Topic: 64.75mm pistons in a cb550

[paulages]

hey guys. i've been posting about my engine work in my project thread, but thought i might get a little more input from the engine builders here. some of you might remember that my 64mm TRW pistons i was running in the 718 ended up out of spec to the cylinder walls after removing the liners to have the porous cylinders sealed. Soos hooked me up with some 64.75mm pistons, which i've been mocking up now.

the piston shoulder comes level with the deck, so piston-to-head clearances are the thickness of the head gasket, which is .043". i clayed one piston and found about .030" clearance between the intake valve and piston, but before i go worrying too much about that, i want to time the cam properly and check the clearances with a dial indicator on the valve stem.


the compression ratio is my current puzzle. i measured TDC and BDC with dyed alcohol, but at TDC there's so little volume i fear that the bit displaced by the spark plug is enough to throw off the numbers.

i decided to measure the chamber volume using plexiglass with a couple holes drilled in it to fill and to allow air to escape. i found 25cc exactly. then, i measured the piston crown volume, but lowering the piston into the bore until the highest point of the crown was level with the top of the bore, and measured the amount of fluid it took to fill it. this is the "X" area below (subtracting the volume in the valve pockets).



the volume of the valve pockets below the piston shoulder is .15cc (the red areas in the diagram below). not much. the volume of the bore with the piston crown level with the top of the deck is 30.62 (64.59mm X 9.35mm). we'll call this "Z." subtracting the valve pocket volume from the measured negative space volume (X), i got 22.35cc. subtracting this from "Z" and we find the dome volume ATDC, or "Y" below, to be 8.27cc.

subtracting the bore volume from the chamber volume (25cc), gasket section volume (3.37cc), and the valve pocket volume (.15cc) I got 20.25cc ATDC.

add this volume to the displacement volume at BDC, and we get 201.5cc. again, close to my real measurements. divide the two, and you get 9.95/1 CR.

i'd like to see it just a little higher, but that will require a different head gasket at this point, and i worry about the intake valve clearance. this bike will hopefully be raced, but is my street bike as well.


any input? am i missing anything?

[bwaller]

Cutting deeper valve reliefs will cost you some CR too, and sounds like that needs to happen. If you use a 0.032" gasket, mill 0.010" off the head, cut the reliefs, it should end up near 10.5:1.

[paulages]

Cutting deeper valve reliefs will cost you some CR too, and sounds like that needs to happen. If you use a 0.032" gasket, mill 0.010" off the head, cut the reliefs, it should end up near 10.5:1.

thanks, brent. knew i could count on you here as well. 

i installed a clutch spring on one intake valve to make depressing it easier, and am checking clearances with the cam timed. roughly timed, there is lots of clearance. i'm about 10 degrees off right now though and have to work in the morning, so i'll have to pick back up tomorrow. i'm crossing my fingers for big fat clearances so i can simply have the head skimmed...

[paulages]

Cutting deeper valve reliefs will cost you some CR too, and sounds like that needs to happen. If you use a 0.032" gasket, mill 0.010" off the head, cut the reliefs, it should end up near 10.5:1.

my math shows that .017" less between the gasket and head will give me 10.55/1 CR. easier to order a .026" head gasket, but quicker to have the head skimmed...

[kayaker43]

Let us know what the pistons are from, and the final mods when you get them done?

[paulages]

Let us know what the pistons are from, and the final mods when you get them done?

i'm not sure what they're from. they say ART on them, and appear to be honda pistons, but if they're 3rd over CB350T pistons, the ring lands are different than STD ones i have in the shop. they came out of a CB750 engine Soos acquired. maybe he'll chime in here. i can certainly measure and describe what i have.

[paulages]

ok, i have the cam timed at 106 lobe centers, and the closest i can find the valve to the piston is .077". i've rotated the engine through several times, checking the clearance every 5 degrees. if .077" is accurate- and i will recheck it several times- then, .017" off and .050" clearance is something i can live with.

i only had a half hour to play around at the shop this afternoon.. hopefully i'll find time tomorrow.

[turboguzzi]

ok, i have the cam timed at 106 lobe centers, and the closest i can find the valve to the piston is .077". i've rotated the engine through several times, checking the clearance every 5 degrees. if .077" is accurate- and i will recheck it several times- then, .017" off and .050" clearance is something i can live with.

i only had a half hour to play around at the shop this afternoon.. hopefully i'll find time tomorrow.

hei paul

late into this, not sure that I am following all your measurements in imperial, so lets just say that as long as the motor doesn't explode, you should be fine

seriously, much bigger bore air cooled ducs live with that CR today,

TG

[paulages]

late into this, not sure that I am following all your measurements in imperial, so lets just say that as long as the motor doesn't explode, you should be fine

seriously, much bigger bore air cooled ducs live with that CR today,

TG

true, some 12/1... can't help but wonder what other factors contribute though. i know some 4 valve air-cooled ducs run such high CRs... they have nice computers telling them when to go bang though, no? believe me, i'd love to push this CR as high as possible, i just don't want something that will not be able to run on pump gas...


what octane do you run at the track?

[turboguzzi]

4valve air ducs? dont think there ever was one other than the failed 70's GP racer....

What kind of octane do you have in your area's pumps these days?

TG

[Doctor_D]

In the states we have 92/93 octane, but it's a combined rating system that uses the Research Octane plus Motor Octane divided by two.  This is lower than the European number for the same gas, because -- as I understand it -- they use only the RON. So, generally the octane number shown in the US is about 5 points lower than in Europe.

Long story short, when we say 92 -- think 97.

[paulages]

do you ever have those... "oh. yeah..." kind of moments?  

i was depressing the valve stem via the tappet adjuster locknut and fighting fitting my screwdriver in there alongside the dial indicator, when it occurred to me that with the 650 valve covers off, i could simply pry the heel of the rocker arm where it contacts the cam until the valve contacted the piston, and simultaneously turn the engine through to find the tightest valve to piston clearance.

anyway, long story short: i have .094" clearance between the intake valve and the piston. i need a .026" head gasket to get me in the 10.5/1 CR range. i was going to have the head decked, but the dome begins immediately and the diameter of the outside of the chamber is significantly less than that of the head gasket area, and without doing the math i believe i'd have to take off closer to .025". i' m annoyed at having to buy another head gasket and wait, but it seems like the best option.

[paulages]

i feel like i'm talking to myself here, but in case anyone cares.. .043" is the thinnest cometic can go in a fiber gasket, but they have .027" available in copper. this will put my CR at 10.6/1, which is what i at least believed it to be with the old pistons. more waiting, but i feel better about this than milling the head. plus, with a copper gasket at least i can anneal and re-use it.

EDIT: i forgot that the 66mm bore copper gasket will not crush like the composite one does, and so the bore of the gasket area will actually be 66mm, not 65.5mm. rechecking the math, this will give me a 10.52/1 CR. perfect. 

[bwaller]

Talking to yourself.... nah we're listening.

I bought some 0.010" viton coated steel base gaskets on Mikes suggestion, I can't remember how many thicknesses they offered. It's something else you might have done to end up with the pistons slightly proud of the deck to bump compression and use a thicker composite head gaskets.

Oh never mind Paul, I don't mean to torture you!  

[Retro Rocket]

i feel like i'm talking to myself here

Always reading Paul....love your work...

Mick

[paulages]

Talking to yourself.... nah we're listening.

I bought some 0.010" viton coated steel base gaskets on Mikes suggestion, I can't remember how many thicknesses they offered. It's something else you might have done to end up with the pistons slightly proud of the deck to bump compression and use a thicker composite head gaskets.

Oh never mind Paul, I don't mean to torture you!  

the problem with having the pistons proud of the deck is that calculating what the CR will be is much more difficult, at least without measuring differently. as is, the flat part of the piston is exactly level with the deck, so it's only the dome that is displacing space in the combustion chamber. i could have still figured it out, but this was easy. either way, i'd have had to order gaskets.

[paulages]

Paul, while you have your motor apart can I come look at what you have done to the internals. My buddy and I have been slowly acquiring parts over the last year to convert his cb500 to a cb550/650/750 I could use some advice on what to do.

sorry, but i don't have much of it apart at this point... the cases are sealed, though the bottom end doesn't really look any different than stock. i can take whatever pictures would help, but i think most of what you need is here:

http://forums.sohc4.net/index.php?topic=40414.0

edit: just noticed you said "come" see... are you in portland?

[paulages]

for a few months. I got a job here so I'm back in the old hood for a while until school starts again.

i'll PM you my shop address and phone number. you're welcome to come look at what's still open.

[turboguzzi]

i feel like i'm talking to myself here, but in case anyone cares.. .043" is the thinnest cometic can go in a fiber gasket, but they have .027" available in copper. this will put my CR at 10.6/1, which is what i at least believed it to be with the old pistons. more waiting, but i feel better about this than milling the head. plus, with a copper gasket at least i can anneal and re-use it.

EDIT: i forgot that the 66mm bore copper gasket will not crush like the composite one does, and so the bore of the gasket area will actually be 66mm, not 65.5mm. rechecking the math, this will give me a 10.52/1 CR. perfect. 

hei paul,

copper gasket works fine for me but for a few buts....

1. dont remember if you have it already, but better go for external oil supply to the cams for less leaks
2. Expect the joint to "sweat" a bit...
3. You'll find you'll need to keep an eye on stud's tension
just my 0.02

TG

[paulages]

thanks TG. i do have the external lines already, so at least the gasket will be less likely to leak. i'll be sure to anneal it well before use.

[bwaller]

Shouldn't have to when new Paul.

I used one on Debs 592 and although it barely shows, as TG mentioned it does sweat even with the X-lines. I may switch back to a composite if it decides to get worse.

[paulages]

Shouldn't have to when new Paul.

I used one on Debs 592 and although it barely shows, as TG mentioned it does sweat even with the X-lines. I may switch back to a composite if it decides to get worse.

on brit bikes i always anneal the copper, even when new, just to be safe. i've been told that one should be able to make an imprint in the copper with your thumbnail if it's fully annealed, but i don't usually actually get them quite that soft. these are very thick gaskets compared to the .027" i'll be using, but there is always a very noticeable difference in how soft the metal is. granted, these are not cometic gaskets... i meant to ask them whether they recommend it or not.