What are the opinions (pros & cons) of forged and cast pistons?
What's the difference between 'A' & 'AA' pistons?
Are Mahle 92mm Piston/cylinders interchangeable (without machining) with 91.5's?
Original Post
Replies sorted oldest to newest
1st oversize cutting of case/heads will accommodate 90.5-92mm bore cylinders (1776 or 1835 cc with stock crank). Not familiar with 91.5mm. Next oversize is for 94mm (1914cc). New trend in pistons is for hypereutectic cast pistons. Before they came out, the preference was forged for light weight and strength with the cast being heavier and weaker - don't believe that hold true now.
My mistake...I'm thinking 90.5, not 91.5
So what's been cut for 90.5 will also accommodate 92mm without additional machining?
What's the advantage of hypereutectic (sounds like a disabling nervous condition) over forged pistons?
Carl, 90.5s are old-school. Some people can't get past this particular "Berg-ism" from the 80s.
I wouldn't bother with 92mm either, just get good quality 94mm and have the case and heads machined to fit. Anything less than 94mm isn't worth doing, machining is the same labor, the size of the hole is just a number to the machinist...
Thanks Danny.
I'm not thinking of rebuilding my type1...I'm just leaning on you guy's experience to clear up some foggy blanks in my knowledge...and I've got a lot of blanks!
Okay, possibly a silly question, but I've been wondering about this.
If he 1914 (1915?) uses the 94's and the stock crank. How do you get the 1968cc motor I saw offered by the "Kaddie Shack"? Is that just the 94's with head work?
I have more gaps / foggy blanks than Carl. 
Ted
'A' pistons refer to the stock piston wrist pin height. Good for 69-76mm stroke. 'B' pistons have a higher pin height (pin closer to crown) for 78-84 stroke. There is also a 'C' height. 82 stroke/5.4 rod/B piston is the easy combo.
AA is just a brand like Mahle. You have to pay attention though, Many aftermarket Bs have the pin 1mm further away from the crown than Mahle Bs. Could affect choice of rod length and shims.
AA is just a brand like Mahle. You have to pay attention though, Many aftermarket Bs have the pin 1mm further away from the crown than Mahle Bs. Could affect choice of rod length and shims.
Originally Posted by TRP:
Okay, possibly a silly question, but I've been wondering about this.
If he 1914 (1915?) uses the 94's and the stock crank. How do you get the 1968cc motor I saw offered by the "Kaddie Shack"? Is that just the 94's with head work?
I have more gaps / foggy blanks than Carl.
Ted
1968cc's is 74mm crankshaft stroke combined with 92mm pistons; an old school way of building a 2 liter before 94's came on the market. Stock crankshaft stroke is 69mm (69x85.5= 1600) and with 92mm pistons makes an 1835, whereas with 94's it's a 1915. 78x92= 2074cc's and 82x92 is a 2180 (very old school!). Using 94's instead of 92's will give 80 to 100 more cc's, depending on the stroke of the crankshaft. Yoda out.
I'd still be leery of taking a 40 year old magnesium case and boring for the cylinder bases fit to where you see the head stud threads exposed (I posted picture before of that). Going with a new aluminum case would be good investment. I wouldn't trust a non-air cooled machinist with doing it for sure. I have same concern on boring out old heads. Guess that's the old school caution on anything bigger than 92 mm P&C. This is where I see the T4 engine's strong suit.
If it's done right on 94s, it's not a problem, even on a 40 year old Mag case. But that is a big if today, I agree there Greg. I thought thick-wall 92s were the same head/case bore size as 94?
Suspect you are correct on the thick wall 92s. Here's good article from John Connoly at AirCooled.
I have 94s, but if I had it to do again I'd go with thick-walled 92s, and as Danny mentioned thick-walled 92s have the same head/case bore as 94s.