Are boring bar witness marks in cylinder cosmetic?

**baja200merk** · 06-26-2015, 05:53 AM

Originally Posted by Dave Strong

Funny thing is all those dimensions that are given are at 68 deg F. Change the temp and it all goes sideways. Thus the assembly room, no dirt all parts at the same temp. Not all about no dirt..

Dave

I build about 30 a year. The shop is 40 in the winter and 105+ this time of the year. Never had an issue but you can see the difference

**powerabout** · 06-26-2015, 06:42 AM

just use a plastic mic lol....

**SunDaze** · 06-26-2015, 06:47 AM

The way it turned out with the shop that did my work in Orlando is that they told me they would machine the bores to what the piston manufacturer specified. They said they did not need the pistons to do the block machine work. They were not going to match specific pistons to specific bores. Again, my application is a stock motor with stock clearances. Nothing fancy. 5600 RPM maximum WOT according to Mercury manual. Auto cut off via software a little above that. I was a little taken back by what they said but they were up front about it and I figured that if I bought a brand new $1700 factory block and 6 factory pistons it is the same situation in that there is no custom fitting done on a stock block between piston and cylinders when purchased as separate components. The manual just states various tolerances (say piston dia, cylinder dia or ring end gap), shows how and where to measure those tolerances and that things must fall inside the tolerances.

It almost seems to me that instead of playing around trying to find a good machine shop, that a person in my situation who maybe wants to refurbish a pair of twins every 6 years or so, it is simpler and cheaper in the end to buy a new factory block, and factory pistons. Using the rebuild approach the machine work was about $450. Shipping down and back was $300 (round trip) and I ended up with a few extra ruined weekends not having the boat to use, wasted several nights in the garage doing the cleaning and assembling and $2300 total sunk cost. If this went right I would have saved about $1k max.

By the way, does anyone know who actually makes the stock factory pistons for Mercury, or do they truly make them themselves? Are they of low robustness/quality compared to Wiseco etc or are they really quite well matched to the expected (non racing) use?

**[email protected]** · 06-26-2015, 04:04 PM

An engine is a engine, and if you don't know what piston to cylinder wall clearance is and how to measure it or you do not have the ability to research it then you are going to have problems with the quality of your assembled part. My point was that as an assembler you are responsible for those clearances being correct. And that is a discussion that any good machinist knows to have with his customer prior to cutting a hole.
you ask what would the clearance be at 1100°. The question should be what is the clearance at 1100° when you have metal on metal dragging across one another? And what does the temperature shoot up to then once there is friction involved?

**flabum1017** · 06-26-2015, 04:24 PM

Originally Posted by [email protected]

An engine is a engine, and if you don't know what piston to cylinder wall clearance is and how to measure it or you do not have the ability to research it then you are going to have problems with the quality of your assembled part. My point was that as an assembler you are responsible for those clearances being correct. And that is a discussion that any good machinist knows to have with his customer prior to cutting a hole.
you ask what would the clearance be at 1100°. The question should be what is the clearance at 1100° when you have metal on metal dragging across one another? And what does the temperature shoot up to then once there is friction involved?

But also, the foundation of the rebuild needs to be solid first.... that machine work was poor..... You should not see those boring lines/ridges after running a couple hours..... not even 500 hours....... even with an experienced mechanic who would have checked all the clearances and chamfers etc and did an impeccable job assembling it, the dingle berry hone job would have still gone away quick leaving a surface that could not retain the oil sufficiently causing the piston to skid..............................

**powerabout** · 06-26-2015, 06:36 PM

Originally Posted by [email protected]

An engine is a engine, and if you don't know what piston to cylinder wall clearance is and how to measure it or you do not have the ability to research it then you are going to have problems with the quality of your assembled part. My point was that as an assembler you are responsible for those clearances being correct. And that is a discussion that any good machinist knows to have with his customer prior to cutting a hole.
you ask what would the clearance be at 1100°. The question should be what is the clearance at 1100° when you have metal on metal dragging across one another? And what does the temperature shoot up to then once there is friction involved?

your talking about car engines

**TEXAS20225** · 06-26-2015, 07:05 PM

personally i have not one single piston that was too big or to little for the hole and i specified what i wanted it bored before i even bought the pistons , we are talking Qc tolerance at most piston wfg company's in a few 10ths , most machine shops are not boring in 10ths

**Chaz** · 06-26-2015, 08:02 PM

Originally Posted by baja200merk

It's all in that text book looking thing oh wait they have them on computers now too kinda like the fancy typewriters that build the Pistons while the machineist is on a smoke break.

Ya know, I wuz doin real good keepin up with the latest in cylinder wall finish .. and the LTOMCtestcenterbouyz had ta dis the grease smudged pages of out dated paper as well as burst my bubble in thinking that pistons were made one at a time in a lathe , and the accuracy from one piston to the next depended on if the machinist needed a smoke break or not ...

It's easy to see ( and criticize ) that the guy went right to a number ( the OP never mentioned current bore size / clearance ) lightly honed or dingle balled the bore and sent it on its way.
I cant think of anyone off hand that finish's a bore correctly , including myself ..

Unless you torque the block and front section together using crank main bearing outer shells in their bore's . Torque a head on one side and a torque plate on the side your honing . Bolt up an adapter plate to the bottom. Then heat the block and honing oil to the engine's normal running temperature , plateau hone the hole . Run the motor on a dyno at 3000 or so RPM for 15 - 20 minutes under a slight load . Shut it off , take it apart , dingle ball the bores , massage the high spots off the piston's by hand , then hang it back together ...

There are some out there that would say doing anything less is being a slacker ...

And you can bet your azz that any mass produced motor is built with a wide range of of "acceptable" parts . The stuff that came out of Mercury Racing was no doubt looked at a bit closer .
The Sunnen or Rottler book ... err ... web site speaks of core roughness , valley depth , peak height , cross hatch angle and load pressure.
I like to think that my collection of fresh std and bottom pressure alum oxide stones can take a steel bore that's .003-.004 small and bring it to less than .001 with a 280 grit stone and sweep the last little %$#@ hair out with a 600 grit polishing stone ...

Chaz = gotz my eye on a Kwik-Way FN and 056 table ...

**TEXAS20225** · 06-26-2015, 08:15 PM

charlie behave yer self

**powerabout** · 06-26-2015, 08:24 PM

I know a guy that did as you said Chaz to find out and yes ended up with a leak down number so low he couldn't believe it after a season of racing
( didnt pull apart and re hone though)

**Chaz** · 06-26-2015, 09:17 PM

Originally Posted by TEXAS20225

charlie behave yer self

Yes sir Tex , just the facts .....

Power , ya know it's a shame that rings aren't offered in a +.005 in every over size like they are in other applications . If ya wanna loosen up the bore a little ( or it's worn some ) the end gap goes with it . Would be nice to be able to get it back with a file fit.
There are those that also say , that the second ring needs more gap than the top.
For decades common sense said that the top ring should have more gap because it was closer to the heat and had a better chance of butting .
Then someone thinking outside the box discovered that if the second ring held better pressure than the top , that it would use that pressure to lift the top ring off it's seat ... I like the idea of an accumulator groove , but how much volume can it actually hold ... LOL

**powerabout** · 06-26-2015, 10:18 PM

Ok
Lets all go 1 ring
Seals better as has less pressure on the back side due to no 2nd ring..lol

**Dave Strong** · 06-26-2015, 10:36 PM

Originally Posted by Chaz

Yes sir Tex , just the facts .....

Power , ya know it's a shame that rings aren't offered in a +.005 in every over size like they are in other applications . If ya wanna loosen up the bore a little ( or it's worn some ) the end gap goes with it . Would be nice to be able to get it back with a file fit.
There are those that also say , that the second ring needs more gap than the top.
For decades common sense said that the top ring should have more gap because it was closer to the heat and had a better chance of butting .
Then someone thinking outside the box discovered that if the second ring held better pressure than the top , that it would use that pressure to lift the top ring off it's seat ... I like the idea of an accumulator groove , but how much volume can it actually hold ... LOL

I was always under the impression the accumulator groove was more a benifit for lubrication than a compression thing. Granted if is full of fuel oil mix will help seal.

Dave