Topic: fin breakthrough

[Don R]

 I'm cleaning this old 72mm cylinder up to send out for repair and thought this pic. was interesting, it illustrates where big bore sleeves break into the fin area. My old buddy Tom used a clear silicone to try to seal the fin/crankcase area from underneath. The light from the bottom illuminates the silicone that protruded through the gaps.
  The sealant was working from the crankcase side but two of the sleeves were either left low or they moved down during use causing the head gasket to leak compression down from above.
 During a leakdown test it would blow bubbles between the fins.

[PeWe]

My 970cc are sealed with tight fitted o-rings with Threebond 1184 aka Hondabond on top. No leak yet after >10.000km.
CycleX 74.5mm OD sleeves so not that huge windows to air vents 1-2, 3-4.
72mm pistons must have 76mm or more OD sleeves.

Covered where I saw sleeves with thin layer of JB Weld on outside before painting.
The triangular vent holes got JB Weld bottoms when sand will enter. Not much base gasket that keep it outside. Risk that hard power clean can press sand inside.

I used 2k clay to make a thin cookie I placed 10mm down as a floor.
 Aluminium had got som holes with small drill before for better grip. Direct when hardened enough to sit filled up with JB Weld epoxy.

Cylinder bored a little wider at  the bottom, 5-6mm deep, 1.25mm wide groove.
O-rings 65x1.5mm, perfect stretch to fit 74.5x1.25mm groove.
Hondabond oversmear cleaned off with acetone and razorblade after pic was taken.

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My 890cc with stepped sleeves 74.2mm OD sealed with Wurth Super RTV only in the gap that sleeve leaves. That silicone feels durable when hardened.

[Don R]

 I am pre-cleaning before sending this late 70's machine work to CycleX for repair or replacement. I'd guess once upon a time the bottom of this one looked a lot like PeWe's cylinder.  The lack of compression seal at the copper head gasket/ dropped sleeves caused them to try some unusually large quantities of goo.
 My thought is Devcon block filler, they used to use it to stabilize wet engine blocks for large overbores. It's still a widely used metal bearing epoxy.
 
 The PO was widely known for his fast cb750 engines but at some point his CBX and Goldwings took over his attention.
  I own an 836 K2 that he also built. That thing goes like stink.

  What a nightmare to clean the silicone out of those pockets, I've been chopping, cutting, o ring picking and milling with a drill bit. A little penetrating oil hasn't hurt but doesn't really release the silicone.

[PeWe]

Weld the block after bore and bore again has been up before on this forum. And dropped sleeves too when not completely pressed down at assembly. Stick up sleeve milled away, engine running warm and sleeve goes down.

I was going to use my JE 71.25mm pistons with Dynoman big sleeves in a stock cylinder. I guess 76-76.5mm OD.

I had contact with a company that could weld. Important to heat it up first, weld and let it cool very slowly.
I changed my mind during the process. Contacted Mike about a billet block, told Dynoman to ship pistons to Mike instead of me.
Total price not much differ when stock cylinder need that much work to get right.

APE 1000cc cylinder job filled the triangular airvent hole with epoxy for make it more stable. Dual o-rings on top of each other around sleeves in bottom.
Also read on this forum.

CycleX 74.5/69mm sleeves are otherwise a good size. Not that big hole, more material left and must be more stable.
70mm pistons can give some power when everything else is in synch singing the same symphony.

My last 2 Dynos 107.5 whp, (92Nm/68 lb ft torque at 6800rpm).
This with Shell V-power street fuel so it must be possible to squeeze out a few more with race fuel. Fuel with more oxygen might need slightly rich blend.

[Tracksnblades1]

I'm cleaning this old 72mm cylinder up to send out for repair and thought this pic. was interesting, it illustrates where big bore sleeves break into the fin area. My old buddy Tom used a clear silicone to try to seal the fin/crankcase area from underneath. The light from the bottom illuminates the silicone that protruded through the gaps.
  The sealant was working from the crankcase side but two of the sleeves were either left low or they moved down during use causing the head gasket to leak compression down from above.
 During a leakdown test it would blow bubbles between the fins.

Does any one still offset bore the cylinder and uppercase for race only engines. A local guy, gone now, used to use l’m thinking the 1100 wrist pins for added strength for the small off centered rod c/L to piston center.  purposely boring the cylinder block lightly offset away from the problem break through areas..? Or am I daydreaming about a different engine..?

[NitroHunter]

I know offset boring 5mm was done on the DOHC V&H Pro Stocker, but not familiar with it done to a single stick.

[Don R]

 I took the cylinder off and noticed a raised wrist pin on the pistons. There are Teflon buttons to support the oil rings. I'm assuming if the pins were moved up, the rods may be stock length. That's a green light to new rods, especially since I have a new set of MTC aluminum rods already.   

[Tracksnblades1]

I know offset boring 5mm was done on the DOHC V&H Pro Stocker, but not familiar with it done to a single stick.

Good to know about DOHC,s…

This old boy was doing the Russ Collins and Cyclex thing years ago.  Big valves, rolled valve guide angle, relocated ports,
Respaced carbs, etc… All the stuff Capt or his buddy’s been doing lately… Only one I knew that built his own green case bearing alignment tool. Cyclex advertises about this too. I don’t remember him doing much other than sohc chopper stuff. He built his own frames and loved the RC Sidewinder.. . Im not sure if it was the performance or the custom pavement mitered  outlet of the pipe he preferred…. I was pretty young then, still playing scratch and sniff.  But I’m sure he rough offset bored them on a old 3PH 5hp Bridgeport.. His son, gone too, was one of my buddies in terrorizing the playgrounds…😜

[PeWe]

Offset boring of CB750 might help the stepped sleeves 74.2mm OD.
My 890cc cylinder had square holes in #3 cyl to air vent.
My 970cc cylinder with 74.5mm sleeves had squared holes in both 1-2 and probably 3-4 too.
If spreading 1-2 and 3-4 apart 1mm it might avoid going thru. Another thing about the other sides holes coming closer.

A 76mm OD sleeve will go thru with much more.
Both my cylinders are earlier models before the wide oil returns, but not that early that were sand casted. Fins look fine and thin.

A shop that can bore, weld were going thru, bore again and prepared to weld-bore again where thin must be the way when using sleeves for 1000cc and above.
My 74.2, 74.5mm OD sleeved cylinders had not that much meat left around the sleeves for offset as the photos above show.

Mike's Billet block were relatively cost effective to get a not leaking and more rigid build.