14 replies to this topic

[dotmatrix]

I have a dail bore gauge that I was playing with in a standard sized 998 engine block.

as far as I can work out the largest diameter was 64,67mm and it was right up by the top of the bore.

I suppose the block was bored to 64,58mm and then a little larger for tolerance. what is the tolerance? 0,2mm? or is the piston that amount smaller?

it surprised me that the largest diameter wasn't down in the middle of the bore but right at the top, can this be correct?

thanks in advance

[Dusky]

Biggest diameter at the top because it gets the hottest there and the pistons expand the most + most wear there(peak cilinder pressures) as far as i know.

[dotmatrix]

Biggest diameter at the top because it gets the hottest there and the pistons expand the most + most wear there(peak cilinder pressures) as far as i know.

 

right. makes sense

[Spider]

Biggest diameter at the top because it gets the hottest there and the pistons expand the most + most wear there(peak cilinder pressures) as far as i know.

 

It also received the least amount of lubrication, in fact, the area between the Top Ring and the Oil Control ring receives next to no oil at the top of the bore.

[Spider]

I have a dail bore gauge that I was playing with in a standard sized 998 engine block.

as far as I can work out the largest diameter was 64,67mm and it was right up by the top of the bore.

I suppose the block was bored to 64,58mm and then a little larger for tolerance. what is the tolerance? 0,2mm? or is the piston that amount smaller?

it surprised me that the largest diameter wasn't down in the middle of the bore but right at the top, can this be correct?

thanks in advance

 

Sorry for the imperial numbers here. These engines are imperial, while they were (loosely) converted to metric at some point, I'm still stuck in the dark ages with them and can only relate to the in imperial measurements.

 

If it's still on a standard bore size (2.543" - 64.588 mm) then it would have been bored to a nominal diameter and then pistons selected to suit.

 

At 64.67 mm (2.546") that's 0.0762 mm (0.003") wear, at which point, IMO, it should be bored to the next oversize.

 

When it's re-bored, most shops will measure the Pistons to be fitted and bore & hone to them. There is a nominal clearance as recommended by the piston manufacturer which the machine shop should follow, rather than any factory data as not all pistons are made the same way or from the same materials. I usually go a little over these sizes, but that's only come from the particular pistons I use and my own long term experience with them. If the machine shop is really on the ball, they'll measure each piston, number them and hone each bore to suit each piston as (and in particular with AE Pistons) there's usually some variation between them.

[dotmatrix]

Moke Spider, on 30 Nov 2018 - 01:10 AM, said:

dotmatrix, on 29 Nov 2018 - 11:20 PM, said:
I have a dail bore gauge that I was playing with in a standard sized 998 engine block.
as far as I can work out the largest diameter was 64,67mm and it was right up by the top of the bore.
I suppose the block was bored to 64,58mm and then a little larger for tolerance. what is the tolerance? 0,2mm? or is the piston that amount smaller?
it surprised me that the largest diameter wasn't down in the middle of the bore but right at the top, can this be correct?
thanks in advance

Sorry for the imperial numbers here. These engines are imperial, while they were (loosely) converted to metric at some point, I'm still stuck in the dark ages with them and can only relate to the in imperial measurements.

If it's still on a standard bore size (2.543" - 64.588 mm) then it would have been bored to a nominal diameter and then pistons selected to suit.

At 64.67 mm (2.546") that's 0.0762 mm (0.003") wear, at which point, IMO, it should be bored to the next oversize.

When it's re-bored, most shops will measure the Pistons to be fitted and bore & hone to them. There is a nominal clearance as recommended by the piston manufacturer which the machine shop should follow, rather than any factory data as not all pistons are made the same way or from the same materials. I usually go a little over these sizes, but that's only come from the particular pistons I use and my own long term experience with them. If the machine shop is really on the ball, they'll measure each piston, number them and hone each bore to suit each piston as (and in particular with AE Pistons) there's usually some variation between them.

thank you very much for your reply and explanation.
no problem using inches.

what does a "nominal diameter" mean in this context?

I have great confidense in the machine shop I use. they have been machining engines for 55 years (the guy is still there) and are very friendly and keen on explaining things to me. but I will make sure to ask about the things you mention here. would be interesting to hear their answer.

[Spider]

A Nominal measurement is in this instance, the sizes given in the blue print drawings and also the workshop manuals etc.It's not an exact - to the 1/10th of a thousands of an inch measurement that is usually used when measuring piston and bore sizes (in this case), or put another way, it doesn't take in to account tolerance figures.

 

As an example, with your standard bores, the Nominal size is 2.5430", but when measured (after machining in the factory), these are likely to be anywhere from 2.5430 to 2.5438" - 8 ten thousands variation. Small but when the running clearances are of the order of 0.001 to 0.0015", a figure of 0.0008" is more than half these clearances.

 

I'm sure your chosen machine shop will do a good job for you, with all those years under his belt

[DeadSquare]

 

Biggest diameter at the top because it gets the hottest there and the pistons expand the most + most wear there(peak cilinder pressures) as far as i know.

 

It also received the least amount of lubrication, in fact, the area between the Top Ring and the Oil Control ring receives next to no oil at the top of the bore.

 

 

But it also gets the least wear, as the piston ring doesn't reach there, and the piston is stationary.

[AKat]

Hi Dotmatrix,

I quickly chucked together these photos and details to hopefully help you out.  Sorry its not an A series, its the Isuzu G200 I have on the stand at the moment but it all works the same.  

Firstly, 99% of the time, the only thing in the manual that you're concerned with is the Gauge Point (where to measure on the piston skirt) and the piston to bore clearance.  Reason being there is obviously going to be differences in manufacturing etc, so you want your piston to fit your bore correctly.  
 

1. (man how good have these forums gotten?  I can do dot points now and everything!)
2. Ok, so here is the spec sheet, it shows me the gauge point (in this case 0.500" up from the bottom edge or 12.7mm), and it shows me the clearance required (in this case its 0.0035" or 0.088mm.... forged pistons often need more clearance than OEM cast ones) 
   
    
    
3. Mark the piston where the gauge point is 
   
   
    
4. Measure the piston and lock the mic
   
   
    
5. Zero the dial bore indicator off the mic measurement.  You want to use the gauge for comparing and seeing the size difference.  We're measuring clearance after all.  So from zero if you get a reading of 0.02mm off your gauge, then your clearance is 0.02mm (or its 0.02mm too tight )
   
   

   
    

6.  

   These are the 3 points for measuring the bore.  Take the size at the 3 planes at the top, middle and bottom of the bore 
   
   

    

    

   And here is the final clearance size 
   
   

   I'd call that as close as it gets is to swearing !  
   
   
    

   Don't get hung up on nominal sizing etc, it will just confuse the hell out of you at this point.  Just use the piston for a gauge and the Dial Bore Indicator as a comparitor 
   
   
   Cheers

[dotmatrix]

 

Hi Dotmatrix,
...

 

Cheers

 

 

thank you a lot for this thorough explanation. really helpfull! I will get data, and try measuring again based on your explanation.

[Earwax]

Excellent explanation and photos A KAT 

[DeadSquare]

They don't put these sorts of things in history books:-

 

 

 

80 years ago, Before the advent of high grade steel for valves, one of the rituals of motoring was "The Decoke".

 

Cleaning the burnt, coke-like, carbon from the combustion area was actually just a by-action to the refacing and lapping in of the valves, that was needed to stop pitted valves burning out.

 

It was also an opportunity to measure the bores, which was simply done by pushing a piston ring, progressively, down the bore and checking the gap with a feeler gauge.

 

If sufficient wear was detected,  "Plus Two" pistons could be fitted.  These were .002" oversize, had a lower top ring and were fitted without a re-bore, the glaze being honed with a device like a rubber jam jar with emery paper wrapped round it, inserted in the bore and inflated with a foot pump.

 

During the war, a set of pistons was almost unobtainable, so worn bores were dealt with by putting the pistons on a lathe and knurling them to "bring them up to size".

Edited by DeadSquare, 05 December 2018 - 12:25 PM.

[Spider]

They don't put these sorts of things in history books:-

 

 

 

80 years ago, Before the advent of high grade steel for valves, one of the rituals of motoring was "The Decoke".

 

Cleaning the burnt, coke-like, carbon from the combustion area was actually just a by-action to the refacing and lapping in of the valves, that was needed to stop pitted valves burning out.

 

It was also an opportunity to measure the bores, which was simply done by pushing a piston ring, progressively, down the bore and checking the gap with a feeler gauge.

 

If sufficient wear was detected,  "Plus Two" pistons could be fitted.  These were .002" oversize, had a lower top ring and were fitted without a re-bore, the glaze being honed with a device like a rubber jam jar with emery paper wrapped round it, inserted in the bore and inflated with a foot pump.

 

During the war, a set of pistons was almost unobtainable, so worn bores were dealt with by putting the pistons on a lathe and knurling them to "bring them up to size".

 

Oh crap.

 

I feel really old now !!  I remember all that stuff.!!

I think it was improved additives in the fuels that eliminated the need for a de-coke, but the Head Gasket sets that are sold here locally are still to this day referred to as a VRS (Valve Regrind Set) Gaskets !!

[nicklouse]

I remember buying De-coke sets in the late 80s.

[AKat]

They don't put these sorts of things in history books:-

 

 

 

80 years ago, Before the advent of high grade steel for valves, one of the rituals of motoring was "The Decoke".

 

Cleaning the burnt, coke-like, carbon from the combustion area was actually just a by-action to the refacing and lapping in of the valves, that was needed to stop pitted valves burning out.

 

It was also an opportunity to measure the bores, which was simply done by pushing a piston ring, progressively, down the bore and checking the gap with a feeler gauge.

 

If sufficient wear was detected,  "Plus Two" pistons could be fitted.  These were .002" oversize, had a lower top ring and were fitted without a re-bore, the glaze being honed with a device like a rubber jam jar with emery paper wrapped round it, inserted in the bore and inflated with a foot pump.

 

During the war, a set of pistons was almost unobtainable, so worn bores were dealt with by putting the pistons on a lathe and knurling them to "bring them up to size".

Mate that's kicking it way back! 
I knew they knurled steam engine pistons, never realised they did it with automotive ones to!  Thank god for the jet engine era and the advent of readily available high temperature steels!