Author Topic: Oversized Piston Help  (Read 162 times)

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samdavies97

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Oversized Piston Help
« on: April 01, 2019, 10:18:24 PM »
Hi

I’m looking to put oversized pistons in alongside reboring my current cylinders. But I’ve noticed two different sizes, one being H=25 the other H=30.
Can anyone please provide me with some info at to what the difference is and what I need to consider when doing this mod?.
I’ve attached a pic to show.



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Rusty's Uncle

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Re: Oversized Piston Help
« Reply #1 on: April 02, 2019, 07:01:30 AM »
I believe that refers to the height of the piston above the gudgeon pin centre. Not sure if you realise that with the piston size you show you will need new special barrels and the crankcase will need boring out to take them.
You may be able to achieve 700cc by using Bis pistons.

michaelodonnell500

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Re: Oversized Piston Help
« Reply #2 on: April 03, 2019, 02:52:11 PM »
I believe that refers to the height of the piston above the gudgeon pin centre.

This is something you would need to consider if changing the stroke of the crank while retaining the maximum piston height. Rusty is right about the piston size. You can have standard barrels bored out to 80mm (bis piston size), but it leaves the bottom skirt of the barrel a bit thin. Anything more than that you will need to buy oversized barrels and have the crankcase modified.
Another important point is that the price of boring out from 650cc to 700cc nearly costs the same as a set of oversized barrels so it's not a budget way to a faster engine.
 
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samdavies97

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Re: Oversized Piston Help
« Reply #3 on: April 03, 2019, 05:14:05 PM »
Ah well I emailed Ricambio about their oversized 795cc piston and barrel set and they’ve told me that no boring or modifying of the crankcase needs to be done.



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michaelodonnell500

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Re: Oversized Piston Help
« Reply #4 on: April 04, 2019, 01:47:47 PM »
That's interesting.
I asked these guys about the 700cc barrels and the kit they sell has a larger diameter outer skirt. The diameter of the outside barrel is mm. 84,50 and the crankcase needs to be drilled to 85,00mm

http://www.fiat500sport.com/engine-assembly.html

I always thought these kits came from the same place, obviously not! I have a set of barrels that I had bored out to 700cc. The guy doing the work thought the material might be a bit thin around the skirt area, but as the piston doesn't travel in that region so he thought it should be ok.  That leaves me feeling a bit more positive about my re-bore then!
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Rusty's Uncle

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Re: Oversized Piston Help
« Reply #5 on: April 04, 2019, 04:34:10 PM »
Ah well I emailed Ricambio about their oversized 795cc piston and barrel set and they’ve told me that no boring or modifying of the crankcase needs to be done.

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You would have had fun trying to get your money back from Ricambio when you found they did not fit  :o
I assume that the 795cc kit would have had 85mm pistons and the standard barrel locating bore on the crankcase is 82.5mm so Ricambio were talking out of their exhaust pipe.  You also have to take into account that the gas escape ring in the cylinder head has to be welded up so expensive alloy welding job then the head will need reskiming. Also at that size you have to consider how you get a seal at the base of the cylinders as the sealing face is virtually eliminated by the machining.
How do I know this ? Because I have a 795cc engine that I rebuilt this year  :)