Britbike forum Forums British motorcycles in general British motorcycles in general Which Comes First, Piston or Bore?

I was looking over my cylinders after running a spring hone through them. Quite frankly, they look pretty good considering they had a piston rusted up in one side that had to be driven out. The pistons I pulled out were .010 over and I know it is going to have to be bored. The question is: do I buy a set of pistons at .020 over in the hope that it will clean up or do I send it out to be bored before I have the pistons in hand?

By the way, when I used to do this I would always fit a set of pistons just to get them perfect.

Buy the pistons and have it bored to suit

But how can I buy pistons when I do not know how much will be needed to bore?

The place doing the review will tell you if you don’t already know.

Unless there’s some damage to the bore I find that the old piston and some feeler gauges give me a reasonable idea. The side of the piston gets little wear, you can measure with feeler gauges against the side of the piston (left/right where there is no thrust wear) then fore/aft where there is thrust wear. But 0.020” Bigger will usually Be sufficient unless the bore has scoring.

This is where it gets tricky. As per my measurements, the .010 pistons I took out were perfect and the bore mics out perfect to a .010 over rebore. I do not think the bike ran long enough after the rebuild to make much of a difference. The bore on the right side is rusty but it is not terminal rust and I think it will clean up. If I hadn't beaten the heck out of the right side piston to get it out I would be tempted to just clean up the bore, use the original pistons with fresh rings just to make it run again.

Most shops prefer to do as Allen G. says.

The shop prefers to have the piston in hand to measure it, and bore the cylinder to the proper specified clearance to fit the piston.

Have the shop measure the cylinder first to determine what the overbore might "need" to be, then order the proper sized piston.

At least that was the procedure 54 years ago when I worked in an automotive engine machine shop.

^^^yes

make it easy. take the jugs--and the cylinder specifications-- to the shop and have them look them over.

then buy the approriate pistons and make sure the shop has whatever spec sheet comes with the pistons about honing

the piston people should have recommendations about how coarse to hone the bores

you may find it difficult these days to get 0.010 oversize pistons. not much of a market left for a manufacturer to carry that wide a selection

Why not hone it to clean up the rust, then check for pitting and if non present then fit your old +010" piston's with new rings.

If it does not work you are down a honing cost and a set of rings, if it works then no rebore and no risk it does not clean up at .020".

+1 for Kommando
Clean the bores first then measure them carefully and take the decision as to death to go up to the next side after you know what you are looking at.
Some shop will charge a setting up fee if you only get a hone done.

Bores can look decent but can easily be .010" out of square to the base flange or out of round or both. Often .010 will not get a cylinder into spec. Why not order 2 sets of pistons, .020 and .040 see which will work. Return the set that you don't need.

1. Get your machinist to measure, or measure yourself if you have the tools and know how to use them, or both.
2. Buy the appropriately-sized pistons.
3. Do the bore job.

Pistons are not always perfect or identical. Boring and honing should be done with the pistons in hand.

('Stands up ready to have bricks thrown at himself')
When I last spoke to a piston manufacturer many yeras ago about this subject, great surprise was the result, how so??
The manufacturer retorted that "As we are the only people that know the actual percentages of the components for the piston alloy and its manufacturing process, we are the only people that know what size to machine the piston to for the correct required running clearances, providing the boring machinist does the job to the actual bore size".
What this means is that 'standard bore' is exactly that - not + or - 'the machinist's guesstimation factor', in the same way as +20 on standard bore is also exactly that.
That's why competition pistons are made with the required clearances and road pistons for the same engine are different and forged pistons are a different size to cast pistons for the same engine.
problem occurs when you are using road pistons for competition use....or different fuel.
My goto Engineer (of almost 40 years) has followed this mantra for years and does not suffer consequent problems when correct 'running in' requirements are observed.
(Waits for bricks to arrive!!)

That is good advice.
When I had a brand new GM 500cc speedway engine built, I asked the builder about running in. "Warm it up then thrash it" was his reply

Here's the truth as I know it. And I worked my way through college boring motorcycle cylinders...

► It starts with measurements. To do it properly, you need to "map" the bore by taking 9 separate readings: top, middle, bottom straight on. Then the same 3 readings 15° to the Left, and again 15° to the Right. Cylinders tend to wear tapered with the larger dia at the top, but they also wear oval because of the front-to-rear slap of the pistons. The mapping is advisable due to this weird wear pattern.



► This is VERY misleading. No wear accumulates on the sides of the piston because the piston is not round, it's oval. The ovality of the piston is not known and so taking a measurement there is useless. And what will you do with pistons that have no sides ?




► You can most probably only bore the cylinder from 10-over to 20-over if you use a Sunnen hone. The problem with boring bars (cyl boring machines) is that some machines bore the cylinder from the bottom, and others from the top. These 2 surfaces on the cylinder are not parallel, thus a bore job from the opposite (wrong) end will hardly ever align the hole with the previous. The answer then is go to the next piston size and try again.

I highly agree that all cylinders are bored to the pistons. Piston sizes vary from maker to maker. So you either need to use a shop that has the pistons in stock, or supply +20 and +30 piston sets to your machinist. This because there is typically no charge to get the cylinder taken two sizes bigger... as long as the cylinder remains in the boring bar. If it has to come out because you need 4 weeks to get the second set, then all the setup has to be done a second time. You may incur a second charge for the next larger hole. Thus, when working with a machinist, supplying both piston sizes up front is the way to go.

Hope this helps.

Some Interesting Info RF’, I didn’t know pistons were oval and not round. Though by how far out of round are we talking? I don’t have a 3” or 4” micrometer to accurately measure the skirt but as I’m able to turn the piston in a bore then this must be a Thou or less.

Many times I have checked for an indication of bore wear by measuring the gap at the side of the piston then turn the piston 90° and check again with the side of the piston against the front/back or the bore. That piston surface should be fairly unworn compared to the front and the difference is values gives a reasonable indication of wear or at least that you are looking for a rebore instead of just a hone and new rings. I didn’t give any indication that this was the master way of doing a Bore
Measurement, I have telescopic and bore gauges but would
Still rely on exact measurements to be taken by the guy doing the work.

I agree Allan, I like to measure the bores myself before taking the work to the machinist, fully expecting him to take his own measurements. It gives me an idea of what to expect, and gives me confidence in my measuring abilities when the machinist gets the same values.

Before I would buy two sets of pistons though, I would want assurance from the supplier that I could return the unused set - pistons aren't cheap these days!

Thanks for the great advice all the way around. Like RF Whatley, I too worked my way through the latter half of my college years boring cylinders but I forgot everything I once knew after decades of mind numbing corporate jobs I have a plan of attack now. I will run the hone through the offending cylinder, get the rough worked out and then take detailed measurements up and down the bore to see where I stand.

I will not be using the original pistons 1) because of the savage beating I gave it to get it free and 2) because the exposure had left the piston crown with what looks like corrosion damage. It looks like the aluminum on the piston has been dissolved in spots or like it rusted away, if that were possible..

Probably been running a bit lean, wrong plugs fitted, timing set wrong or an air leak at the carb. Some fuel additives will give a brown coating to piston and combustion chamber which could be mistaken for rust.

I rather think that the piston maker had one too many funny tablets in his college years.
While the actual amount of thermal expansion will vary with the chemical composition of the piston, the difference is not all that much and insignificant when you look at the larger clearances used for air cooled engines .
The bulk of cast aluminium pistons will be cast from an alloy of roughly 12.5 % Si.
These are sold under all sorts of names like Lo-Spand , No Spand , Spandex etc etc
The trick behind them is the silicon phase contracts as it heats up while the Al phase expands and at 13.5 % they cancel each other out, when at operating temperatures , however the inverseness is not linear so the piston expands then contracts .
Alloys over 12.5% Si are neaar impossible to cast so very few go over that amount of Si but they can be forged .

Now forging alloys are a totally different chemistry because you can use a lot higher Si content add to that things that make the ally very difficult to cast like Mn can be added

In the Mahle Piston book you will find tables of piston alloy copositions and a bit further on the co efficients of expansion for all of the.
If you want to do the maths you will see the difference is around 0.0001" for a 60 mm piston from the highes to the lowest.

PS lots of interesting things on this download site for the engineering mind.

A competent machinist can clean them up without removing a significant amount of liner material. THEN measure them, THEN recommend the next course of action based on your cylinder's available overbore pistons.