New AA P&C's if youre interested

Discussion in 'Mech Tech' started by Gnasha, May 9, 2019.

  1. Hi, just recieved the above AA 96mm P&C's. I'm pleased to say visually they look good.

    The AA pistons ( 408g) weigh less than the Mahle piston (498g) and the AA gudgeon pin is shorter and as a result lighter. The pistons come with a graphite skirt with spaces top and bottom of the skirt on opposites sides to take measurements from.

    These are my findings:

    Clearances, Piston to Cyl.
    No1, 0.05-0.03mm, Ovality 0.015mm
    No2, 0.07-0.05mm, Ovality 0.02mm
    No3, 0.06-0.04mm, Ovality 0.02mm
    No4, 0.06-0.06mm, Ovality 0.01mm

    Gudgeon pin center to piston crown:
    I measured these from top of pin bore (pin out) to piston crown and I added half the diameter of the pin, is this the best way to measure this distance? accurate enough?

    N01 35.15mm
    No2 35.12mm
    No3 35.115mm
    No4 35.125mm

    Weighing the pistons, rings, pins, and circlips (proper clips with flat face and not just wired) and rearraging them for closest matching weights I was able to get them to within 0.6g. This was a good start for balancing I thought. Gudgeon pins were a good push fit. in the past some pins were machined from the outer ends leaving what can be an imperceptable ridge in the middle,this was a stress raiser and eventually failed. These were constant size full length.
    Ring gap clearance:
    Top 0.014 - 0.016mm
    Mid 0.012- 0.015mm

    Ring clearance still to do, I forgot:rolleyes:

    Cylinder heights, case to cylinder face and cyl, head to cyl face. I took 4 measurements from each cylinder.

    No1 91.04, 91.04, 91.05, 91.05mm
    No2 91.05, 91.0, 91.0, 91.03mm
    No3, 91.04, 91.03, 91.02, 91.04mm
    No4, 91.02,91.04,91.02, 91.02mm

    The cylinders themselves are clean and honed and look good albeit not black but a slighltly lighter colour.
    The fins do look slightly smaller in comparison to VW



    Vw on the right

    I'd appreciate any feed back or question, hope this is of interest

    Attached Files:

  2. We used the AA 96 x 66mm pistons / barrels back when we built our 1911 type 4 (back in 2014 / 2015) and found them equally as good then as it looks like they are now.

    Our conrods are weight matched to within 0.1g of one another (we have precision measuring equipment to do the checks with), they have the V notch added, the combined rod / piston lengths have been matched as per Tom Wilson's book.

    Ours kicks out a nicely driveable 96 bhp at the flywheel, which gives Gusbus a nice driving experience :D

    Little details like what you have listed all make for a better built motor imo, I think the measurements you have made are accurate enough, you could go to one more decimal place but is it necessary? For stress measurements we make at work, we go to 3 dp at most, anything further heads towards inaccuracies in the verniers and micrometers.
    rob.e and Poptop2 like this.
  3. Thank you for the response. When I come to balancing I have a scale that will give an accuracy of 1 decimal place, if I can get to 1DP i'll be happy. It cost under £10, (china) repeatability is good and its good for relative/comparision weights. never checked the accuracy for one offs.

    Did you wash P&C's before assembly? Washing up liquid? It looks like the bores have already been honed, however, I think i'll still rub down with some fine emery, wash and do the white paper test for cleanliness?
    MorkC68 likes this.
  4. We did wash everything before assembly with strain gauge solvent cleaner and the light blue paper towels then blew gently with an airline. I guess fairy liquid is okay, I dont see why not - we used what we did as it doesnt dissolve immediatly and allowed any debris to dissolve onto the paper towel.

    I think your scale will be more than fine, repeatability is the main thing!
    Valveandy likes this.
  5. As a comparison I’ll post the dimensions of my KB pistons and AA barrels later but to answer your question about cleaning. You must clean all new parts before assembly, I use paraffin; washing up liquid contains too much salt. Don’t use emery or any abrasive on the barrels, leave the honed finish so the rings bed in. And during the final assembly use some oil on the rings and nothing or just a little WD-40 on the bores.
    Last edited: May 9, 2019
    snotty likes this.
  6. Your dimension all look good but don’t forget the ring gaps – check what the piston/ring manufacturer recommends for an air-cooled engine, don’t have them too tight.

    These are mine.

    Pistons: KB Hypereutectic alloy
    Dimensions were the same within a couple of 10ths”.
    Compression height (Gudgeon pin centre to piston crown): 1.11"/28.21mm
    Crown diameter: 3.754"/95.36mm
    Skirt diameter: 3.777"/95.95mm
    Weight, bare: Circa 390g.

    Gudgeon pin: 22mm dia, spiral lock retainer, not just a circlip.
    Weight: 101.5g

    Piston, pin and rod total weight: 1085.6g to 1086.0g

    Piston rings: Hastings.
    Ring gap top: 0.028"/0.71mm
    Ring gap 2nd: 0.018"/0.46mm

    Cylinders: AA Biral (Finish honed by DRD Racing Heads)
    ID Top: 3.779"/95.98mm
    ID Bottom: 3.7794"/95.99mm
    Length (deck to case): 3.614"/91.79mm
    Deck height: 0.061” (1.55mm)
    Cylinder base shims: (1/2) 0-089"/2.26mm, (3/4) 0.091"/2.31mm (the crank c/l is not exactly on the case c/l).
    Gnasha and MorkC68 like this.
  7. Good reply thank you, looks like I still have some work to do, as I said I still have my ring clearances to do, what were yours? did you check the gaps top and bottom of the cylinder?

    There are so many various ways experienced people prepare their P&C's just prior to fitting. I have always used wd-40, but recently read the solvents flash of quickly and can leave a varnish. Each to their own I suppose. I'll use a mild soap soloution in hot water, electronic cleaner, paper test, and a light coating of oil on the bores.

    Do you oil the rings in place just prior to install?

    Thank again.
  8. Also check for casting material between the fins of each barrel and remove as much as possible so the air flow is good around each 1
    I spent quite a while with a file and hacksaw blade...
    Gnasha likes this.
  9. thanks for that, I did see a few turning swirls on the top which I removed but never looked between the fins. I'll check them.

    Thanks again
  10. My cylinder bores had little to no taper so I checked the ring gap where access was easiest – at the bottom and I used the piston upside down to keep the ring square in the bore. Gaps are up there^. Top: 0.028"/0.71mm, 2nd: 0.018"/0.46mm

    For an air-cooled engine KB recommend a ring gap of 0.0075” per inch of bore diameter (3.78” (96mm) x 0.0075” = 0.028”). That’s probably generous but I’d rather have a slightly larger gap than a ring seize in the bore.

    Whatever you put in the bores prior to assembly will be burnt off as the engine starts; I only use WD-40 to prevent flash rust. If you use oil keep it to thin film and definitely avoid lathering the bores with assembly lube that I saw someone on the Samba had done recently.

    I use engine oil in the ring grooves prior to installing the rings and then just a little more on the rings themselves before fitting the barrel. Incidentally, I fit the pistons on the rods and then the barrels on the pistons because the pin bisects the oil control ring groove.
    IMG_6655 (Medium).JPG

    The cylinder tins needed a small modification to clear the larger fins on the Biral cylinders.

    IMG_4881 (Medium).JPG
  11. Attached are my ring clearances in there respective grooves. I think the top ring clearances are a little tight according to Mr Bentley. Your thoughts word be appreciated, all measurements are in millimetres.

    Talking of piston, cylinder and rod installation. As a trial I tried the piston with rings fitted in the cylinder to see the clearances for fitting the pin. Well, at one of the cut outs at the base of the cylinder the oil rings have to be exposed to give clearance to fit the rod. Is this normal? does the ring gap positioning and arrangement present any problems with these cutouts?

    . IMG_0814.JPG

    Thank you
  12. Be careful using standard specs and tolerances with non-standard parts but if the piston manufacturer says use standard clearances then you’re okay to use them. Your ring gaps are too tight even with standard specs.

    The piston doesn’t go so far down the bore at BDC to expose the rings.
  13. Yes I understand during normal operation the rings aren’t exposed. However, to fit the gudgeon pin the oil scraper ring becomes exposed.
  14. Look at my pistons^. If yours are like that you’ll have to fit the oil scraper ring after the pin is installed, but it looks you can fit the pin with the scraper ring installed – fit the rings, drop the piston into the cylinder and fit the pin. Or fit the top rings, fit the pin then the scraper ring and fit the piston into the bottom of the cylinder – that’s what I do.
    Last edited: May 12, 2019
  15. I’ve just had another look at your ring clearances, my mistake, you’re of course talking about side clearance not end gap. 0.04mm side clearance is slightly tight but okay IMO – the ring doesn’t grow much in width.
    Last edited: May 12, 2019
  16. Thank you, I was to say the least concerned and was ready to pen a Mal to AA :rolleyes:
  17. You should still ask AA what end gap they recommend, they will probably say just use the standard but it’s worth asking the question.

    Incidentally, do you know who the ring manufacturer is?
    Gnasha likes this.

  18. Now, do you mean end gap or side clearance ? :D I think you mean ring clearance in the groove. Yes, I think ill send them a note an ask their views.

    The rings came fitted to the pistons and pistons inside the cylinders. No paper work at all inside thebox.
    77 Westy likes this.
  19. Heres some pics of the narrow gap between oil ring groove and gudgeon pin, is this a normal occurance during the assembly process?

    IMG_0817.JPG IMG_0819.JPG IMG_0822.JPG IMG_0820.JPG IMG_0821.JPG
    art b likes this.
  20. End gap is the gap between the ends of the ring when it’s in the cylinder. You’ve measured the ring side clearance in the groove but don’t install those pistons without checking the end gaps, it could end very badly if you do.
    IMG_4885 (Medium).JPG

    It’s interesting that AA have put such a large cut-out in the bottom of your cylinders, it’s much bigger than the window in the block that it lines up to. And my cylinders from AA didn’t have cut-outs at all, I had to cut them.

    You’ll probably find it easier to install the pistons from the bottom of the cylinder; I think the scraper ring will come out of the bottom as you fit the pin otherwise.

    IMG_6664 (Medium).JPG

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