Thanks
Rick OBrien
65 FHC in FL
Was it just at a machine shop for boring and cylinder liner work?
If so, and if it were my engine block, I would take it back.
Dennis
69 OTS
It’s just very inconvenient to tear apart all the pristine assembly to this point and take it back to the shop.
Rick
I agree but with piston expansion etc, I would not want to risk not having it correct and the piston possibly scoring.
I just don’t have the knowledge and would not want to chance having to pull out the engine at a later time
Dennis
*1000!
Every bore needs to be rechecked and measured using a bore gauge, and long feeler gauges.
Sorry should have been done before it was out of the machine shop back up now or suffer the consequences. Lucky you noticed thank your lucky stars
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Wasn’t it mentioned earlier in this thread that the machine shop had already honed the bores .001" to achieve proper piston clearance? So they obviously hadn’t bothered to actually measure them.
I would want to measure each bore in at least six places before I installed the pistons.
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Correct Clive. I was wondering if anyone would remember that. I kept the backstory out of this for simplicity.
There are 2 shops involved. Shop 1 bored the cylinders +.030 to achieve the .0011/.0017" clearance I mistakenly gave them from Bentley back in 1984 for OEM pistons. Shop 2 (a month ago) honed out another .001 to achieve the proper AE clearance. Actually, they were not working to a clearance (AE doesn’t supply that) but rather to an absolute bore diameter (3.6550/3.6560") that came from the AE instructions. They hit the low end of this tolerance. I suspect shop1 left the big ridge and shop 2 just knocked another .001 off it. It would appear neither shop did a comprehensive measurement.
The silver lining might be there is still .001 left along the entire cylinder to play with while trying to remove the ridge.
Aren’t you glad you brought this up Clive!
Rick
This is what I don’t get.
I only have an inside mic. so it is quite laborious for me to take measurements of a bore. But even so, I usually measure a bore six ways (three across, three longitudinally).
A proper machine shop, on the other hand, ought to have a bore gauge, so it would have been quick and easy to take measurements. But it seems they didn’t.
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Hi again Rick,
It has been a long time since I did my pistons but my knowledge is that machine shops should always work with a piston to bore clearance not a general spec.
I think my machine shop did a clearance for each piston of 0.0015 in each bore. Given manufacturing tolerances of pistons, the pistons were marked as to which bore they should go in.
I think the piston to bore measurements were always done on the thrust side of the piston, at the bottom edge of the piston, (but I could be wrong on this as it has been a while. Forum may wish to comment…). And this measurement was done with the piston at various heights in the bore.
I checked all these measurements myself when the block came back. In addition to checking at various heights, I did the same measurements after rotating the piston 90 degrees, and then another 90 degrees, etc. I also checked all piston ring gaps at various heights in each cylinder before putting them on the piston.
I suggest it would be a good idea for you to check the clearances in all cylinders before you send the block back.
I doubt that the machine shop left 0.001 “to work with” in each cylinder. This would mean that you managed to get each of the pistons in with a 0.0007 bore clearance. For me, it took a lot of fiddling to get the pistons to slip into a bore with 0.0015 clearance. If they left you clearance of 0.007 I think it would have taken a little more time orient each piston to slip into the bore
I think that what you want is the that bores are within spec now with (say) 0.0015 piston/bore clearance in each cylinder and all the shop needs to do is remove the 0.002 error in the offending cylinder. The big question is how does the bore transition from (hopefully) spec and then narrow by 0.002 ? If it was me, I would measure the bore clearance very carefully, to see how it transitions and narrows.
Dennis
69 OTS
I absolutely concur: if the machine shop sent this out in this condition, I bet a dollar to a doughnut they’ll try to weasel out of it. So go armed with data for each cylinder and probably photos too.
In addition to Paul’s insight, one option after determining clearances don’t work is to take the block to a top machine shop and ask to have the corrective work done without bad-mouthing the offending shop to the top machine shop. A really good shop may take on the corrective work if they don’t think they might get bad-mouthed also. This path costs more money but less emotional aggravation to arrive at the correct solution and seems less likely to lead to a second machining where other problems arise.
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Yes, this is general practice but AE (in 1984) took a much simpler (and obscure) approach. They do not tell you the desired clearance. Instead they tell you the bore to shoot for. They claim the pistons are so precise the proper clearance will result. But in fact its even more obscure. In their words:
“Cylinder bores must be finished to the nominal size (as shown on the label of the carton) plus the required oversize, to a tolerance of -0.000/+0.001 inches”
The original nominal size is 3.625 and my oversize is .030 so the final result is 3.655/3.656". This is the target I gave the machine shop. They came in very close to the low limit of 3.655. So this is why I say they have approx another .001 available and still stay in tolerance.
However, like most people, I was interested in the calculated clearance. So I measured every piston very carefully and got a range of 3.6530/3.6533" This range against the bore tolerance yields a calculated clearance range of .0017/.0030"
I plan to give the shop the pistons and have him shoot for a fitted clearance of about .0024 and this time along the ENTIRE length of the bore!
For reference here’s the carton label:
Rick
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Hi Rick,
I am not an expert on this. All I have is my experience which was about a 0.0015 clearance from piston to bore.
I think that race engines might have more piston to bore clearance because of higher temperatures causing more piston expansion but Jaguar must have a reason why they specified their range of piston to bore clearance which is less than the target you mentioned of 0.0024
Dennis
This was the original clearance I gave to shop 1. It was straight out of the Bentley manual (.0011/.0017). However I later learned this is valid only for the original split skirt pistons. My oversize AE pistons are solid skirt and require a larger clearance.
Rick
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OK
Good luck!
Dennis
Hi Rick, unless you had split skirt pistons, your original piston to wall clearance would be fine. For later solid skirt pistons I have found .00225-.0025 to be ideal. I had the same problem with my first rebuild.
The machine shop gave me .001 clearance and I was using shorter skirted newer pistons. The engine ran for 3 minutes before screeching frozen to a stop.
Good luck,
Allen
Dad unfortunately found that particular problem out in the late 50s when he did his first overhaul on a Jaguar, and never made that mistake again.