Author Topic: Factory based cylinder for 73mm pistons.  (Read 3789 times)

0 Members and 1 Guest are viewing this topic.

Offline gschuld

  • Master
  • *****
  • Posts: 2,346
Factory based cylinder for 73mm pistons.
« on: April 10, 2018, 08:32:22 pm »
Fair warning, this will be LONG!  :o (sorry in advance)

So in the interest of pushing boundaries ::), I’d like to see if I can reliably put 73mm pistons in a factory cylinder.  The safe bet is to stay at 72mm or smaller in a factory cylinder, but I like a challenge...

For some reference, the pics below are of my vintage RC prepared big bore factory cylinder(at current 70mm bore) with their standard RC sleeves.

The sleeves are 3.000” OD(which I believe were the standard for (70-73mm at RC).  FWIW, this leaves roughly .100” wall at 71mm and .080” wall at 72mm.

.080” wall is generally accepted as the minimum wall thickness for acceptable sleeve rigidity(resistance to distortion).  Going to 73mm with these sleeves gives roughly .060”, not good.  It seems RC got away with 73mm pistons in those sleeves in their special RC cast block cylinders because they were essentially a solid block of aluminum with little more than cosmetic fins.  So presumably the great rigidity of the block itself helped helped keep bore stability in check with the extra thin walls of the RC 3.000” OD sleeves.

Now when you run sleeves that thin(+/-.060” at 73mm bore) in a factory block, it becomes a potential problem.  Most obvious is the break through of the air gap areas between 1/2 and 3/4 when boring the cylinder to take larger 3.000” OD sleeves.  The silicone injected in there(after the new big sleeves are installed) is intended to deal with the newly created oil seepage problem, but does nothing to support the integrity of the block/sleeve itself.  So the new unsupported area in that section of the cylinder block doesn’t give even support for the sleeves, so combined with extra thin sleeve wall at .060” and the natural result is piston bores that don’t want to stay round as the sleeves are inclined to distort at least somewhat.  This explains why RC in one of their early catalogs specifically tried to discourage customers from using 73mm pistons in factory cylinders with their sleeves(many did it anyway and lived to tell the tale)

After coming up with my attempt at a solution for the 1/2 3/4 gap issue, I discovered that my idea wasn’t so new after all.  I came across a post from Mec from Austria from 10 years ago where he has seen the entire 1/2 3/4 gap areas filed with Devcon with good success for big bore road race engines.  This does two things, if the gap is filled COMPLETELY(without voids) it both seals the break through areas from oil leaks AND should be sufficiently rigid and machinable that is acts like a solid casting in this area, so it should have a very positive effect on supporting the sleeves.  I used Marine Tex BTW, but the results should be comparable. 

So, doing some basic math, I figure I need 3.035” OD sleeves to allow a reasonable piston/bore clearance at 73mm to end up with .080” wall or greater.  At .080” sleeve wall, and “solving” (hopefully) the biggest weak link with the 1/2 3/4 gap break through areas, is it reasonable to assume I can get away with 73mm bore and be confident that the sleeves will hold their shape(at .080” wall)and stay nice and round for me?  I seem to have 1/8” or greater on the rest of the oil return spaces bordering the RC 3.000” OD sleeves, so going to 3.035” OD shouldn’t effect that much.

FWIW, I’m pretty sure Brian(Bear) has been running 73mm pistons in a modified factory based cylinder with something like 13:1 compression, stroker crank, big cam, running methanol with a dyno reported 135 RWHP thrashing around the tracks of Australia in a sidecar racer(about the most punishing use for a sohc4 race engine I can think of) and somehow hasn’t seemed to have had trouble with his bore stability.  Granted, methanol has a great natural cooling effect so that may help some by keeping cylinder/sleeve temps down.

If anyone managed to stay awake through all that and has enough energy left to offer some insight or thoughts, I’m all ears. ;D

George



« Last Edit: March 22, 2021, 01:28:42 pm by gschuld »

Offline dragracer

  • Really Old Timer ...
  • *******
  • Posts: 5,020
  • CB750F Dragbike
Re: Factory based cylinder for 73mm pistons.
« Reply #1 on: April 11, 2018, 12:14:38 am »
Thanks for your efforts George. This could save a budget builder a few $$ if he knows how to press out the donor sleeves. I'll keep this in mind when I freshen up my red F model. The sleeves are worn out at this point and crying for attention.

Offline gschuld

  • Master
  • *****
  • Posts: 2,346
Re: Factory based cylinder for 73mm pistons.
« Reply #2 on: April 11, 2018, 03:10:37 am »
Frankie,

Yes, there is more to the story regarding sleeves, etc.  But for now, I’ll keep the subject contained a bit.  I’m most concerned at this point on whether by attacking what I have surmised to be the two major reasons taking a factory cylinder to 73mm is risky that I can now get away with it and not have bore distortion issues.  Perhaps in drag racing, there is less concern here given the very short run time of the motor(measured more in minutes of hard run time than hours or miles) but for other applications such as road racing, boosted engines, or street use is entertained, good bore stability is certainly a benefit for a healthy reliable engine.  But it sure couldn’t hurt for drag engines too😉

Mike’s killer Billet block is basically the ultimate designed in answer for both proper cooling and bore stability up to 72mm(plus it’s so damned pretty👍) the cast RC block has bore stability in spades but zero effective cooling capacity.  For those attempting both with a factory cylinder, it gets a little more challenging.

Offline bear

  • Vale Bill McIntosh ......"illegitimi non carborundum"
  • Old Timer
  • ******
  • Posts: 2,570
  • Leeton in Australia
Re: Factory based cylinder for 73mm pistons.
« Reply #3 on: April 11, 2018, 04:02:00 am »
Good post George, ;)

I'm impressed with your efforts, well done.
As a bit of a postscript, we had a go at the RC cylinders some years ago.
Unfortunately used in our application they seemed to be susceptible to cracking .

Cheers,
Brian
 
The older I get the faster I was.

Offline gschuld

  • Master
  • *****
  • Posts: 2,346
Re: Factory based cylinder for 73mm pistons.
« Reply #4 on: April 11, 2018, 04:48:22 am »
Brian,

I have seen RC cast blocks being used in road racing application from back then, but it seemed pretty rare.  I’m certain Russ Collins didn’t have road racing in mind when he had those cylinder blocks produced.🧐.  Then again, your application seems to be particularly severe😳

George

Offline Tom

  • Enthusiast
  • **
  • Posts: 134
Re: Factory based cylinder for 73mm pistons.
« Reply #5 on: March 20, 2021, 07:34:40 pm »
Hay George, interesting subject. I just bought an old drag bike and was surprised to find 73mm pistons in it. They used an early sandcast barrel the one without the extra head bolt between 2/3 and have drilled and tapped the barrel for the extra bolt. I have in the past had a RC 970 70mm engine I built back in the seventies it suffered from oil leaks from between 1/2 and 3/4 where you could see the sleeves, later I had another 71mm engine that had been gooped up with JB weld and it leaked oil bad too, then I bought an RC Cast block but no good for the street. On this bike I just bought this engine has no oil leaks I figured it was a 900 67mm engine but upon opening and finding 73mm MTC pistons and the top of the sleeve is grooved and fitted with a paino wire squish ring. These early barrels are much thicker and have more meat for big sleeves. Cheers Tom
'59 T120.,'70 T120R Had one new.,'69 CB750 Oz Sandy Ruby Red.,'69 CB750 Oz Sandy Blue Green.,'70 CB750 K0 Candy Gold.,'71 CB750 K1 Valley Green I bought new.,'71 CB750 K1 Candy Garnet Brown.,'76 Rickman Honda CR750 R.C. Eng Cobra Powered. Two CB750 Choppers a Amen Saviour and a a Santee rigid arm Softail. A ‘72 Z1 Jaffa, and lotsa fun Honda Monkey bikes.

Offline 754

  • Really Old Timer ...
  • *******
  • Posts: 29,058
Re: Factory based cylinder for 73mm pistons.
« Reply #6 on: March 20, 2021, 08:16:51 pm »
If i can find my sleeves I will measure , i think yhey were bigger diameter.....han the sleeves in a 970 block i have .
 And I know they needed additional machining.. If I recall . But i know diamter was larger. 
Maker of the WELDLESS 750 Frame Kit
dodogas99@gmail.com
Kelowna B.C.       Canada

My next bike will be a ..ANFOB.....

It's All part of the ADVENTURE...

73 836cc.. Green, had it for 3 decades!!
Lost quite a few CB 750's along the way

Offline Medyo Bastos

  • Master
  • *****
  • Posts: 1,520
  • Gusto mo titi ko?
    • project
Factory based cylinder for 73mm pistons.
« Reply #7 on: March 20, 2021, 09:05:04 pm »
I have a set of 73mm ID sleeves i took off an RC big block. I can pull them out from under my bed


Sent from my iPhone using Tapatalk
« Last Edit: March 21, 2021, 09:17:13 am by Medyo Bastos »

Offline johno

  • Expert
  • ****
  • Posts: 1,302
Re: Factory based cylinder for 73mm pistons.
« Reply #8 on: March 21, 2021, 02:46:04 am »
Good post George, ;)

I'm impressed with your efforts, well done.
As a bit of a postscript, we had a go at the RC cylinders some years ago.
Unfortunately used in our application they seemed to be susceptible to cracking .

Cheers,
Brian

Bear correct me if I'm wrong but your best engine had a custom cast barrel ??? lucky bugger ;D
johno
GRASSHOPPER SOHC HONDAS ARE THE MEANING OF LIFE.

Offline bear

  • Vale Bill McIntosh ......"illegitimi non carborundum"
  • Old Timer
  • ******
  • Posts: 2,570
  • Leeton in Australia
Re: Factory based cylinder for 73mm pistons.
« Reply #9 on: March 21, 2021, 05:43:48 am »

Bear correct me if I'm wrong but your best engine had a custom cast barrel ??? lucky bugger ;D
johno

Our best engine was our little engine it was a joy to race but we used a std barrel with the air gaps filled for it.
But our big engine used a solid barrel, it was the last of a couple that a bloke sand cast for me many years ago.
That engine was a monster, all it wanted to do was bit me on the ass. I never really raced that engine, I wrestled it.
It was a fast big track engine though. ;D
The older I get the faster I was.

Offline gschuld

  • Master
  • *****
  • Posts: 2,346
Re: Factory based cylinder for 73mm pistons.
« Reply #10 on: March 21, 2021, 09:01:22 am »
The custom cast cylinders like Bear used and Japauto made were ideal (for castings anyway) as they offered both more physical material to support the sleeves and a good bit of fin material for cooling.

Mike’s billet block goes another step fwd of course.  Even more fin area, Laverda like, and a modern Nikasil bore up to 72mm.  It’s a shame it’s not allowed for road racing almost universally😕

For most vintage road racing classes that I am aware of, rules stipulate the cylinder blocks must be OEM.  RPE in Australia went so far as to bore out the 1/2 and 3/4 sections completely and inserted hand cast slugs of Siamesed aluminum. Then bored for big sleeves and redrilled for studs.  Yikes.

The casting Bear used for his big motor were very nice 👍

George

Offline Medyo Bastos

  • Master
  • *****
  • Posts: 1,520
  • Gusto mo titi ko?
    • project
Re: Factory based cylinder for 73mm pistons.
« Reply #11 on: March 21, 2021, 09:19:56 am »
I have a set of 73mm ID sleeves i took off an RC big block. I can pull them out from under my bed


Sent from my iPhone using Tapatalk
LA Sleeves logo










Sent from my iPhone using Tapatalk

Offline 754

  • Really Old Timer ...
  • *******
  • Posts: 29,058
Re: Factory based cylinder for 73mm pistons.
« Reply #12 on: March 21, 2021, 09:50:18 am »
3 inch and a few thou.
Maker of the WELDLESS 750 Frame Kit
dodogas99@gmail.com
Kelowna B.C.       Canada

My next bike will be a ..ANFOB.....

It's All part of the ADVENTURE...

73 836cc.. Green, had it for 3 decades!!
Lost quite a few CB 750's along the way

Offline Don R

  • My Sandcast is a
  • Really Old Timer ...
  • *******
  • Posts: 19,079
  • Saver of unloved motorcycles.
Re: Factory based cylinder for 73mm pistons.
« Reply #13 on: March 21, 2021, 11:10:02 pm »
 I was thinking some of the shops bored the block and welded up the holes, then bored for the sleeves again.
No matter how many times you paint over a shadow, it's still there.
If you love it, set it free, if it stays it's probably one of my 750's.
 CEO at the no kill motorcycle shop.

Offline grcamna2

  • Not a
  • Really Old Timer ...
  • *******
  • Posts: 14,017
  • I love to restore & travel. Keep'em Going Strong !
Re: Factory based cylinder for 73mm pistons.
« Reply #14 on: March 21, 2021, 11:23:43 pm »
subscribed
75' CB400F/'bunch o' parts' & 81' CB125S modded to a 'CB200S'
  I love the small ones too !
Do your BEST...nobody can take that away from you.

Offline Tom

  • Enthusiast
  • **
  • Posts: 134
Re: Factory based cylinder for 73mm pistons.
« Reply #15 on: March 22, 2021, 03:33:31 am »
There were some goodies in the engine I got, MTC 73 mm pistons, RC golden rods, Lightened crank, a welded cam marked 369 which I believe is from an Australian local cam shop an Ivan Tighe copy of a RC327 and other bits under cut dogs trick clutch etc and a set of Dellorto carbs on a Lynx M301 manifold, I called Lynx in Melbourne today and new manifold rubbers are still available so I will get some.
'59 T120.,'70 T120R Had one new.,'69 CB750 Oz Sandy Ruby Red.,'69 CB750 Oz Sandy Blue Green.,'70 CB750 K0 Candy Gold.,'71 CB750 K1 Valley Green I bought new.,'71 CB750 K1 Candy Garnet Brown.,'76 Rickman Honda CR750 R.C. Eng Cobra Powered. Two CB750 Choppers a Amen Saviour and a a Santee rigid arm Softail. A ‘72 Z1 Jaffa, and lotsa fun Honda Monkey bikes.

Offline madmtnmotors

  • When did I get to be a
  • Really Old Timer ...
  • *******
  • Posts: 6,033
  • Sunny Central Florida
Re: Factory based cylinder for 73mm pistons.
« Reply #16 on: March 22, 2021, 10:22:22 am »
So in the interest of pushing boundaries ::), I’d like to see if I can reliably put 73mm pistons in a factory cylinder.  The safe bet is to stay at 72mm or smaller in a factory cylinder, but I like a challenge...

The sleeves are 3.000” OD(which I believe were the standard for (70-73mm at RC).  FWIW, this leaves roughly .010” wall at 71mm and .080” wall at 72mm.

The math threw me at first. I'm guessing you meant "0.10" wall at 71mm"?  8)
TAMTF...


Wilbur



Projects:
"Evolution": http://forums.sohc4.net/index.php?topic=100352.0
"P.O. Debacle": http://forums.sohc4.net/index.php/topic,126692.msg1441661.html#msg1441661
F2/F3 O-rings: http://forums.sohc4.net/index.php?topic=113672.msg1300721#msg1300721
Cam Tower Studs: https://www.mcmaster.com/#93210a017/=t19sgp
Clean up that nasty harness: http://forums.sohc4.net/index.php?topic=137351.msg1549191#msg1549191
                                           http://forums.sohc4.net/index.php/topic,148188.msg1688494.html#msg1688494
                                           http://forums.sohc4.net/index.php/topic,139544.msg1579364.html#msg1579364
                                           http://oldmanhonda.com/MC/wiring750K1.html
Charging system diagnosis: http://forums.sohc4.net/index.php?topic=1012.msg8345#msg8345
Get the manuals: http://manuals.sohc4.net/cb750k/
The Dragon: http://forums.sohc4.net/index.php?topic=45183.msg1571675#msg1571675
Headlight Switch: http://forums.sohc4.net/index.php?topic=113986.msg1283236#msg1283236
Branden's leak free top end thread: http://forums.sohc4.net/index.php?topic=107040.0
Engine Lifting Made Easy: http://forums.sohc4.net/index.php/topic,58210.msg1684742.html#msg1684742
                                      http://forums.sohc4.net/index.php/topic,100352.msg1675840.html#msg1675840
Static and Dynamic Timing: http://www.hondachopper.com/garage/carb_info/timing/timing1.html
Airbox Gasket Replacement: http://forums.sohc4.net/index.php/topic,114485.msg1290000.html#msg1290000
"Café" : http://forums.sohc4.net/index.php/topic,84697.msg953814.html#msg953814
PD Carb Choke Linkage: http://forums.sohc4.net/index.php/topic,100352.msg1669248.html#msg1669248
                                    http://forums.sohc4.net/index.php/topic,110931.msg1248354.html#msg1248354
                                    http://forums.sohc4.net/index.php/topic,48858.msg515204.html#msg515204
Follow up on your damn posts: http://forums.sohc4.net/index.php/topic,144305.msg1791605.html#msg1791605
Taiwanese Cam Chain Tensioners:  http://forums.sohc4.net/index.php/topic,155043.msg1774841.html#msg1774841
Gumtwo Seat Cover: http://forums.sohc4.net/index.php/topic,164440.msg1897366.html#msg1897366
Primary Drive: http://forums.sohc4.net/index.php/topic,166063.msg1919278.html#msg1919278
Tank Latch: http://forums.sohc4.net/index.php/topic,165975.msg1919495.html#msg1919495
Shorten your forks: http://vintage-and-classic-honda-s.456789.n3.nabble.com/How-to-shorten-forks-td4042465.html DO NOT CUT THE SPRINGS!
Clutch How To: http://vintage-and-classic-honda-s.456789.n3.nabble.com/How-to-change-and-adjust-a-clutch-SOHC-td4040391.html
Late model K7/K8/F2/F3 front sprocket cover removal: http://forums.sohc4.net/index.php/topic,178428.msg2072279.html#msg2072279
630 to 530 conversion: http://forums.sohc4.net/index.php/topic,180710.msg2094423.html#msg2094423

Sent from my Tandy TRS-80!

Offline gschuld

  • Master
  • *****
  • Posts: 2,346
Re: Factory based cylinder for 73mm pistons.
« Reply #17 on: March 22, 2021, 01:29:08 pm »
Corrected it above, thanks.🙄

Offline 754

  • Really Old Timer ...
  • *******
  • Posts: 29,058
Re: Factory based cylinder for 73mm pistons.
« Reply #18 on: March 22, 2021, 04:29:53 pm »
Found my sleeves, they are also at 3.0 plus a few thou.
 Pretty sure my  970 is a bit smaller diameter..
 The bores srecaround 69mm.
Maker of the WELDLESS 750 Frame Kit
dodogas99@gmail.com
Kelowna B.C.       Canada

My next bike will be a ..ANFOB.....

It's All part of the ADVENTURE...

73 836cc.. Green, had it for 3 decades!!
Lost quite a few CB 750's along the way

Offline sohc boy

  • Hot Shot
  • ***
  • Posts: 346
Re: Factory based cylinder for 73mm pistons.
« Reply #19 on: March 22, 2021, 05:11:00 pm »
There were some goodies in the engine I got, MTC 73 mm pistons, RC golden rods, Lightened crank, a welded cam marked 369 which I believe is from an Australian local cam shop an Ivan Tighe copy of a RC327 and other bits under cut dogs trick clutch etc and a set of Dellorto carbs on a Lynx M301 manifold, I called Lynx in Melbourne today and new manifold rubbers are still available so I will get some.
.....good score there Tom .. you buy just recently ?
i live by the voice inside my head

Offline MRieck

  • Really Old Timer ...
  • *******
  • Posts: 10,436
  • Big ideas....
Re: Factory based cylinder for 73mm pistons.
« Reply #20 on: March 22, 2021, 07:01:16 pm »
The custom cast cylinders like Bear used and Japauto made were ideal (for castings anyway) as they offered both more physical material to support the sleeves and a good bit of fin material for cooling.

Mike’s billet block goes another step fwd of course.  Even more fin area, Laverda like, and a modern Nikasil bore up to 72mm.  It’s a shame it’s not allowed for road racing almost universally
😕

For most vintage road racing classes that I am aware of, rules stipulate the cylinder blocks must be OEM.  RPE in Australia went so far as to bore out the 1/2 and 3/4 sections completely and inserted hand cast slugs of Siamesed aluminum. Then bored for big sleeves and redrilled for studs.  Yikes.

The casting Bear used for his big motor were very nice 👍

George
One of my customers in Europe races 2 of my blocks. They have held up great after 3 seasons and make the engine assembly more rigid. I believe they are used in endurance racing.
Owner of the "Million Dollar CB"

Offline Tom

  • Enthusiast
  • **
  • Posts: 134
Re: Factory based cylinder for 73mm pistons.
« Reply #21 on: March 22, 2021, 07:16:50 pm »

[/quote].....good score there Tom .. you buy just recently ?
[/quote] yes last weekend, super happy with my purchase, wil clean it up and put it back together for my Rickman with one of the new fandangled cam chain tensioners.
'59 T120.,'70 T120R Had one new.,'69 CB750 Oz Sandy Ruby Red.,'69 CB750 Oz Sandy Blue Green.,'70 CB750 K0 Candy Gold.,'71 CB750 K1 Valley Green I bought new.,'71 CB750 K1 Candy Garnet Brown.,'76 Rickman Honda CR750 R.C. Eng Cobra Powered. Two CB750 Choppers a Amen Saviour and a a Santee rigid arm Softail. A ‘72 Z1 Jaffa, and lotsa fun Honda Monkey bikes.

Offline T1m32sh1n3

  • Full Member
  • *
  • Posts: 76
Re: Factory based cylinder for 73mm pistons.
« Reply #22 on: March 23, 2021, 05:49:22 am »
Tom, what can chain tensioner?

Offline gschuld

  • Master
  • *****
  • Posts: 2,346
Re: Factory based cylinder for 73mm pistons.
« Reply #23 on: March 23, 2021, 06:00:06 am »
The custom cast cylinders like Bear used and Japauto made were ideal (for castings anyway) as they offered both more physical material to support the sleeves and a good bit of fin material for cooling.

Mike’s billet block goes another step fwd of course.  Even more fin area than stock, Laverda like, and a modern Nikasil bore up to 72mm.  It’s a shame it’s not allowed for road racing almost universally
😕

For most vintage road racing classes that I am aware of, rules stipulate the cylinder blocks must be OEM.  RPE in Australia went so far as to bore out the 1/2 and 3/4 sections completely and inserted hand cast slugs of Siamesed aluminum. Then bored for big sleeves and redrilled for studs.  Yikes.

The casting Bear used for his big motor were very nice 👍

George
One of my customers in Europe races 2 of my blocks. They have held up great after 3 seasons and make the engine assembly more rigid. I believe they are used in endurance racing.

That’s good to hear.  All across the US and Australian vintage racing organizations require factory cylinder blocks from all I’ve seen.  Europe has their own thing going on with multiple organizations doing their own thing.  I’m glad there is a road racing outlet for your blocks....they must be pleased with them.  The superior heat shedding capabilities of that block must be very welcome in big bore 1000-1026cc endurance bikes.
« Last Edit: March 23, 2021, 06:11:54 am by gschuld »

Offline Tom

  • Enthusiast
  • **
  • Posts: 134
'59 T120.,'70 T120R Had one new.,'69 CB750 Oz Sandy Ruby Red.,'69 CB750 Oz Sandy Blue Green.,'70 CB750 K0 Candy Gold.,'71 CB750 K1 Valley Green I bought new.,'71 CB750 K1 Candy Garnet Brown.,'76 Rickman Honda CR750 R.C. Eng Cobra Powered. Two CB750 Choppers a Amen Saviour and a a Santee rigid arm Softail. A ‘72 Z1 Jaffa, and lotsa fun Honda Monkey bikes.