Topic: Cracks in Engine Casing

[Roscoe]

Hey guys,
Is this a problem?






I have been cleaning off the old paint before repainting the engine for my 78 CB750 F2 and noticed these. There appears to be no gaps and no oil has been seeping out. Can I ignore them or is it a major issue? They look a bit like stress cracks.

[Aaron J Williams]

Not to worry, they are just casting marks from the look of them.

[hymodyne]

couldn't you magnaflux the case to check for cracks? I may be calling it by the wrong name; I had the block of my old triumph gt6 submitted to a process that exposed hairline cracks  before  I rebuilt the engine.


hym

[CBJoe]

Here's the post about the casting marks.

http://forums.sohc4.net/index.php?topic=31143.0

Joe

[scondon]

 My engines have looked much the same and "cracks" were in damn near the same spot as yours. No worries, paint will hide them

[Roscoe]

Thank you all very much for the replies. You have certainly taken a great weight off my shoulders. As it is for the f2 the finished engine will be satin black so hopefully it will hide the cracks. Thanks again for all your help. I don't know what I would do without this forum. Bloody glad I found it.

[754]

What is the fourth number from the right side going to the left? Is it diecast or sandcast?

Cracks like that a common on diecast later in the runs..

[bwaller]

couldn't you magnaflux the case to check for cracks? I may be calling it by the wrong name; I had the block of my old triumph gt6 submitted to a process that exposed hairline cracks  before  I rebuilt the engine.


hym

Hym, I had a 68 GT6, fun ride.

[hymodyne]

I rebuilt the engine and then got divorced... I still have the engine. I hope to rebuild a GT6+ around it once I get this bike of mine built like I want it.

hym

[merc2dogs]

 You normally get marks like that from a mold that's not warmed up all the way, generally morning shift stuff
 
 Worked at a couple diecasting shops and morning shift would throw the first few castings into the re-melt pile  because of the way the cold molds would 'crack' the castings. Mondays would be a real pain as EVERYTHING was cold, and often everything for the first hour and a half was re-melt.

 Normally not a problem as mentioned earlier.

Ken

[JAG]

If it looks like what I had, it really is no big deal, just a screw up from the factory, and seemingly the reason why they painted it..  I stripped mine all down to begin polishing, and found those lil buggers...

Can't polish it away, nor no amount of sandpaper will get rid of it. The only option I was left with was to paint that section back with ceramic high temp paint..

[Steve F]

They're probably similar to (if not the same as) "knit lines" like that which is experienced in plastic injection molding, where the material goes around an obstruction within a mold and rejoins, leaving those lines.  The material is slightly cold at the leading edge of the pour, and then has trouble rejoining with itself.  A hot mold will lessen the appearance of these lines, but slows production.

[Roscoe]

What is the fourth number from the right side going to the left? Is it diecast or sandcast?

Cracks like that a common on diecast later in the runs..

Its B750GE-1005159. By all that I have found out its a 1978 f2 model general export.

[Terry in Australia]

Don't worry about it mate, as has been said before it was common on CB750 engines, and if everybody here could look closely at their bare unpainted engine cases they'd probably see those "cracks" too. Cheers, Terry.

[jtb]

Ditto Terry and Scondon.  Mine had the same texture when I had it stripped for painting.  (77 cb750f)