Discussion in 'Mech Tech' started by Gnasha, May 9, 2019.
I think your ring gaps are too tight but if that’s what AA said to use so be it. Side clearance is fine.
I don’t understand why you’re concerned with fitting pistons from the bottom of the cylinder. Just use a ring compressor like the one I made, pic up there ^ and push the cylinder over the piston -arrange the ring gaps to avoid the cut-outs in the cylinder.
With my pistons I don’t have a choice; the scraper ring covers the pin so I have to fit the piston to the rod before the rings and cylinders.
@Gnasha ..How are your AA pistons holding out and where did you get yours from?
I recently bought a set of 2056cc AAs from Cool Air VW. Took about 2 weeks to arrive
Im still waiting for the cases halfs to return from machining
Flat top 96’s with a standard cam might not end well – what deck height and CR are you aiming for?
I was advised previously to aim for 8:1 CR. The pistons are flat topped apart from a cutout for valve clearance
I've not calculated the shim thickness yet
Yes, a standard cam needs about 8.0:1 static CR; the problem is bus heads have a relatively small combustion chamber volume and work with dished pistons, when you fit flat top pistons you have to fit cylinder base shims to get the CR low enough and end up with a large deck height. This reduces squish and efficiency – and power. Try to keep the deck height to 1.5mm and increase the combustion chamber volume if necessary, but don’t fit cylinder head gaskets.
Incidentally, you don’t need cut outs for valve clearance.
I have similar pistons 2056 with a scat 25 cam. is it preferable to alter the CR in the head first or use shims first under the barrel? Is there any "meat" to cut away in the head?
Compared to a standard cam a Scat C25 needs a higher static CR to work well – I have 9.0:1 – and for good squish and the best engine efficiency deck height should be 1 to 1.5mm IMO, although of course the engine will still run with a much bigger deck.
I suggest you adjust the deck height to 1.5mm (or slightly less) with cylinder base shims if necessary and use one of the CR calculators to obtain the combustion chamber volume for the CR you require. Then open up the heads, but only remove the minimum amount of material from the combustion chamber roof, take the sides out from around both valves to the edge of the cylinder mating face.
The pic is my 1800 head with 41mm inlet and 34mm exhaust valves opened up to 62cc.
I should have mentioned my 2.o is going to push a Karman Ghia
The 96's for 1911 kits for 1800's all look like flat top pistons..... are they to be avoided or do cylinder base shims do the job... just wondering
Sent from my SM-G930F using Tapatalk
I know . The way the engine is built is the same whether it’s pushing a bus or a car.
It depends on what CR the cam you use requires and the combustion chamber volume of whatever heads you use but generally if you fit flat top pistons in an engine that had dished pistons the CR will to too high, especially if you use a standard cam that needs a low CR.
Use cylinder base shims to set the deck height - or tweak the CR (but stay within 1mm to 1.5mm deck). Open up the combustion chamber if you need to for the CR you want but don’t use head gaskets to increase the volume.
Separate names with a comma.