It started with a pop

[AndrewSlater]

Yes thanks for the continued help and info everyone.

Ideally the third piece would be milled but I don't have access to one so still thinking this one through.
I agree with Neil that it is wise to confirm that the new pump cover lines up well with my third piece and allows the oil gears to run smoothly.

I have been re-reading parts of the 'Secrets of the Inner Circle' and watching copies of Harry's video.
I've also dipped in this useful rebuild thread which has a lot of detail regarding the engine.
https://rennlist.com/forums/912-forum/7 ... ad-28.html

I've been measuring and remeasuring my head chamber volumes. The cylinders in each head are closely matched, but the heads less well matched.
I don't see any evidence that the previous builder had compensated for this anywhere.


A lot of threads seem to have problems measuring the volume accurately as they introduce bubbles into the chamber.
I've not really had any problem with that and it is quite a repeatable process, with repeatable results to 0.1cc.

If you don't have an accurate bourette - you can always weigh the amount of water required - 0.1g = 0.1ml.

I plan on measuring the piston dome volumes next - AA pistons claim they are 17cc but best to check for real.
Once I have those figures I can work out how to best use the heads with regard to shims etc for a reasonable compression ratio.

AA pistons claim the kits are for a CR of 9.2:1, though clearly that depends on many factors.
A lot of the American builders seem to aim for a CR of 8.8 due to the poor gas in America.

Plenty still to do.

[neilbardsley]

My naive take is that 8.8 compression ratio is a bit low? I know that Mike Norton, on the 912 BBS board and very experienced air cooled aircraft engine builder, likes to be safe but 9.2 is ok here in Europe?

Ps I've found that when reading forums not all advice is equal for example I would put myself as the bottom of the pile but if you ever see anything from Vic Skirmants, Jack Staggs or Tim Berardelli they are at the top of pile.

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[Adrian3]

hi Andrew, the air in the chamber problem occurs when measuring the effective head volume by filling up via the plug hole....please let us know how you measure the dome volume, i hear it's not easy

[AndrewSlater]

Yes I agree Neil that a CR of 8.8:1 seems a bit poor - I would like to get closer to the range 9.1:1 - 9.3:1.

I decided to try to calculate what my previous configuration was running before moving on to the new P/C set.

First I had to reinstall a couple of rods back onto my crankshaft and repopulate the case.
The first snag here was that if I fully torque the case back up the crank seems to bind, so I had to leave it fractionally loose for the crank to turn. A problem for another day - surely the case hasn't suffered from stress relief so quickly!?

So I put piston #1 back on with cylinder #1 which allowed me to measure the deck height.
I then made a jig to sit snuggly on top of the cylinder to allow me to make a mould of the piston head.


I had both modelling clay and plaster of paris available to see which worked best, but started with the plaster of paris.

After an hours curing I had a mould, that I quickly sealed and then tried measuring the dome volume.
You can see the mould is raised as the original deck height was approximately 2.2mm




I measured it to be 22.2cc, but then the Maestro seems to give 15cc for a NPR big bore kit piston dome volume - very different - not sure why.
Maybe I need to file the mould down to the cylinder head level and remeasure it.

I am a little confused as to whether I need to know either
a) the full dome volume and simply the deck height, or
b)I need to know the partial dome volume at the deck height I am intending to run.

Anyone know the answer?

If it is b) then shouldn't the manufacturer provide a graph of partial dome volume against deck height?

[neilbardsley]

Article here from Ron of Precision Matters about how to

https://www.912bbs.org/forum/threads/aa ... ing.50196/

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[AndrewSlater]

Hi Neil, yes that had come up in my searches several times. I've read quite a lot on the 912BBS board but most of the threads aren't very conclusive or useful.

I think I have got my head around what I need to try to measure - how I do that is a different thing.
I did contact AApistons earlier in the week but had zero response from them as yet.

Still going to try to get some numbers for my old P/C set before doing the same for my new ones.

I'll try to post a drawing when I have some more facts.

[robs912]

Hi Andrew
If you gonna onto aa website (or many others)they have a compression ratio calculator.
All you need is your head cc size,bore,stroke and piston dome cc which is is 17cc.
From all this you can work out what shims to use to get the correct deck height to give you a 9.1 compression ratio.
At least that's what I did. Hopefully it's right?
Mine is just under 9.1 to 1 and it seems to run and pull ok.
Cheers
Rob

[AndrewSlater]

Hi Rob,

yes I decided to calculate the CR and the dome volume from first principals and that seems to stack up with the online calculators.

I am considering trying to measure the dome cc of my actual pistons but I expect they will be very close to the declared 17cc, although one of the contributors to 912BBS states they are always 17.1cc+.

I calculated the C/R for piston #1 on my previous set up as the deck height was quite high when I measured it and I got a CR of around 8.5:1 - so I expect the car was setup for a low CR as the Americans do.

My plan is to start to reassemble the engine and then make a further set of deck height measurements to decide on the final shims.
I am probably going to try to re-volume one head chambers so that both chambers on each side are the same size, although both heads will be slightly different.
I expect to have different shims dimensions on different sides to equalise the small difference in head volumes from one side to another.

I think I have just enough clearance to accommodate a slender shim on the high volume side, and a more standard one on the low volume side.

[AndrewSlater]

I've been jumping around on the rebuild a little in the last few days, but one of the jobs has been to inspect and measure my new piston and cylinders set.

The first thing was to drop all the pistons out of the cylinders to strip them and clean them.

As supplied the piston rings looked like there was a pattern but having reviewed the data they weren't very consistent.
Three had the oil ring at 12'oclock and one at 6 o'clock.

Two of the pistons have to be rotated 180 degrees to keep the wrist pin offset correct, so I would have expected two to have had the oil ring at 6'oclock.

Reviewing the Maestros videos he fits

Oil ring gap : 12o'clock
Ring 3: 6 o'clock
Ring 2: Either 3 or 9 o'clock ( to point to the exhaust valve )
Ring 1: Either 3 or 9 o'clock ( to point to the inlet valve )

The pistons and cylinders have now been cleaned a number of times - they were quite dirty as supplied.
I've checked all the rings in bores and the ring gaps are all 0.25-0.3mm so all good.
In terms of the new pistons they nominally weight 361g each, the wrist pins 120g and the rings with wrist clips 58g.

The pistons themselves were matched to within 0.4g, the wrist pins with 0.1g but the variation in ring sets was 0.6g.
My rods were matched to within 0.4g so again pretty good.

So with a bit of careful mixing of components around and an element of luck I have managed to balance every rod and piston combination to within 0.1g without any further machining. That's good enough for me.

Encouraged that everything was good to go, it was time to built up the crank following the Maestros instructions.



The only slight departure I made was to use Miller's Assembly Lube rather than the secret sauce - hopefully the Porsche God's won't notice!



All the rods drop at the same rate when I run the Maestros drop test - so it all looks good.

[neilbardsley]

I think since you got the sets within .1 of a gram the Porsche god's will forgive you not knowing how to mix the secret sauce

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[AndrewSlater]

I didn't say that I didn't know how to mix the secret sauce, I've got all the secret ingredients ready on standby, straight from the Maestro's cookbook.
Just thought I would give the Millers Assembly Lube a try - seems pretty good albeit expensive.

I might use the secret sauce when I assemble the case next - just to keep my conscience clear.

[AndrewSlater]

I decided to take a brave pill and attempt the modification of my third piece for the Precision matters oil bypass kit.

I started by marking out the piece and then with a fine dremel bit started a notch on the centreline.
Once this was started I moved over to a sanding wheel that was just over the American 1/2" instructions.
To appease the Porsche gods I like to think of it as 13mm.

Surprisingly the dremel sander takes the material quite quickly, so I had to do it slowly slowly with lots of checking.

Within a short time I had it done, and I had modified the oil pump gasket as per the instructions.


I will still need a bit of a trial fit to make sure my oil pump gears sit happily in the precision matters filter plate - but job mainly done!
Although looking at this photo I notice I forgot to alter the other side of the gasket - doh!

On a roll with the third piece I decided to fit the new bearing.
Some guides show a bearing press, others use a lot of heat on the third piece whilst freezing the bearing, whereas the Maestro whacks it with a hammer.
I contemplated buying a 6 tonne bearing press but decided that if a hammer is good enough for the Maestro then a hammer it was.

I heated the third piece to around 90 degrees C with my hot air gun, whilst also cooling the bearing down to around -20 degrees C with some pipe freeze.

I carefully grabbed the cold bearing placed it in the oiled and hot third piece and as I reached for the hammer it just slid in under gravity!
Crikey that worked well!
Result - now the Porsche gods were definitely smiling upon me.

[Nine One One]

Andrew,

Which bit do you have to remove material from on the third piece?

[AndrewSlater]

You have to remove material from the left oil port ( outlet ) to allow the new angled oil channel pipe to fit and mate with the new part.
The angled pipe is just about shown in the first photo, next to the head of the dremel. The cutout is right at the bottom left of the last photo.

[AndrewSlater]

I had previously put the case back together with all the old bearing etc, to try to confirm my deck heights for CR calculation.
However at the time I couldn't quite get the case back together without the crankshaft binding. I suspected I had got the flywheel bearing slightly off.
Having checked it again it surprising how far off you can be and think it is going back OK.


New hole made by the dowel pin - not even close to the correct one - whoops!


Since rebuilding the crank it was time to put some new bearings in the case and get it bolted up ( taking more care especially with the flywheel bearing this time ).
Took my time and got it all back together following the Maestros instructions and sneaked up on all the torque settings.

I now have the case halves together and the crank turns nice and smoothly.


With the case back together I spent quite some time carefully checking and rechecking my deck heights.
Both sides are identical and spot on 1.00mm so happy days.