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The standard bore on model 23 Briggs engines is 3.000. I measured an NOS Standard piston, part # 99947, an NOS .010 piston, part # 99948, and an NOS piston, part # 99949. Briggs recommends boring the engine from the standard bore size of 3.000 to 3.010 for a .010 over piston, and to bore to 3.020 for a .020 piston. These three pistons all measure about .011 smaller than the Briggs recommended bore size. I tried three different sets of calipers and came up with the same results. Seems like a difference in size between the bore and piston of .011 is a bit excessive to me.
 

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I went thru all that with dads engine when I rebuilt it, Initially I thought I had gotten ahold of a mismarked piston assy till I wound up with two if them the same exact size. I ended up picking up a .030 over piston to do dads motor .020 over from stock size. I will never have a motor machined again without having the piston on hand.

I read somewhere one time that a good rule of thumb for boring air cooled engines is to start with .002 clearance then ad .001 for every inch of bore to get a good clearance. If you need to find a home for that .030 over piston I am pretty sure there is room enough here on the shelf :)
 

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Discussion Starter · #3 ·
Thanks for the info, Pete. I did a bunch of research, but there really isn't a whole lot of info out there. Most people just say, follow what Briggs recommends. I pick up these pistons when I can Pete, since originals are NLA. The std. piston will replace a galled up one in a low hours ZZ engine, the .010 piston is for the Page, and the .020 piston is for a 23dfb I'm rebuilding now. The .030 piston is for the G Master Jr. I'll keep my eyes peeled for you, though!
 

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One thing to remember is that engines were designed alot looser back then. Its been 40+ years but I think that a piston for a 283 Chevy was 0,011" smaller than the bore. Things had to be looser because the oils were not as good. I never checked but I was told that pistons were made alittle bit oval shaped. I used to do my measurements to 0.001" but now that I'm playing with Kohlers I've got to measure to 0.0001". It seems too tight to me.

In 1970 I had a 1964 Chevelle 300 Deluxe, 2 dr sedan with a 283 as my drag racing car. I rebuilt the engine but put it together at its looser ends of sizes. My brother had his 1957 2 dr sedan with a 327 that he had built real tight. Mine started easy and ran fine. His would start hard when cold or very hot. If the battery got weak it wouldn't start. Several times I had to tow start him. I drove the car once and definitely don't like driving the 55 to 58 Chevy cars. I preferred my smaller Chevelle and several Novas. I stayed in the habit of putting my engines together a little loose and and have not regretted it for 45 years.

Good Luck, Rick
 

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Discussion Starter · #5 ·
Thank for the info Rick, and that's the info I was looking for. I came across an old post on a car forum, where a person mentioned that an old cast iron Briggs with a 3" bore ran a .010 piston to wall clearance. This confirms exactly what you are saying, Rick. He also had many years experience in the engine building field, and the topic of the post was piston to wall clearance. Oh, and I do miss my old Chevelle!
 

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Discussion Starter · #8 ·
I always brought the block and new pistons to the machine shop when getting the engine bored, also. This was back when I was rebuilding small block and big block Chevy engines. It's been years since I've messed around with those and miss it. Nothing like firing up a built big block Chevy with open headers for the first time...the sound, the smell...AHHHHH! I need to do something with these.
 

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I use to have a set of 12.5:1 pistons for a small block!
 
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Discussion Starter · #10 ·
12.5:1, dang! These are TRW LS-6 454 type pistons. I pulled them from an engine that turned out to have super low miles on it from the looks of things. The engine was in a pick up, and I picked up the engine for cheap, not knowing the condition of the internals. I thought it was just a stock engine and was surprised to see the domes on the pistons when I removed one of the heads! The skirts are in great shape.
 

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DR. Bolens
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I agree with the others on here that tolerances were alot looser back then then they are today, we've come a long way

I went thru all that with dads engine when I rebuilt it, Initially I thought I had gotten ahold of a mismarked piston assy till I wound up with two if them the same exact size. I ended up picking up a .030 over piston to do dads motor .020 over from stock size. I will never have a motor machined again without having the piston on hand.

I read somewhere one time that a good rule of thumb for boring air cooled engines is to start with .002 clearance then ad .001 for every inch of bore to get a good clearance. If you need to find a home for that .030 over piston I am pretty sure there is room enough here on the shelf :)
What number do you need? I may have a couple here on the shelf!
 

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If Pete is looking for a .030 over piston for a model 23 Briggs, the part number for the piston assembly would be 99950. One model 23d engine, type number 0129, such as the engine in a '64 Allis Chalmers B-10 or '64 Simplicity Landlord, used a different piston, pin, and connecting rod. An .030 over piston for them would be part number 294679.
 
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