The AP1 and AP2 Honda factory pistons have the same 30mm compression height.

The Wossner AP2 piston is designed to use the longer AP1 rod so has a shorter compression height of 26.7mm. You run 153mm ap1 rods, 90.7mm ap2 crank and 26.7mm compression height. This is different to the Honda AP2 setup.

There is another thread where a chap mentions using the Woessner pistons on FRM bores without any problems.

https://www.s2ki.com/forums/index.ph...=695378&st=100

Finally, the H22 piston uses a 31mm compression height and 22mm pin. To put these in a F20C, you would need to remove 1mm off the top of them, which pretty much reduces them to a flat top -1cc piston, and you would need your rods bushed down to 22mm pins as well. You could run them unmodified with a 2mm head gasket ( 1.4mm thicker than standard) , but the compression ends up pretty much around 10:1 anyway.

For a FI setup the low compression Woessner pistons would be ideal if we can be confident in them running on FRM reliably.

 

^ oh I see what you're saying. Well this is good news indeed.. thanks for clarifying that. Now, the dude in the other thread says that these pistons need to run looser than stock due to different metallurgical processes. How is this determined? is that recommended by Woessner?

 

I am all for a low compression frm compatible piston, If I knew there was one that worked off the shelf I would have them in my motor already. However that being said I would be skeptical of Wosimer pistons for the following reasons.

1. They offer pistons in bore sizes not recommended by honda. Max bore according to honda is +.25mm, not +1mm.

2. They dont say what grade of aluminum they are made from.

3. They dont talk at all about piston coatings, which the stock pistons have.

4. They dont specify wall clearances, which is majorly important depending on material grade. The guy running these said he had high oil comsumption because he was running .003 in wall clearance. This is over 3 times the stock wall clearance. I dont imagine his motor will be anywhere near as reliable as a motor running stock clearances.

 

0.003" clearance is pretty much the norm for forged race pistons. This is also what Mahle recommended for their H22 pistons. My F24 which used these pistons used less oil than my stock F20C at approx 12,000km.

Stock pistons cant expand anywhere near as much as forged otherwise they would jam in the bore.

One could argue, even allowing for different materials and expansion rates, the stock pistons are on the tight side. Just count how many people have problems with pistons picking up on the bores, and how sensitive they are to bore squirter cooling. Running a little more hot clearance reduces the likelyhood of the piston picking up on the bore.

I'm pretty sure the Woesner online pages mentioned skirt anti friction coatings. As for the material, as long as they are durable and they specify the appropriate clearances, why is the a problem.

 

The "dude" is me
When I get the pistons from a german vendor (not Woessner himself) they were sended to me without any specification or installation manuals.
So I used the clearances stated in the Honda manuals -> 0.03mm (0.001"). The pistons stuck and I could hardly turn the crank.

So I contacted Woessner directly and had a longer call with Mr. Woessner himself about these pistons. He told me that a clearance of 0.08 - 0.09mm (0.003") is needed and the pistons are coated.

The other statement concerning the manufacturing process is due to my personal expierences (I'm an automotive engineer) and some other race buddies.

The Woessner pistons (87.5mm) are still running well in my setup. Oil consumtion is a bit higher as normal. But what are we talking about? My Oil catch can has not been drained since 10k miles....

One word about max. bore: who said that this can (should) not be over .25mm? Is there a Honda Bulletin or so available? My guess is -as I have written on the other thread- Honda makes .25mm oversized pistons only because there is no need for them to prepare for a bigger diameter. They are not a tuning company.

regards
Bernd

 

Material for the S2000 pistons is 2618-T6, forged material special moly coated. Other pistons also from Woessner are made of 4032-T6.
The 2618 alloy has appr. 12% higher strenght than 4032.

2618 aluminum expands approximately 15% more than 4032 aluminum, thus the piston-to-wall clearance has to be 15% greater.

 

So, these pistons ARE safe to run in our FRM sleeves...? I am looking to get the 8.5:1 CR versions for my AP1. I will most likely stronger con rods as well (high boost). Do these guys make them to order, or do they have a stock to pull from?

 

Interesting news here...
Mahle should make a set for us also...

 

I thought Inline Pro sold FRM friendly lower compression pistons? In fact, I'm 100% sure they do; Ya sent me pricing months ago when I was still thinking about lowering my compression...

 

you say that 2618 expands more than 4032 so that would mean that the 4032 piston needs to be tighter when cold no? But earlier is was said that the 4032 pistons need to run "looser" than the oem 2618.... it seems like a contradiction, or maybe I'm not understanding right

Please clarify