Topic: Robinson TBA 1240 Big bore stroker build

[RAZZ]

Hi just fund this build fra Robinson TBA maybe other than me that like to Watch https://www.facebook.com/208192209229327/posts/4562125883835916/?d=n


Sent from my iPhone using Tapatalk

[RAZZ]

https://fb.watch/9Rpfff3DVg/


Sent from my iPhone using Tapatalk

[simon#42]

thanks razz that kept me busy for an hour !

[PeWe]

I could not see how much they had to weld to keep all new oil vent/leak holes shut and bore again.
75mm bores must end up in 78mm sleeves, 1.5mm sleeve thickness I have heard is minimum.
Not much room left to make a groove at the bottom around sleeves for o-ring/sealer.

Also if the 2 large triangular holes for went on each side of the cam chain tunnel (which makes sand to land on basegasket) are filled completely with epoxy to make the block more rigid.

Interesting to know how they fix all that.

Even CycleX relatively small 74.5mm OD sleeves perfect for 70mm pistons open some in the cylinder block. Vents on 2 and 3.
Depending on block with more or less additional pores.

Edit: Another look revealed spots that look welded as it must have been done.
Maybe the entire areas between 1-2 and 3-4 welded.

[dragracer]

Surely would be nice to own a complete setup for tis cylinder. ead , pistons and all.

[PeWe]

Head would be interesting to see too.
If they use >34mm in valves to feed the big bores.

I guess this is still their working website
https://www.rpengines.com.au/racing/cb750-big-bore-engine/

[slikwilli420]

Head would be interesting to see too.
If they use >34mm in valves to feed the big bores.

I guess this is still their working website
https://www.rpengines.com.au/racing/cb750-big-bore-engine/

Need a lot more than 34 intakes to feed that bore size properly.

[RAZZ]

Just Saw this crazy engine from RPE..it sounds fresh and williing to work   

https://m.facebook.com/groups/cb750motorcycles/permalink/10159598828791442/?mibextid=KsPBc6


Sent from my iPhone using Tapatalk

[MRieck]

Just Saw this crazy engine from RPE..it sounds fresh and williing to work   

https://m.facebook.com/groups/cb750motorcycles/permalink/10159598828791442/?mibextid=KsPBc6


Sent from my iPhone using Tapatalk

I don't do Facebook but I have to wonder about engines that large with OEM cylinders despite cylinder offset, inserts etc etc. That head could use 37 or 38mm intakes with exhaust 2mm less. Tall order.

[Tracksnblades1]

A while back on Mr Robinson website there were pictures of 2 cylinder oval sleeves machined to fit in the 750’s block..
I’m pretty sure there were offset pistons and rods in the real big cc build.. he advertised billet stroker cranks and Carrillo style rods too..

[MRieck]

A while back on Mr Robinson website there were pictures of 2 cylinder oval sleeves machined to fit in the 750’s block..
I’m pretty sure there were offset pistons and rods in the real big cc build.. he advertised billet stroker cranks and Carrillo style rods too..

I purchased a set of those rods.....they weren't anything like Carrillo's. Very heavy and the piston pins wouldn't fit in the bores. One of the pin bores was bored offset as well. I was able to return them but by the time  I paid for shipping back to Australia it was a loss.

[Tracksnblades1]

A while back on Mr Robinson website there were pictures of 2 cylinder oval sleeves machined to fit in the 750’s block..
I’m pretty sure there were offset pistons and rods in the real big cc build.. he advertised billet stroker cranks and Carrillo style rods too..

I purchased a set of those rods.....they weren't anything like Carrillo's. Very heavy and the piston pins wouldn't fit in the bores. One of the pin bores was bored offset as well. I was able to return them but by the time  I paid for shipping back to Australia it was a loss.

That is good to know. Thanks for sharing..

I’ve tried following RPE after watching one of his engine in the down under of races..😁
It’s good to see them back going again.. Being self employed myself, it can be extremely obvious that you can only be as good as your worst employee is, for that day…Some bring their home drama to work with them..

I was hoping there would be another 70mm billet long arm attempt in the future…
The welded strokers back in the day ran well around here, until pieces wanted out..😳😩

[Tracksnblades1]

https://fb.watch/9Rpfff3DVg/


Sent from my iPhone using Tapatalk

You think the Machinist is using more than the opposite corners to secure the block to the upper crankcase..?
Wish I could see the back of the upper crankcase better…. Wonder if they’re boring the upper at the same time…?
Long quill..?

[PeWe]

It should go thru to the air vents between 1-2 and 3-4.
An old cylinder with Henry Abe stepped sleeves had holes to vents. I think sleeve outer was 74.5mm

Same with another cylinder CycleX stepped sleeves 74.2mm dia.
And finally my 970cc cylinder with CycleX sleeves, 74.5mm dia.
All earlier cylinders with small oil return holes. All with holes to vent. Maybe one cylinder only had holes to 3-4 vent.
Oil WILL weap if not sealed.

A 72mm bore will have at least 76mm outer dia for 2mm thick sleeves. I have heard that 1.5mm is minimum.

What about cylinder with all that material gone?

My 970 cylinder got a wider milled part for tight sitting o-ring to seal, 77 mm (1.25mm groove 5-6mm deep). On top of that Three bond sealer (Same as Honda bond)

 

I filled the bottom of the triangle vent due to the risk that sand and dirt could find its way into engine.
The base gasket is an RCS, 78mm holes I earlier used with the billet block that has ~77.5mm dia "sleeves".

I think that the entire triangular hole can be filled with epoxy to make it more stable.
My cylinder got ca 7 mm thick bottoms in the triangular holes. Might help with stability too

How will a stock cylinder keep stability after it has been bored to at least 76mm?
77mm sleeves for 73mm pistons?

The Billet block however has no weakening air vents. More meat and  wider cooling flanges.

Edit:
I think Brian, "the Aussie side car racer" said they welded the air vents completely shut when boring to bigger bores.
 1200cc with stroked crank?

[Tracksnblades1]

Looks good Per…

What epoxy did you trust to seal er up..?

[PeWe]

Epoxy putty for the bottom that hardened within 10 minutes.  A thin cake I made and put in place.
Filled up with std JB Weld epoxy.
I drilled a few small holes in the hole sides for the epoxy to grip in.

Did the same for a head where someone had done helicoil fix drilling too deep, no bottom where oil can leak out.
Fixed a bottom with 2k putty to keep thin flowing JBWeld in place as a final seal.

Held a flat thin still thing from outside between flanges, pressed in a little putty to create a thin bottom. I do not trust the strength of the putty only.

Did the thread with Wirth thread insert before hardened too much.

[MRieck]

Epoxy putty for the bottom that hardened within 10 minutes.  A thin cake I made and put in place.
Filled up with std JB Weld epoxy.
I drilled a few small holes in the hole sides for the epoxy to grip in.

Did the same for a head where someone had done helicoil fix drilling too deep, no bottom where oil can leak out.
Fixed a bottom with 2k putty to keep thin flowing JBWeld in place as a final seal.

Held a flat thin still thing from outside between flanges, pressed in a little putty to create a thin bottom. I do not trust the strength of the putty only.

Did the thread with Wirth thread insert before hardened too much.

Good idea using the putty first. If not that JB will flow everywhere......what a mess!

[PeWe]

2k Putty good to have together with the JB Weld epoxy

[MRieck]

The billet block with a 6mm stroke and 72 mm bore was about as big as I imagined would be practical....and that's stretching the word. I would only endorse Marine Crankshaft for welded cranks.

[PeWe]

The billet block with a 6mm stroke and 72 mm bore was about as big as I imagined would be practical....and that's stretching the word. I would only endorse Marine Crankshaft for welded cranks.

1123.7cc
73mm pistons make 1155cc (69mm stroke)

All extra torque must be welcomed for  racing

71.25mm pistons and stock stroke fine for street (1004.7cc)
I have reached a few hp more  with 970cc,  107.5whp.
That setup runs too well to be taken apart for the comeback of the billet block, now  with gas ported pistons.  Thinner gaskets as the 970 has and same 125-75 cam timing can (should)  enter +110whp


The RPE bike on methanol had 120whp according to their videos.

A good stroker crank with rods must cost some. Or if stock size rods are used with "stroker" pistons.
But if 120whp is the goal....

All that could be worth it if getting 140whp
- How to reach that with carbs, no N2O nor other race fuel ?

[Don R]

 My 72mm cylinder had some kind of silicone in the triangles that's nearly impossible to remove. Two sleeves were low in the block and didn't seal against the head gasket. During a leak down test it could blow soap bubbles between the fins.

[PeWe]

I think APE fills the entire triangular hole.
Air vents 1-2 and 3-4 intact.
http://forums.sohc4.net/index.php/topic,159763.msg1834608.html#msg1834608

Edit:
I remember another thread where Big Jay wrote they used double o-rings stacked on top of each other to seal the sleeves.

Maybe as my 970cc cylinder with a wider bored part, +2.5 mm dia making 1.25 mm wide groove.
I used 1.5 x 65mm o-rings. Nice stretch around 74.5mm sleeves making a tight fit into the 1.25 mm groove. A rounded tool to press the o-ring down cross wise.

If 76.0 mm sleeves a little bit bigger o-ring to not stretch it thinner not sealing that well.
66.3mm when increased by same percentage 76-74.5= 1.5.
1.5/74.5=0.02 (76 is 2% bigger than 74.5)

65 x 1.02=66.3mm  66mm should fit too. My o-rings where really tight.
+14.6% stretch of the o-ring. It must sit tight around sleeve.

My cylinder is still tight, no leaks thru the air vents I know got holes when bored for the 74.5mm OD sleeves . Today 17.419km

I think that Nitrile is more sticky than Viton, seal better and easier to shape for this purpose.

(All my Viton o-rings have a more plastic feel. )

https://www.o-ring-stocks.eu/o-ring-66x1.5-nbr-nitrile-70-shore-a-black-ors12152

[Old Scrambler]

You BIG BLOCK guys may need an oil-cooler.............just listed one on ebay and referenced above in performance parts for sale.