Topic: Factory based cylinder for 73mm pistons.

[gschuld]

Fair warning, this will be LONG!  (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.

George

[dragracer]

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.

[gschuld]

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.

[bear]

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
 

[gschuld]

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

[Tom]

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

[754]

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. 

[Medyo Bastos]

I have a set of 73mm ID sleeves i took off an RC big block. I can pull them out from under my bed


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[johno]

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
johno

[bear]

Bear correct me if I'm wrong but your best engine had a custom cast barrel lucky bugger
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.

[gschuld]

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

[Medyo Bastos]

I have a set of 73mm ID sleeves i took off an RC big block. I can pull them out from under my bed


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LA Sleeves logo










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[754]

3 inch and a few thou.

[Don R]

 I was thinking some of the shops bored the block and welded up the holes, then bored for the sleeves again.

[grcamna2]

subscribed

[Tom]

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.

[madmtnmotors]

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"? 

[gschuld]

Corrected it above, thanks.🙄

[754]

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.

[sohc boy]

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 ?

[MRieck]

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.

[Tom]

[/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.

[T1m32sh1n3]

Tom, what can chain tensioner?

[gschuld]

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.

[Tom]

Tom, what can chain tensioner?

MessnerMoto http://forums.sohc4.net/index.php/topic,171811.msg2000132.html#msg2000132