Topic: cb500 slipper pistons

[turboguzzi]

Hello All

Dissasembling now the bike in preparation for the next racing season. Motor is appart already and has held amazingly well, other than new primary chain, pistons and rings, there's not a lot to rectify. These little 500's are really over engineered.

But.... racers are seeking constantly for an extra edge and seeing how weighty and hefty the pistons are compared to the ones on my GSXR, I have sarted to search for slipper type pistons. These usually have shorter wrist pins (less weight) and also thinner rings for less friction and less weight too. Stock rings are 1.6mm thick, racing ones are usually 1-1.2mm.

my limitations are that I am already on +0.50, i.e. can fit 56.5 or can go up only to 57 bore as I am running in the 500cc class (+3.5% allowance of rebores), so 57 is the max I can go.

Thing is that havent been able to find any slipper type pistons with these bores. PeterC told me that he goes for XL125's but I've seen that other than having a domed crown, they are full skirt like the original CB500's.

Have contacted a few aftermarket comapnies that make 57mm big bore kits for the current honda CRF100 but no response yet and I'm not sure if it has a 15mm pin or if they'll sell to me just the pistons without the big bore cylinder.

Any ideas/experience with sources? 

Tnxs in advance
TG

[JLeather]

Dynoman.net makes custom pistons.  I'm bouncing ideas off him right now for custom pistons for my turbo engine build.  Price comes out to about $700 a set depending on what ya want.  Not cheap, but not that much more than a regular forged Wiseco set and they're fully custom forged pistons.

[bwaller]

Why not try Terry at teamhansenhonda@comcast.net. I can't say for sure he can help, but it's another option.

[turboguzzi]

hey! thanks for the tips, will try them even if dynoman doesnt seem to cater for the smaller fours.

Got some advance on my side, a company making pistons in the far east seems to have something pretty nice, 1mm rings, wrsit pin is shorter by 5 mm, let's see how it goes. It's a pisotn meant for some sort of east market only 125 honda. thye sent this drawing.

kidnapping my own thread here but thought that some people might be interested to see that the squish ring idea from "the gentleman's express" article does work. you can see how all around the squish ring there are no deposits, meaning that it really does throw all the mixture into the main volume of the combustion chamber. The dark deposits are because I'm running without valve stem seals and the engine does suck in some oil. there are slightly more despoits near the spark plug as with 35 degrees of advance the piston is not yet near enough to TDC for the squish to take effect so early. I'm up to 12.5:1 CR and even in hot conditions there's no pinging. tempted to go up to 13:1 for next racing season.

Any other ideas regarding 56.5mm slipper pisotns are welcomed

TG

[MRieck]

I'd run stem seals. All that oil in the cylinder combined with really high compression will cause detonation. What are you running for fuel?

[turboguzzi]

am running 98 unleaded + octane booster, we are not allowed AVGAS to race here. in any case, was indeed thinking about fitting seals now. what you see in the pic is the result of 3 races since i last pulled the head. not nice but not a disaster either as I pull the head every two three races and clean. i am running stock valve guides and some people here in sohc4 thought that no seals was good insurance against valve sticking considering that this engine lives mostly between 8-12K rpm.

TG

[MRieck]

 Bronze guides would be nice especially on the exhaust side. Oil control is important and no seals/slipper piston combination offer little in the way of control.

[turboguzzi]

Bwaller
Contacted terry and indeed he's an ultra helpful guy, sent me some amazing pics of his bikes and superb CB400F motor.
Regretfully, even he admited that the pistons I'm looking for are very hard to find if not unexisting.

Trail Bikes Honda seems to have a 57mm that could be perfect but looks like I scared them when I wrote them that I want only the pistons from their big bore CRF100 kit for a 1972 CB500.... they havent even answered...not nice.

does anyone have an idea what wrist pin dia a CRF 100 might have?

Mrieck, currently valves guides are OK but in case I need to redo them I was thinking about the K-Line solution. ever heard how they hold up? sounds interesting.
http://www.cylinderheadshop.co.uk/K-line.html

Cheers
TG

[bwaller]

Turboguzzi,

Yes, Terry has built a giant killer with that beautiful 400F. Did you follow the Motorcyclist magazine vintage CB750 roadrace effort with Mitch Boehm? In one article he describes an absolute battle with a guy on a 400F based racer that makes you think bigger isn't always better! Terry was trying to convince me to use a 400 instead of the 500/550 I had planned for a race platform.

Good luck with the CRF100 slug. If that doesn't work, surely between the other Japanese manufacturers one is using a more modern piston in the size you're looking for....... I'm not sure how to help you find it though.

[crazypj]

I don't have anything that small either.
Got some VF500 but they are 60mm and some Yamaha ones at 62mm
What about blanks for VF400? (just a suggestion, don't know bore size)
 The modified piston from gentleman's express is almost exactly the same mod I've been doing for years on CB750 piston for 591 kit. Would have saved me some time if I'd found it 30 yrs ago 
PJ

[MRieck]

Mrieck, currently valves guides are OK but in case I need to redo them I was thinking about the K-Line solution. ever heard how they hold up? sounds interesting.
http://www.cylinderheadshop.co.uk/K-line.html

Cheers
TG

Those liners are automotive based consequently they are made for low rpm use. The bronze certainly helps in regard to heat dissipation. For the  cost I'd replace the guides with bronze pieces. Unfortunately I don't know offhand who supplies them. They can be custom made by Kibblewhite or others....I just don't know the cost.

[turboguzzi]

Thanks MR, will look into KB guides.

BTW, looks like I had a breakthrough with pistons with a japanese company who seem "willing" to modifiy an existing core.

We'll see.

Regarding the crazy 400F, I admire the work and pieces but it looks painfully expensive! No way could I afford one. My CB500 racer cost pennies to put together. Yoshima quoted just a similar engine at 2600$ in 1977 dollars. cant even think what it'd be now. anyone wants to do the math?

cheers

TG

[ttr400]

Hey Turbo, any pics of that 400F motor?

[MRieck]

Thanks MR, will look into KB guides.

BTW, looks like I had a breakthrough with pistons with a japanese company who seem "willing" to modifiy an existing core.

We'll see.

Regarding the crazy 400F, I admire the work and pieces but it looks painfully expensive! No way could I afford one. My CB500 racer cost pennies to put together. Yoshima quoted just a similar engine at 2600$ in 1977 dollars. cant even think what it'd be now. anyone wants to do the math?

cheers

TG

TG   ttr can really help you out. He is the man when it gets down to this stuff. Period

[ttr400]

Thanks for that MR.

I have hunted all over for suitable 56mm pistons to use in my 400F projects, have not had much luck so far.  I have a couple of leads and some race guys in the UK also looking.
The CB125J was a popular choice std 56.5mm with a decent dome etc, these are now no longer available. the CB250 piston is another one but it has a very high dome and might create clearance problems.

I have contacted Wiseco about making me some and they basically didn't want to know saying they can't get rings....but I do know guys in Holland that bought 3 ring full skirt 56mm pistons from them with rings and all. Maybe someone could check with them again.

 Also spoke to a very helpfull chap at JE pistons, He explained that they would have to machine them from a larger forged billet, there would be set up and design costs etc plus these would be without rings. Around $150.00 per piston

I am mow taliking to a co in the UK that will make the 2 ring slipper piston with rings,pins clips the whole kit at around 150.00 pounds each. bit expensive, but when you need em you need em. there is also min qty's involved.

Turbo the piston in the pic with sizes, if you compare it with the yoshi 2 ring slipper' yoshi in brackets.
Height 47.1mm (47.6)
Dome 1.9mm (3.3)
Center of pin to top 22.2 (23.5)

Kevin

[turboguzzi]

Bwaller, Kevin
From this year on, the CB400F are allowed into the vintage 500 class I race since it has become more than 30 years old model. Still, after all the reading I done on them (no experience) it looks like it's a lot of effort to make them competitive and even bigger effort to make them stay together.... See the article in sohc4. Was also quite surprised to see that their stroke is pretty long, 50 mm against 50.6 for the CB500, so not too much to be gained in terms of keeping piston speed down at higher rpm.

On the other hand, with their 51mm original bores, original valves and seats must be smaller than the 500's so in order to breath well with 56mm pisotns at 12-13K you are looking straigh away into major headwork. So I guess that after fitting carillos, bigger valves and seats, maybe re-angling the valve guides too, etc, you will end up with a strong 490 motor, six speed gearbox and maybe 20 lbs less weight. Not sure about the handling though, with a single dwontube it looks a tad less rigid, but again didnt ride those. what's sure is that I cant afford the thing!

Kevin, dont know if this applicable to your 400 but I managed ot get 12:1 on my CB500 even with stock flat top pistons by maching the block by 1.2mm and using a 0.5mm thinner copper gasket for an overall 1.7mm more comp.

BTW, what wrist pin dia does the 400f have?

cheers
TG

[GroovieGhoulie]

400F should have a 13mm gudgeon pin.  Yoshima's and RSC pistons and such had 15mm pins, with the rods to match.  Carillo makes rods for the 400F, but I'm sure they come in 13mm sizes, so then the problem comes, where to get competition-grade, 16K-rpm rods with a 15mm small-end?

[bwaller]

GG,  for rods ttr400 can probably save the day. Team Hansens also offered me a set of beauty rods! I expect both these will have a 15mm small-end.


And Turboguzzi I agree with you about the cost of a done-up 400. I'm only thinking of getting back in the game for my enjoyment, which probably won't include "selling the farm" to get there. Those 400 race bike sure are pretty though!

[ttr400]

TG thanks for that info, I normally go for 10.5-11.0-1 CR as we don't have acces to decent race fuel anymore. I also built a 400 motor with the CB500 flat pistons, was a pretty strong motor.
GG, Carrilo and the rods from Team Hansen can be ordered with 13 or 15mm small ends.
I have a set of Honda RSC rods in my racer with the 56mm 2 ring slipper pistons. 10.5/1 cr.
The little 400f's handle real nice, I think it's probably the best handling of the Honda 4's.

I was chatting to a guy in the UK a couple of weeks ago who races a worked on CB500, his bike was out of action so he road his mates 400 racer, he lapped 2-3 secs a lap quicker..same track. I'll see if I can find a pic of his 400, it's an ex Isle of man racer from the late 70's that he used to race back then and found the bike again and brought it back to life.

Photo of a 400F knife edged crank with Carrilo rods.

[bwaller]

We've all seen Kevins very sharp 400F race bike, here are a couple pics of Team Hansens bike. I'm sure Terry wouldn't mind my putting them up here.

[ttr400]

Yep those little RC replicas are certainly beautiful machines, In fact that is the bike I got to ride at Mosport in Canada. It has a 350F motor and was a blast to ride.

Found a pic of Daves CB400F ex Isle of man racer.

[turboguzzi]

cool bikes! wish mine looked half as good. Strange how those 29mm CR's look bigger than the cylinder head!

I can certainly believe that with similar power potential, six speeds and 20lbs less weight, a 400/490 would be faster indeed. still, seems like the ticket for a real fast 500 vintage honda is a twin as Todd Henning proved time and again. The italian 500 championship I compete in was won this year by a very trick CB500 twin, though most of the other guys down the final rating were on 500-4's.

TTr, I think that if you manged to build into your 400 a squish area ring like i did in mine you could go up to 12:1 even on pump gas, that's at least my experience. Domed pistons for high CR create the dreaded "orange peel" combustion chamber shape that is very prone to detonation. Care to send a pic of your combustion chamber? have no idea how a 400's look.

TG