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3.9 V8 bores and bore gauge measurements


duncmc
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My brain is starting to hurt!

I have stripped down my spare 3.9 V8, and at first glance it looked good. But I didn't trust myself. So I bought a bore gauge which reads in 0.5 thou increments.

According to the Rover V9 rebuild manual, the bores need to be re-bored if the are out of round by over 0.5 thou. Fair enough.

So I measured my bores, top, centre and deep in each bore and at 90 degrees. Lots of measurements.

post-28226-0-04578500-1357418873_thumb.png

What is bugging me is how can I be sure that these measurements are truly correct when the measurement error of the bore gauge is 18 micrometers, or 7 tenths of a thou!? The combined potential measurement error for two measurements is 14 thou! Sure some of my cylinders have measurements that are out of round by 1 thou, some by 1.5 and 2 measurement are out by 3 thou. But are they correct, I think I will re measure in the morning and then have another think.

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if your going to trouble of thinking rebore then are you going top hat liners , otherwise bit of waste of time JMHO

Exactly, and then its a case of I might as well get a 4.6 to top hat.

I am mainly trying to figure out if I can reassemble as an interim engine without a rebore to replace my current 162K worn engine. It could be close you see, if the readings I have taken are not correct or factoring in the measurement error.

I have just read a bit in David Hardcastle's book "Tuning Rover V8 engines" where he says "when getting a block rebored insist that the machine shop can bore to an accuracy of +/- 1 thou over the whole bore"!

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I am sure many peeps will sayi am barking

But a good rough guide is on the liner top itself and

The fact the piston does get to the top

So

Finger in bore and 'feel' the lip at the top vs were the piston goes up and down and it's the worst of any of the 8

Nothing = fine

A very very small ridge that can barely be felt = new rings

A big lip that you can click with your fingernail = liners time

If all I'd ok the check for any pick up - this will be a bottom to top grove where a ring debris or carp has grooved the liner and will affect the seal if all ok the finally

Check pistons top purr edge for burn or picking up if all clean ANC no marks the you just need rings

Buy good rings AND gap them

Hth I will now wait for the dissers !

Nige

No it's not high tech but better than many will say

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What type of bore gauge do you have and have you any means of calibrating it?

We bore and hone cast iron A series Mini blocks for racing and achieve top to bottom accuracy within 0.0002" and ovality within 0.0002". We have both Bowers bore micrometers and a comparator gauge for checking. The Bowers gauge is checked against its sizing ring so we know we are on size, the comparator then gives us the roundness.

The Bowers bore micrometers have three 'feet' at 120 degrees whereas the comparator has essentially three 'feet' close together and another 'foot' opposite the cluster of three.

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What type of bore gauge do you have and have you any means of calibrating it?

We bore and hone cast iron A series Mini blocks for racing and achieve top to bottom accuracy within 0.0002" and ovality within 0.0002". We have both Bowers bore micrometers and a comparator gauge for checking. The Bowers gauge is checked against its sizing ring so we know we are on size, the comparator then gives us the roundness.

The Bowers bore micrometers have three 'feet' at 120 degrees whereas the comparator has essentially three 'feet' close together and another 'foot' opposite the cluster of three.

Zoltan, I think you have hit exactly what I am was worrying about on the head. I just hadn't worded it as such. The bore gauge that I bought is only a cheap thing, cost about £30 (http://www.agl-distribution.co.uk/2-to-6-inch-imperial-dial-bore-gauge.html?___store=agldist&___store=agldist). And I have no way to calibrate it myself. I am setting it to the spec bore using my cheap digital vernier micrometer from Machine Mart, zeroing the gauge in that, and then looking for how much deflection I am getting. And it has the potential to be out by 7/10 thou, so I don't think I can trust it to measure 0.5 thou accurately.

I am sure many peeps will sayi am barking But a good rough guide is on the liner top itself and The fact the piston does get to the top So Finger in bore and 'feel' the lip at the top vs were the piston goes up and down and it's the worst of any of the 8 Nothing = fine A very very small ridge that can barely be felt = new rings A big lip that you can click with your fingernail = liners time If all I'd ok the check for any pick up - this will be a bottom to top grove where a ring debris or carp has grooved the liner and will affect the seal if all ok the finally Check pistons top purr edge for burn or picking up if all clean ANC no marks the you just need rings Buy good rings AND gap them Hth I will now wait for the dissers ! Nige No it's not high tech but better than many will say

Nige, given the above and without taking my block to a machine shop just to be measured, I think your suggestion makes sense. This is what I looked for initially before I bought the bore gauge. I have no ridges at the top or bottom, but there are some pick up marks. Its going to need a hone before the new rings anyway, so I am still going to end up having to take it to a machine shop! I just like to know what I have before I go.

I am going to go and retake three measurements now for each and average them, just to see how much variation I get.

Then I will have to speak to the machine shop, thinking of Chessman Engineering in Coventry.

Thanks Both for the advice. It helps. :)

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I might have got it wrong but According to the measurements you have posted above, the smallest size there is 3.698 and the biggest is 3.701. (That's ignoring the occasional 4th figure of half thous). That's only 3thou difference between all 8 cylinders, not bad for a used block.

The maximum difference on any one cylinder is 2thou, and your worried about finding a machine shop to get an accuracy of less than 1thou??

Or have I read this threat wrongly??

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I might have got it wrong but According to the measurements you have posted above, the smallest size there is 3.698 and the biggest is 3.701. (That's ignoring the occasional 4th figure of half thous). That's only 3thou difference between all 8 cylinders, not bad for a used block.

The maximum difference on any one cylinder is 2thou, and your worried about finding a machine shop to get an accuracy of less than 1thou??

Or have I read this threat wrongly??

I think you have read the thread wrong. I wasn't worried about finding a machine shop capable of working to 1 thou. It was the fact that the Land Rover Overhaul manual for the Rover V8 specifies that if the bores are out of round by more than 0.5 thou half way down their bores, the engine needs a re-bore. The problem I had was how could I measure that accurately with my bore gauge which is accurate to +/- 0.7 thou. AND the David Hardcastle book was suggesting 1 thou was OK.

I have now taken 4 lots of each measurement and averaged them out. I get 1 cylinder to be bang on, 5 cylinders within 1 thou, and 2 to be within 2 thou out of round.

With the above in mind, and considering that the bores look OK, with only very faint pick up marks that can't be felt, and that the compression test where good before I stripped it down, (a lot better than my current engine in the car) I don't think it needs a re-bore.

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I would agree, your worst bore is 2 thou out, they'll be fine.

All you need to do is Flexhone the bores to remove the glaze. This will allow new rings to bed in properly.

Have you measured the pistons? Do you plan on re using them?

No, I haven't measured the piston yet to check the clearance. And I am hoping to reuse the pistons. I need to get a bigger micrometer first. One of the tasks for this week.

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I don't know how much useful measurement will be possible on the pistons themselves as the skirts are actually not round but a complicated elliptical shape to account for expansion and contraction. That will give you all sorts of anomalies in your measurements. You are probably better off stopping at measuring ring gaps. Having 8 parallel and round bores with a good cross hatch hone pattern with a set of correctly sized new rings will make more difference.

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The Overhaul book says to measure the pistons at 90 degrees to the gudgeon pin and 8mm up from bottom of the skirt, only the one measurement. So I'll do that. I am going to measure the rings also. As a back up I have seen in one of the Rover manual somewhere putting the piston in the bore and measuring the gap with a feeler gauge, so I can check with that too.

You have probably guessed that I haven't overhauled an engine before. I did help my Dad when I was a kid though.

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Generally speaking with the RV8 , any more than 3 thou ovality is rebore time.

Up to 2 thou or so is just a light glazebreak and new rings. The ovality will be on the thrust side of the bore.

Other than a bore gauge, an internal micrometer is the best tool.

However, another way to measure which is very accurate, is to use an old fashioned engineers method. Cut a piece of 1/8th brazing rod to the same size as the bore diameter and then carefully dome the ends with a file so the length of the rod is the bore size. Then insert it into the bore at different points and use feeler gauges to measure the wear.

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