Briggs and Stratton.
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Old 08-23-2020, 07:44 PM   #39
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Default Re: AutoLite 216's in Offy heads

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It was Sir Harry Ricardo. I believe the Harley racing flathead was the final and best application of his theory and worth emulating. The same can't be said for Briggs and Stratton.
Haha. Sounds good!
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Old 08-23-2020, 08:20 PM   #40
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Default Re: AutoLite 216's in Offy heads

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It was Sir Harry Ricardo. I believe the Harley racing flathead was the final and best application of his theory and worth emulating. The same can't be said for Briggs and Stratton.
I gave my book "The High-speed Internal-combustion Engine" by Sir Harry R. Ricardo to a grandson. He loves it
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