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Allan G, kevin
Total Likes: 5
Original Post (Thread Starter)
by Big_Jim59
Big_Jim59
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.
Liked Replies
by Irish Swede
Irish Swede
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.
2 members like this
by Twin Pot Phil
Twin Pot Phil
('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!!)
1 member likes this
by RF Whatley
RF Whatley
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.


Originally Posted by Allan G
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)...

► 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 ?

[Linked Image from us.mahle.com]


► 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.
1 member likes this
by Mark Z
Mark Z
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!
1 member likes this
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