Rebore - .040= + Compression ?

G day all,... Can any one tell me how much the compression is raised by a simple rebore, or better still, from standard.
My Combat is going back together with a .040 rebore.
I am already concerned about compression ratio for these old girls, and don't want any more than necessary to keep the cam happy.
Pistons and gaskets, are from RGM, who were very good to deal with.
Thanks AC.

A rebore won't change the compression ratio. EDIT: shouldn't change it much :? . Now if you skimmed the head or decked the barrel, well, that's another story.

AussieCombat said:

G day all,... Can any one tell me how much the compression is raised by a simple rebore, or better still, from standard.
My Combat is going back together with a .040 rebore.
I am already concerned about compression ratio for these old girls, and don't want any more than necessary to keep the cam happy.
Pistons and gaskets, are from RGM, who were very good to deal with.
Thanks AC.

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Boring a combat .040 oversize will raise the compression by .33 point. If it was 10-1 before it will be 10.33 to 1 after unless the deck height of the piston has been changed. Jim

Interesting non-obvious phenomena to me Jim. How does boring a bit bigger diameter increase the CR if the stroke and cam and gaskets remain the same?
A bit better inhaling through same size valves?

Thanks Jim, I thought it had to do something. My pistons are 5.0 Millimetre ( .200 " ish ) thick on top, surely I can take .040" off, which I think is equivalent to one point, without hurting them and still have the engine working well.
AC.

Actually Hobby, I thought, before I asked, that a bigger hole and no change to stroke, would lower the comp, but a friend said the same as Jim. I'll take that.
Thanks again...AC.

Ok I"m glad you asked as the increase in filling efficiency by bigger bore will apply to my special going from 750 to 920 with same 750 valves/ports but ridiculous aggressive over lapped cam, which could use some help to start and idle down low.
Both my 2 Combats had .040 over jugs when I got them but no detonation in summer loaded climbs on 91 octane and full 28" timing. Hope yours gives nice deep sharp exhaust notes too.

Boring a 750 to +0.040 is changing the bore and stroke from the stock 73mm x 89mm to 74mm x 89mm.

Stock engine is 745cc,
+0.040 engine is ~765cc.

So apart from being illegal for 750cc racing, you have gained ~10cc per cylinder.

A moments thought reveals you are now compressing slightly more mixture into the same space, so the compression must go up, a shade. We'll take Jims word for the 0.33 number. Has no effect on the cams, of course.

hth.

displacement = {pi X rad squared} X stroke
compression ratio = displacement + chamber volume divided by chamber volume

Stock cylinder displacement is 372.48 cc
.040 oversize cylinder is 382.76

average Combat combustion chamber is 41cc

372.48 + 41= 413.48 / 41 = 10.08 cr

382.76 + 41 =423.76 / 41 = 10.335 cr

Since this is a Norton your actual compression may vary by a half point or so unless it has been worked on and then who knows. Jim

ps: add 1cc to the chamber volume for every .25mm increase in gasket thickness or decrease in compression height.

hm, ok, makes sense now d/t packing more mix into same head chamber. Implies Peel may have to use standard thick gasket not to detonate top off and fudge spark timing for 91 octane, maybe. But then again small chambers burn faster so maybe a wash.

My 72 Ran measured 8.7:1 C.R. , BSA advance unit gave extra 50 % retard. same fully Adv. 36 Deg. Total .
Combat Cam marked ' SS ' set 5 Deg early . .016 Clearance !! and 32 Mk 2 Amals . Idled at 500 rpm all day.
went like um , fast . Bit of megaphoneitis @ about 3.200 .But camed up ( 1/2 WFO + ) ran astonishingly from 3.500
or even 3.000.
Also had hard advance springs , and 23 tooth sprocket .
Not fussy on throttle setting below 3.000 , but above that reqired ones attention .

Riding quitely touring , if you got it down to 60 mph , itd be back at 80 in a few minutes if you didnt keep an eye on things.
Needed to clean clutch periodically with that gearing too .

W&S valve spring set . Would go straight past 7.000 to , though I never let it.saw v7.2 once or twice .

Matt Spencer said:

My 72 Ran measured 8.7:1 C.R. , BSA advance unit gave extra 50 % retard. same fully Adv. 36 Deg. Total .
Combat Cam marked ' SS ' set 5 Deg early . .016 Clearance !! and 32 Mk 2 Amals . Idled at 500 rpm all day.
went like um , fast . Bit of megaphoneitis @ about 3.200 .But camed up ( 1/2 WFO + ) ran astonishingly from 3.500
or even 3.000.
Also had hard advance springs , and 23 tooth sprocket .
Not fussy on throttle setting below 3.000 , but above that reqired ones attention .

Riding quitely touring , if you got it down to 60 mph , itd be back at 80 in a few minutes if you didnt keep an eye on things.
Needed to clean clutch periodically with that gearing too .

W&S valve spring set . Would go straight past 7.000 to , though I never let it.saw v7.2 once or twice .

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Hobot, have you got a brother?

Seems so as Ms Peel Combat behaved similar to Fullauto's description, constant attention to feather throttle or quickly excess ticket to jail speed even climbing steeps loaded to the gills kept trying to shift to 5 th over drive to ease the too easy acceleration. The pull got an extra piston worth @ 6800 and steepened till I got scared by 7500. Only one's more surprised by 2nd gear thrills were the hi hp sport bikes climbing the sky between turns while Ms Peel merely barely floated front
leaping out ahead of them - and with the rod links and fork mods didn't have to let off blasting towards and out of apexes. Accidental combo in my case so no claim to be smart tuner but am so sold on smallish passages if not going for land speed max out. Seriously only took one place over 45' lean and game was over and back to solo thrills. Seriously if not for freeway like sections here for speeds over 135 my 750 Combat with standard 750 head was plenty for me. Glad to know I'm not the only one flabbergasted by Norton potential even in today's crowd. hobot

I thought that original manufacturers oversize pistons for rebores had reduced crowns to maintain the standard compression, unless specified higher ?

Cash

My emergency backup clapped-out 750 barrel was recently bored out to plus .060"--there was a surprising amount of meat left--which means the new displacement is 776.43cc and a new compression ratio of 10.47-to-1 with a standard head milled .040"--plus the .0045" bore clearance. It should be interesting to see what kind of performance I get with 932's, 32 X 30 mm manifolds, a Combat cam, balanced pipes and Dunstalls.

It looks like there's plenty of metal left for another couple of overbores--maybe even out to 77mm on this particular barrel--has anyone done that?


Tim Kraakevik
kraakevik@voyager.net
'72 Combat, '74 RH10 850

I tried one at .080 once and it cracked above the RR hold down bolt. I had a customer bring one in recently that had been bored .200 over size and sleeves had been installed to go back to stock.. It was cracked and leaking at both rear bolt holes. Jim

My emergency backup clapped-out 750 barrel was recently bored out to plus .060"--there was a surprising amount of meat left--which means the new displacement is 776.43cc and a new compression ratio of 10.47-to-1 with a standard head milled .040"--plus the .0045" bore clearance. It should be interesting to see what kind of performance I get with 932's, 32 X 30 mm manifolds, a Combat cam, balanced pipes and Dunstalls.

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With that fast flow bigger bore 10+ CR Combat set up it should run out from under and away from most sport bikes until ya snick 3rd around 90 mph and top 130 mph.