Topic: Another 836 piston questions - Valve pockets vs valves - Who made them? -

[PeWe]

I have purchased a new set of old 836cc pistons that look fine on eBay. Not much more $$$ than only new rings to my old pistons. (+150)

The engine is a K6-76/77, ported old type K-head with bigger inlet valves, 34 instead of 32mm.

Only one important thing.  The valve pockets versus the size of the valves.  The inlet valves are the bigger F2, 34mm. The corresponding pocket in piston is not so much bigger, let say 1mm around if the valve is perfectly centered in the pocket. The exhaust valves however, are OEM size and have more clearance. 

- What do you think, is the pocket too small for the 34 mm inlet valve?  Do I have to open up more? See attached photos.(I'll check this physically with some kind of clay in the pockets put everything together (without head gasket) with valves , cam... and rotate the engine to check how the marks will look in clay when valves enter the pockets at max lift.)

- Next question if anybody know what kind of piston it is? It look almost like my old Russ Collins 836.
Differences:
New pistons are cast , not forged as Russ Collins.
Markings inside only RC,  HP6450.  Russ Collins marked with RC ENG
Normal piston pin clips, not teflon.
Valve pockets smaller, aligned with valves and not too wide as many other pistons have.

[lucky]

I have purchased a new set of old 836cc pistons that look fine on eBay. Not much more $$$ than only new rings to my old pistons. (+150)

The engine is a K6-76/77, ported old type K-head with bigger inlet valves, 34 instead of 32mm.

Only one important thing.  The valve pockets versus the size of the valves.  The inlet valves are the bigger F2, 34mm. The corresponding pocket in piston is not so much bigger, let say 1mm around if the valve is perfectly centered in the pocket. The exhaust valves however, are OEM size and have more clearance. 

- What do you think, is the pocket too small for the 34 mm inlet valve?  Do I have to open up more? See attached photos.(I'll check this physically with some kind of clay in the pockets put everything together (without head gasket) with valves , cam... and rotate the engine to check how the marks will look in clay when valves enter the pockets at max lift.)

- Next question if anybody know what kind of piston it is? It look almost like my old Russ Collins 836.
Differences:
New pistons are cast , not forged as Russ Collins.
Markings inside only RC,  HP6450.  Russ Collins marked with RC ENG
Normal piston pin clips, not teflon.
Valve pockets smaller, aligned with valves and not too wide as many other pistons have.

So is the engine a 1976 or a 1977???
"ported old type K head" .....Is it a K cylinder head?
What year is the cylinder head?

The 1978 K models had larger valves too.
Lots of questions.

I would call Ken at Cycle-X and see if he can help you.
He deals with these issues all the time.

The 1977-78 cb750,s have a wider cam chain galley and there could be a fitment issue. Also the cylinder head cover is different on the 77-78 models
and does not inter change with earlier models.

I assume your cylinder head cover fits the cylinder head .
If you had a photo of the top of the cylinder head cover without the cam chain cover then we could tell what year the cylinder head is.

I am wondering about the piston crown height first -valves later.

[PeWe]

The bike is a 1977 in registration, K6 which was produced 76 - spring 77 with the old K head, modified with F2 inlet valves. 33,5 OZ valves could have been another alternative.
( NOT 77-78 F2- head with different combustion chambers, bigger in and ex valves.)

Piston crown look good. See attached photo where the new piston is compared with an old RC836 that the engine was prepared with at the very beginning. Pistons later blown, replaced with Action Fours 836 with lower lower compression.

[754]

 77K should have small valves. Valvecovers are different on late engines, but if they cant be interchanged..its the first I have heard of it..

 Part of proper engine building, should include claying pistons, and checking clearances... IF you change valve size, cam. pistons.. or combinations thereof.
 There is a large litter of damaged parts left on the trail, from builders who said.." I heard it should/will work"

 If you dont take the time to check now.. gonna be more work later if it goes South..

[PeWe]

The engine has used the head as it is today during appr.18650 miles. Only thing new now is the pistons, 3:rd set of 836cc pistons since the engine was modified 1984.

Heureka! I could not give up when it must be something writtten about this. Very interesting information found.      V8 or a 51.015 cubic inches SOCH ... so what?

http://www.rehermorrison.com/blog/?p=186     The first step in the Reher-Morrison method is to determine whether the valve pockets are located properly. A discarded valve that fits your cylinder head makes an ideal tool. Cut off the head of the valve and turn the stem to a point. Preassemble the engine with your bare cylinder heads (remember to use a previously compressed head gasket), put masking tape on the ring lands to center the piston in the bores, a bring the piston to 10 degrees before or after Top Dead Center (it doesn’t matter which at this point) and drop your homemade punch into the intake and exhaust guides. Give the punch a gentle tap to mark the valve stem centerline on the piston and then remove the head.

To check the valve pocket location, remove the cylinder head and set a pair of calipers to the radius of the valve head (for example, for a 2.500-inch diameter valve head, set the calipers at 1.250-inch). With one point centered on the punch mark, swing the other point around the valve eyebrow. If the caliper hits the edge of the valve pocket, so will the valve. I recommend a minimum of .050-inch radial clearance between the edge of the valve and the pocket .............  It continues here with how to verify the angle of the valve head and piston pocket with a simple home made tool.

Found more about piston clearance in all ways. http://www.duncanracing.com/TechCenter/TECHHowtoCheckPistontoValveClearance4-6-10.pdf

 Valve pocket circumference Valve to piston, Intake and Exhaust: .040” minimum ... This might be a guideline for a CB750 too.

More here, valve pockets   http://books.google.se/books?id=M_NDJSrpGc0C&pg=PA85&lpg=PA85&dq=valve+head+diameter+%2B+clearance+piston+pockets&source=bl&ots=71fV_2SVEc&sig=dz33AO63UOMHjQuSOUfLreK_muY&hl=sv&sa=X&ei=jVGcT4_6DorR4QTBm_2pDg&ved=0CCgQ6AEwAQ#v=onepage&q&f=false

[754]

 the punch method is usually what we do before flycutting pistons... or I mean is done before I get them..

[PeWe]

Punched the pistons today.  Pistons on without rings. Used old cylinder base gasket, no head gasket when I just put the parts together in position, guides to fix cyl and head in correct pos though.
The method to make a punch of an old valve was something I reconsidered since I know how hard a valve is and difficulties to make the tip in the center without proper machining.

I found a good tool, a drill. 6,5mm perfect fit in the guides. Worked fine to punch the pistons. A little bit too close on inlet valves vs pocket wall. I'll adjust just for sure, grind 1mm away half of the pockets on all inlet pockets. 34mm inlet valves need space.

I also tested with valves in, pistons at top, how much the valve could move up-down to be sure that the piston has good clearance to valve at top. IN 4,55-5,55mm, EX 3,1-3,5mm.

Next is to open the engine completely, check the primary chains for slack. Maybe better to replace when the engine is open. It's a lot of stuff I need to replace just for sure when the engine with crankcase is opened. cylinder studs, chains, con rods. $$$$
I went to a dealer here for HD cylinder studs.
NO, but they had 40% discount on lether clothings so I got new jacket,  trousers and gloves, $750US. End sale due to final close down.

[lucky]

Punched the pistons today.  Pistons on without rings. Used old cylinder base gasket, no head gasket when I just put the parts together in position, guides to fix cyl and head in correct pos though.
The method to make a punch of an old valve was something I reconsidered since I know how hard a valve is and difficulties to make the tip in the center without proper machining.

I found a good tool, a drill. 6,5mm perfect fit in the guides. Worked fine to punch the pistons. A little bit too close on inlet valves vs pocket wall. I'll adjust just for sure, grind 1mm away half of the pockets on all inlet pockets. 34mm inlet valves need space.

I also tested with valves in, pistons at top, how much the valve could move up-down to be sure that the piston has good clearance to valve at top. IN 4,55-5,55mm, EX 3,1-3,5mm.

Next is to open the engine completely, check the primary chains for slack. Maybe better to replace when the engine is open. It's a lot of stuff I need to replace just for sure when the engine with crankcase is opened. cylinder studs, chains, con rods. $$$$
I went to a dealer here for HD cylinder studs.
NO, but they had 40% discount on lether clothings so I got new jacket,  trousers and gloves, $750US. End sale due to final close down.

Thats why a lot of those dealers are out of business. Jusy fluff. Hat,key chains,clothing, cleaning products but where are the serious parts and expertise???

[754]

 Be careful punching the piston, transfer punches should work well, shim with tape if needed.. If you put thick or several layers of masking tape on, you may be able to mark them without making a permanent mark in the piston...

[PeWe]

The expertise is here at SOHC forums! I like to read explanations from guys that have used and worked with these machines since early 70's. They really know what they are talking about.

Plenty of new and old parts for CB 750 to order, mostly from USA.