Overbore pistons

[Crazcnuk]

Hi all.

I just picked up a set of overbore pistons, barrels etc, from Andreas.

The pistons are marked RK Eng 75K on the underside. They are domed, and measure 2.945in dia. .

This makes them even larger than the Wiseco 1196cc kit (2.933in)

Anyone have any experience with RK pistons? I am going to need a head gasket and possible rings when I install these in the bike and need to find where to buy them.

I have been searching the internet but can find nothing on these.

[TopCatGr58]

Nope, no experience, but that's only 1/3 of a mm total diameter, only a 1/6th mm larger all the way around than the 1196 wiseco ones, so I would think that the COMETIC brand head gasket for the 1196cc sized pistons would work okay? You might try contacting COMETIC and see what they say, perhaps it would be okay, or they could make a custom one for your size!?
T.C.

[Crazcnuk]

I am pretty sure it would work, as well, if I have no other choice. I have time, though, before I will be putting it together, so I figured I would check around.

I am mostly concerned with the compression ratio these pistions will create. They have a fairly hefty dome on them, whereas my stock pistons are flat.

In the manual you could get 9.0:1 compression in US models, and 9.2:1 in European. Wiseco big bore sets are 10.25:1.

[Goin Commando]

I am mostly concerned with the compression ratio these pistions will create. They have a fairly hefty dome on them, whereas my stock pistons are flat.

Clay is your friend, when you install this set make sure that you have plenty of clearance- piston to head, valves to piston. There is a thread on this subject here somewhere.
This set sounds like a true performance package, I hope it works well for you.
The company you seek might be called RK Performance, but dont be surprised if your new/old piston set is obsolete.

[Crazcnuk]

Not too worried about clearance. I will check all that as I put it together.

More interested in whether these may or may not be 11:1 or better, as that requires a lot more care after it's together. things like premium grade fuel, heat issues etc.

If they are no more than 10.5:1 that would be great.

Another question, would it be worth it to locate a set of 78-9 Camshafts to go with the Overbore, compression and 1980 head?

[TopCatGr58]

Hey Cnuk,

With domed pistons and the larger 80 valves, I think you might be a bit more concerned about piston to valve clearance, and the earlier years have more lift that could put them even closer??
T.C.

[Crazcnuk]

Yes, that's why it would be nice to find someone with prior experience with this particular setup. There are a lot of things that may crop up.

However, if this is a performance piston set, then it was likely designed to go with a whole host of changes, ie larger valves, larger camshafts, higher rpms, etc. and may work with the '78 cams and '80 valves, but require stiffer valves springs, or whatever... or maybe a completely different cylinder head!


Cometic does make a headgasket with a 2.953" bore P/N H0153SP3050C for this application.

[andreashweiss]

I was not aware this bike had the large cylinder kit when I picked it up. Only noticed it when I took it apart. Rest assured it had a stock 78e cylinder head. Valves, springs,cams, etc were stock.Connecting rods were stock so was the crankshaft. I send the carbs to someone on the list and I think he said it had some bigger jets. The engine was not damaged and turned over just fine. andreas

[Crazcnuk]

Yes, the pistons and all are in good shape. It looked like he had it running a bit rich (carbon buildup in a short period of running). It will be interesting once I put it together and get it out in the spring.

You brought up another question though. I have been looking at the Jetting recommendations in other posts, and see no mention of increasing the jet size to compensate for the added displacement ...

So far I have 4 into 2 aftermarket exhaust, K&N single filter in the stock box (looking at drilling it out) and 1208cc displacement with, (guessing) 10.25+ or better compression.

This bike is a decker with Windjammer IV, stock bags and trunk.

Also wondering if anyone has tried advancing the cams at the chain sprocket for the usual 3deg advance that most gas engines seem to like? Used to do this on V-8s as a matter of course using offset cam keys.

[DiverRay]

The jetting "should" be OK on your bike. mains and all are changed as the airflow changes. The cams can be adjusted, as I think MAXIMAN has done his. The trick was to have the cam sprockets slotted so they could be adjusted.

[Crazcnuk]

Wiseco also has a headgasket that will work.

W2723 for $58CDN has a 2.992 bore.


If it's possible to bore these bikes out that far, then I guess we are pretty safe with the 1196 kits!

That would make the XS 1244cc, or 76cuin.

[Ken Talbot]

The headgasket that came with the Wiseco kit I just installed was in fact a Cometic.

[Crazcnuk]

Which makes it strange that they both sell different sized gaskets for the same application.

Wiseco says that the gasket they sell is for the 1176-1196 overbore kit, and yet the gasket has a far larger bore than the larger of the 2 kits. It's even larger than the Cometic gasket I was quoted, 2.992 vs 2.953. Considering that the 1196cc kit is 2.933, the gasket they quoted me is pretty large.

Only thing I can think of is that maybe Wiseco uses the Cometic gasket in thier kits, but the one they are quoting me is a different gasket, that does not come with thier kits.

The Cometic gasket I was quoted is a dead soft copper head gasket. Is that what comes with the overbore kit?

[Ken Talbot]

The Cometic gasket with my Wiseco kit is a fibre material with metal rings for each of the bores.

[Crazcnuk]

Yes, just got confirmation back from Wiseco that the gasket they are quoting is a composite gasket.

This starts another question, Which is better, Dead-soft Copper or composite!

I have solid copper gaskets on the headers on my 502 Monte Carlo and quite like them.