When to bore out a cylinder?

[kpike52]

As part of my Hustler rebuild I am letting the high school small engine shop class check out my 16hp B&S. On Cyl A they found a broken ring, piston skirt chipped out and the cylinder wall was scored. We were worried that this could have been going on for a while and the cylinder A would need to be re-bored. I measured both bores @ 90 degrees and got the following diameters.
Cyl A, top 3.440/3.438, mid 3.440/3.438, bottom 3.437/3.434.
Cyl B, top 3.437/3.435, mid 3.438/3.440, bottom 3.437/3.434.

Both cylinders look the same to me. So do both or neither requiring boring?http://www.6x6world.com/forums/image...ies/hmmmm2.gif

[mudNmallards]

This is back from my days as a machinist when I used to build engines so maybe some of it is foggy and I don’t have my machinists handbook handy.
You need to compare those readings to either the OEM bore and piston dimensions to get proper fit or in the absence of that the bottom piston skirt to get the right fit for a given piston.

The “rule of thumb” we used to use when nothing else was available was .004 per inch of piston.

A real quick check I used to do was take a new ring and coat the cylinder with dychem or Prussian blue ( or a magic marker if nothing else was available). Check the end gap of the piston and use the piston to push it down. If the scratch left voids or you lost end gap it was going to be bored and sleeved. A strong light would reveal any oval wear of the cylinder as well. That was not a quantative check but it would tell you real quick if your rings would seal and that’s where we would bore rather than try to hone all that out.

Not knowing any of the proper clearances for that particular engine and the fact you say they are scored and have about .003 between cylinders and between .003-4 out of round overall- I would recommend boring and sleeves.

[kpike52]

Thanks. That is good advice.

[thebuggyman1]

Take your block to an engine builder that will do the boring....they have a special tool for measuring the bore at any given spot inside it.....not just the 3 spots. It is nearly impossible to tell how the wear has occured also. Your cyl could be worn .006" over size, but if that .006 all happened on one side, thats actually .012" before it's straight again. Too big for a .010" over piston....it would have to go to a .020" piston. If you are going to put it back together, you might as well do it right.
I just had the same problem with the 40 horse for my bigfoot project. One cyl was .008" worn, but by the time it was straightened out it was .013-.014"......So, the .020" over pistons are on order.

[fmints]

good info Jim!!

[kpike52]

Sounds like I should plan to have both cylinders checked and then bored by a builder, after that buy the new pistons and rings.

[mudNmallards]

Actually the other way around. Make sure you buy the pistons/rings and let the machine shop do the fitting to the actual parts going into the block.

Also, make sure they check the valve guides while they have it- those usually wear substantially too

[kpike52]

You are psychic! I talked to the shop today. They will estimate the new size and I'll order them.

[Tomo]

HI
First dismiss the score untill the cylinder is honed ,occasionally marks in the bore will come out ,then remeasure .
Second u did not mention which was the thrust side measurement in your supplied data . Although this can be guessed by looking at your measure ments [providing u have written them in the same order ] Thrust side is the measurement taken across the crank line . This measurement will always have more wear than the measurement takken inline with crank.
Second BUY A CYLINDER BORE SCOPE way easier than manual way
With a worn engine .002--.003 borderline

PS check crank clearance to con rod
Best way new crank,rods,pistons/rings
The cranks in vanguards if the main journal is ok u just grind conrod journal undersize

tomo

[mudNmallards]

Tomo is absolutely correct ( but thats a job for the machinist)- some other things to consider

mic the cam lobes and shafts ( along with their recesses)- if they are worn, repair/replace them too. Many running problems attributed to carbs are actually the cam and its wear.

I would also recommend ( if there is enough base metal) to install/fabricate valve guides as well.

Just dont do a half overhaul