Topic: Pistons & Project Akiyama - to lighten or not to lighten...?

[lordmoonpie]

I'm almost at the stage of engine build for my RC162 replica project - Project Akiyama! There are three major areas for me to sort before I can start mockup and engine build, rear brake, rear suspension and pistons. The stock pistons look OK and are good for 19,000 rpm. I have an opportunity to have some pistons made that are identical to stock in terms of the crown but will be 25% lighter. My feeling is that anything that is a reciprocating mass would be better if it was lighter but given that they are expensive items, what's the view on whether this saving is worth having or not?

I want my engine to reach 20,000 rpm, some I know of reach 22,000 so less piston mass just has to be better.....or does it? What's your view?

[Sam Green Racing]

Simon, there are a few people that have tried that and found that the pistons hit the valves when exceeding Hondas peak power revs, it's due to con rod strech. They're your expensive pistons, clatter them if you must.

Sam.

[turboguzzi]

Do your mean piston speed calculation, it's the best predictor for engine health, try to stay under 25 m/sec for god's sake....

MPS = 2 * Stroke (in meters) * RPM / 60

TG

[MRieck]

 Always go for the lightest piston/rod combination you can afford. Obviously it has to be structurally sound. Personally I think Carrillo should make the CB750 rods in the "A" beam design....they would be well under 300 grams per rod. I figure probably280 grams per rod. Nice.

[Doctor_D]

Try to stay under 25 m/sec for god's sake....

Thanks, I was trying to recall the piston speed limit the other day.

In any case, don't think I'll ever run the CB750 to 12,254.7 rpm's - unless I miss a shift... 

[754]

 I have run close to that.. or to that.. with stock rods..

 more times than I could count..

 Early big bore kits for 750s used modded CB 350 2win pistons. these on a cammed 350 can easily hit 12 -14,000 rpm..

[lordmoonpie]

I have run close to that.. or to that.. with stock rods..

 more times than I could count..

 Early big bore kits for 750s used modded CB 350 2win pistons. these on a cammed 350 can easily hit 12 -14,000 rpm..

Well the 250 engine stock rods are good for 22000 rpm for sure, thee's very few paraders who change from stock rods. I need to do that calculation though up above :-)

[turboguzzi]

I have run close to that.. or to that.. with stock rods..

 more times than I could count..

 Early big bore kits for 750s used modded CB 350 2win pistons. these on a cammed 350 can easily hit 12 -14,000 rpm..

Well the 250 engine stock rods are good for 22000 rpm for sure, thee's very few paraders who change from stock rods. I need to do that calculation though up above :-)

how long do you think it'll take you?



TG

[MRieck]

I have run close to that.. or to that.. with stock rods..

 more times than I could count..

 Early big bore kits for 750s used modded CB 350 2win pistons. these on a cammed 350 can easily hit 12 -14,000 rpm..

Well the 250 engine stock rods are good for 22000 rpm for sure, thee's very few paraders who change from stock rods. I need to do that calculation though up above :-)

how long do you think it'll take you?



TG

You just had to ask that question TG.

[lordmoonpie]

I have run close to that.. or to that.. with stock rods..

 more times than I could count..

 Early big bore kits for 750s used modded CB 350 2win pistons. these on a cammed 350 can easily hit 12 -14,000 rpm..

Well the 250 engine stock rods are good for 22000 rpm for sure, thee's very few paraders who change from stock rods. I need to do that calculation though up above :-)

how long do you think it'll take you?



TG

You just had to ask that question TG.

Ages - I'm well in the middle of fitting the Dyna 2000 to the CR750 now plus started on the XV750 engine build....it's a hard calculation to make when you have such distractions 

[lordmoonpie]

I still haven't done the calculation BUT, a bit more research seems to indicate that changing the stock pistons is a no-no and they're good for 22,000 rpm so why bother. Yes I can get lighter pistons but at a cost of £200 GBP each and months of waiting for a super slow responding company, I'm not going to bother.

The research further reveals that these little engines are nigh on bomb proof out of the crate. A commercial builder takes donor engines and only changes the conrod bolts and amazingly the head bolts on a full rebuild. Even the bearings tend to be OK after 40,000 miles as a road bike. The head bolts in these are funny dodechahedral headed things and appparently they are torqued to a point where once loosened they don't go back without risk of metal fatigue. They can apparently sit on the bench and just go "ping" and shear of their own accord!

So - no progress on the 750 ignition but I have rebuilt the XV750 engine up to a  point where I'm waiting for pistons from Japan (might be a while!) but the good news is that all the conrod bolts and head bolts are on their way to me for the 250 engine so the build starts this week I hope and pictures will finally follow. I think I have most of the parts to get the lump together so it'll be quick for me (that's a relative term!!). Of course I haven't yet decided whether to go for the modified central sump which is a major re-engineering job. The idea is to take the offset stock sump and replace it with a central one allowing symmetrical exhaust pipes....one day I'll take the easy route 

[turboguzzi]

what about some pixies?

 

TG

[lordmoonpie]

I might have to do pictures of the super clean pistons! The donor engine had done a few miles, a few thousand miles but the pistons were OK size-wise. The tops though were very sooty and had a thick coating of hard carbon on them. Someone recommended that I tried getting this off using a brush on oven cleaner carbon remover that was kind to aluminium. Brushed it on, left for 24 hours and washed it off with a little help from an old toothbrush - bingo! Bright shiny carbon free piston crowns.

Now that was a good tip whoever told me originally...