Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[Webby the Bear]

So, chatting to a buddy today and convo got to upgraded Pistons and general block work.

This, as usual  ,posed a few questions if anyone would care to help a bear out 

1.) honing - my understanding of this is where you are essentially 'roughing up' the bore surface to create tiny little places where oil wil stick to. Is this right, what situation would this be necessary and is the surface not already like that or is it smooth?

2.) boring - yes, my posts are I know  is this where you increase the perimeter of the bore to accommodate new, bigger Pistons? Of course I can see why you'd do that but what other considerations are there other than "throwing in new rods"? I presume you'd have to machine a bigger cylinder head area? I assume you'd also have to upgrade your bearing shells, caps and bolts also? Any other considerations?

3.) bore measurements - I did this a couple of times in college with a DTI ( if that's right). So this is obviously straight forward in that you're measuring and comparing diameter against spec. However is it possible that the bores have increased in size? If so why would that be?

4.) adding a sleeve - is this when the bore has increased so much (leaving combustion leaks) and a sleeve put in to effectively bring back the original piston diameter / perimeter size

Tia 

[hotel21]

Here's my understanding, really based on very large, three storey, 2 and 4 stroke marine Diesel engines of a number of years ago.

1). Honing can be used for both roughing and smoothing a cylinder surface. If the bore has been machined (to a slight oversize for example) then honing polishes off the worst of the tooling imperfections. If the bore is well used then the action of the movement and pressure of piston rings polishes and glazes the bore surface such that combustion gasses easily pass by the rings and vice versa, either engine or cylinder oil gets burnt in the process and can gum up rings. Honing in this case busts the glaze on the surface allowing oil to lubricate piston movement as it tends to 'stick' to the honed surface and not slide off the polished bore. 

2). Boring. Is used to either increase cylinder capacity or to return the bore to parallel. Providing the piston, in its normal designed stroke, does not go proud of the block then no adjustment of the head is required. Sometime a thinner head gasket is required to bring back correct compression ratio.

3). Bore measurement. Cylinders can and do become worn out of parallel with the greatest wear at the top of stroke extremity of the top piston ring. Measurement is done so that it can be consider in or out of design tolerance which may dictate replacement rings (they also wear in use remember) or a rebore. If replacement rings then the top ring is generally stepped so that it does not contact the cylinder wall immediately above the compression ring. If it contacts this original bore then it can shatter rings as they are (or were years ago) very brittle. Also it's why you size new rings by placing them in the cylinder and measuring the ring end gap and adjusting (filing) as required.

4). Sleeving. Not something I am familiar with in engines but seen it done in large piston pumps. Any overboring might result in breaking through the cylinder wall so over cutting and sleeving brings the assembly back into spec.

Sorry if a tad long winded but hope you get the gist.

 

[Webby the Bear]

Mate, that's absolutely bang on, thanks for the detail 

Just a couple of Q's on that...

With the honing. . . . Is this something that can be done with emery or are we talking precision, specialist tooling? Is this something that should be done as part of say a head gasket job?

Out of parelell. . . Is that essentially out of round / oval?

Thank you 

[hotel21]

Start with out of parallel.

Pistons are generally very slightly a smaller diameter than the bore. Talking fractions here. The piston rings exert outward force within their grooves onto the cylinder wall. The ring ends when it's assembled, are offset from each other to provide a distorted path for combustion gasses so hopefully reducing blowby.

The outward pressure of the rings onto the bore wall will then wear the outer surface of the rings as the rub on the cylinder walls, as will the cylinder walls obviously. The wear will be non existent above the top compression ring as the compression ring never reaches it and, if the rings do their job, keep the piston central in the bore and thus also not touch the sides. Thus, no wear there.

Below where the top ring reaches will be most wear as combustion gasses try to escape. It wears the side. You have seen the result of a failed head gasket and possible groove cut in head or top of block?  As the piston moves down, pressure in the most general of terms, reduces thus wear is reduced.

Measure the bore in increments from top to bottom and you will see that the extreme top and extreme bottom are basically at factory standard. It increases from bottom to top until it reaches the step created by the top ring.

[BazaJT]

As I understand it-but I'm probably wrong!- is that honingremoves the "glazing" from the cylinder walls which occurs over time/use.Measuring the bores as you say tells you whether the bores are within specs.and should be done in more than one place to check for things like ovality,as I believe there is a thrust side and a non thrust side?Boring at least used to be done in increments of 10thou,so you'd get pistons marked +10,+20 etc.but each engine can only be rebored a certain number of times to preserve oil/waterways etc. before the block is basically scrap.No need I think to change rods bearings etc.unless you're after building a racing engine or something.Sleeving often done on alloy engines to I suppose stop the softer alloy being worn away.As said I could be and probably am miles out on this.

[hotel21]

Baza - pretty much, yup. Didn't want to go into wear side and ovality too early!

[hotel21]

Honing and glaze bursting are similar but different. Glaze bursting can be done with emery and oil but you have to be carefull of contamination of carbide dust from emery into guts of engine if pistons are still in place.

Honing is generally done with a specialist honing tool with Pistons out.

[hotel21]

https://coxengineering.sharepoint.com/Pages/Boreglazing.aspx

Not the best but a quick google.

[Webby the Bear]

Guys, I've read and re read all what you've out and that all makes total sense.

I presume one side will wear more depending on which side the inlet valves are?

[hotel21]

Side wear is due to crankshaft rotation and crankshaft throw. Combustion pressure is trying to push the piston straight down the bore. The throw of the crankshaft is tiring to turn up and down movement into rotary. It will exert  a degree of sideways-pressure on the  piston on the down stroke and can wear on the cylinder wall.

Wear here can, over time, result in what's known as 'piston slap' where wear is to an extreme and piston is floating about side to side, normally with gudgeon pin wear as well.

[Webby the Bear]

That again makes perfect sense.

So withheld most engines as they turn clockwise we're looking at the right hand side of the cylinder wall (as you look at the engine in situ) being the wear side?

[hotel21]

Generally speaking, yes, but not all engines are the same.

The ones I cut my teeth on were two stroke diesels that put as much power running backwards as forwards!   

[hotel21]

http://contentinfo.autozone.com/znetcs/content/how-to/en_US/0996b43f/80/a0/11/e8/f15_25.gif.

[Webby the Bear]

  thanks mate I'll give them links a read now 

[hotel21]

This is how I used to measure bore wear........


http://farm2.static.flickr.com/1012/1402400925_0841df035c.jpg?v=0.