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Cylinder to piston clerance!

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Mr, B~MI

04-26-2000 03:31:15

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Briggs 12 Hp, 281707 model engine, Does anyone have specifications for clearance? I had one rebored and it slaps and uses as much oil as it did before they bored it and it was worn bad enough that they bored it .030 over, After running the engine and using 6 oz. of oil per hour since rebuild I tore it down again to see what was going on. The cylinder to wall clearance is .007 comparing piston measurements to cylinder measurements. These measurements were done with a good Vernier and may not be 100% accurate but certainly do come close. The shop that did the work says he never compared the piston to the bore, just bored it .030 from specs of original bore. I am doubting his word because I have a paper that came back with the engine that has his writing on it and gives his measurements, .0066 difference between the two measurements. But before I go to see him I need some moral support and hard data on required clearance. Thanks, Mr. B

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Mark R.

04-29-2000 22:03:07

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Re: Cylinder to piston clerance! in reply to Mr, B~MI, 04-26-2000 03:31:15

Mr. B, I don't know the spec's for your engine, but after reading the responses, a couple thoughts came to mind. These are based on past esperience with automobile engines. 1. Was the finish of the cylinder appropriate to seat the rings? 2. Now that the engine has been bored (too big??), maybe you could knurl the piston skirts to increase diameter (carry oil well too), and help piston slap. I have heard of mechanics rebuilding engines and using chrome rings in cylinders that were real smooth/slick, and the rings would not seat well. The ring material is probably pretty standard for your engine type, but could this be a problem too? Good Luck, and please let us know how you make out. Mark R.

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dubba dub

03-13-2001 07:43:11

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Re: Re: Cylinder to piston clerance! in reply to Mark R., 04-29-2000 22:03:07

i know how to knurl a piston!!

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Mr.B~MI

04-27-2000 10:11:48

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Re: Cylinder to piston clerance! in reply to Mr, B~MI, 04-26-2000 03:31:15

Ok Guys, Here is what I have to this point. The crank shaft is showing almost no wear, I am trusting my memory now but it seems it was just .00025 flat and no taper. It has new connecting rod and it will only go in with the side cut on the cam side, otherwise you could never assemble it and that was the way the original came out. The piston is new and so is the pin both fit the pin very snug. The notch is up on the piston crown just like the old one was, The rings are new one piece rings, Top ring has the opening on the top, the other two rings are staggered to each side but on the upper extreme. The end gap I checked on the compression ring was over .009 and less that .010. The knock per say is piston slap,( Suspected it was anyway) notice mostly hot and idling especially with the slightest throttle, But was detectable cold as well. that does not bother as much as the fact it burns as much oil now as it did prior to the overhaul, the engine has no more that 326. hours on it since new. I am sure this hour meter was installed when a mouse built a nest in the cooling fins and he burned up the original. The valves were worn slightly but not extremely lose in the guides. We had those ground and seats touched up. the vent valve seemed Ok, although I haven't inspected it since it was test run. This gent it matriculas with care of his engine and changes his oil much more regular than most. Keep in mind that this has run only about 3.5 hours since rebuilt in my shop. But I still think the skirt clearance is excessive-- but I haven't herd from Briggs yet. AND-- Now that I have received some specs I see it is within limits. The bore is 3.467 according to my measure, The piston is 3.460 as I measure it. A far as my experience-- I have been doing some of this since I was 16 years old and I just retired just over a year ago. But I don't claim to know it all, that is why I asked for advise. And I didn't plan to do this one, But a friend needed some help so here I go. Thanks for your comments-- any further input will be appreciated. Mr.B

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Rod

04-26-2000 19:35:22

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Re: Cylinder to piston clerance! in reply to Mr, B~MI, 04-26-2000 03:31:15

Guys Guys - no excuse for not having a $10 book (pn 270962-6/99) that
covers the engines. The days of a good seat of the pants lawnmower
mechanic went the same way as a good shade tree auto mechanic. Too
many makes, models and variations. "ALWAYS RESIZE TO EXACTALLY .010"
(.25mm), .020" (.51mm) or .030" (.76mm) OVER STANDARD BORE SIZE AS
SHOWN IN TABLE NO.1, SPECIFICATIONS" [not shouting, that's the way it
is in the book] The above quote is from Sec. 11 pg. 2 of the Briggs manual. Std. bore is 3.4365"/3.4375 + .030" = 3.4665"/3.4675". The machine shop used the correct procedure. Did they achieve the correct finished bore dia. is another question.

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Gary

04-27-2000 16:45:19

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Re: Re: Cylinder to piston clerance! in reply to Rod, 04-26-2000 19:35:22

No doubt that will work 99.9% of the time, we like to custom fit because its not unusual to find a few thousands variance in the piston. I wonder if they may have not bored it true (say a few thousands over at the mid stroke)

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Mr.B~MI

04-28-2000 05:52:26

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Re: Re: Re: Cylinder to piston clerance! in reply to Gary, 04-27-2000 16:45:19

Gary---I checked the bore for taper on the two thrust sides, it was right on the money using a inside micrometer. The bore looks good too but I do wish he would have given me just a smidgen closer tolerance. I was wanting this to run for the rest of this gentleman's life time. Ha Ha HA-- That's a lot to expect but it can be done with Amsoil and a good engine to start with. But I still think if I can find and solve the oil consumption problem it will run as long as the deck holds up. I just re bushed everything on the chassis and installed lube fittings on everything too, New wheel bushings and fitted them with seals and caps to help keep the dirt out and lube in. Thanks for the thought-- Mr.B

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Gary

04-26-2000 18:47:56

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Re: Cylinder to piston clerance! in reply to Mr, B~MI, 04-26-2000 03:31:15

Whats the end gap on the rings? A rule of thumb is .003 for every inch of bore diameter. A local shop here bores without the piston and they work. My father-inlaw has been a Briggs machnic for over 40 years and will not bore without the replacement piston on hand so he can fit it- his work better!

Oil consumption can also come from either a crankcase breather problem or a poorly sealed crankcase. goodluck

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Jim S.

04-26-2000 07:45:14

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Re: Cylinder to piston clerance! in reply to Mr, B~MI, 04-26-2000 03:31:15

I don't know the B&S info you need, but I can say the following without qualification:

1.) Any shop that bores a block and does not compare the piston to the bore should not call itself a machine shop!

2.) Same goes with turning cranks and not having the oversize rod there to compare it with.

3.) Any machine shop that does not have on hand the specifications for clearances on these parts and does the work anyway does not deserve the name.

4.) These are sloppy, sloppy shop practices. You have an excellent case...they screwed up, and ruined your block in the bargain!

See www.briggsandstratton.com, click on Instant Answers or Technical Support in the window in upper right. They should get you the specs. There may be other places on site where they are available, too.

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don

04-26-2000 13:14:06

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Re: Re: Cylinder to piston clerance! in reply to Jim S., 04-26-2000 07:45:14

hey Jim why condemm the machinist ? .006 - .007 isn't all that bad there could be more messed up in that moter than you or I know , is the piston , rod , rings, installed the correct way ? was the crank checked for out of round ? the assembly of the moter is only as good as the guy putting it together if he doesn't have a book to know what the side clearence is how does he know the rest is right ?

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Jim S.

04-27-2000 07:50:55

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Re: Re: Re: Cylinder to piston clerance! in reply to don, 04-26-2000 13:14:06

I made no comments regarding the assembly of the engine because I have no information about that to comment on.

I maintain that the machine shop practices I initially listed are sloppy. Bores and cuts should be made to fit the new part to be used. That means it needs to be on hand when they are done. Machine shops should have on hand the proper specs when performing the work.

I can see where it might be thought that in #4, I jumped the gun a bit. It would be better if I could see the engine myself before making that statement. But if the shop practices he describes are actually what happened, I have serious concerns about the quality and reliability of that shop.

My bottom line: Like any shop, machine shops vary in work quality. Don't choose blindly. Ask around to find out who is getting the majority of the small engine work from the pros.

Your other point about the owner not having a book to reassemble by has validity. On the other hand, the book is not always necessary if a knowledgeable and experienced individual is doing the assembly work. I make no comments about the knowledge level of Mr,B~MI. I don't know him.

Lastly, in #4, the block is indeed ruined if it is max overbored and bored over the maximum allowable limit. That's highly possible if the piston is not on hand to size to, and the block is "blindly" bored out over the existing bore size.

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