Topic: When do HD studs become necessary? (cb550)

[tvs avery]

I bought a spare engine for my cb550f that Ive been tinkering around with and slowly collecting parts for; big bore kit, 650 cam, ported head and intake, cb750 carbs. At this point, are heavy duty studs necessary? Or just precautionary? I may have the head milled just a tiny bit to bump compression as well. What are some sources? Dynoman has them I believe, anywhere else? Thanks for any experience and input!

[dave500]

not really needed,the stock ones are fine.

[Bankerdanny]

Why would you want to install 750 carbs? The standard carbs should flow more than enough air for the setup you are describing.

[tvs avery]

So out of curiosity, when DO the HD studs become a necessary item? When one runs a really radical cam? Or very high compression? I thought maybe the bigger 750 carbs would be welcome with the increased displacement of the big bore kit. My clymer manual 'claims' it is an upgrade to the 550 carbs even on a standard engine. But, If you guys think it wouldn't help at all, I'd be more than happy to leave the stock carbs on. Perhaps just rejet for the extra cc's? Thanks for your input!

[TwoTired]

So out of curiosity, when DO the HD studs become a necessary item?

-When you increase compression ratio.
-When you increase effective compression ratio.
-When you increase standard HP output more than 20%.
-When you routinely blow head gasket compression seal rings and the head and cylinders are known flat.

When one runs a really radical cam? Or very high compression? I thought maybe the bigger 750 carbs would be welcome with the increased displacement of the big bore kit. My clymer manual 'claims' it is an upgrade to the 550 carbs even on a standard engine.

You might consider CB650 carbs like the PD 50.  With it's accelerator pump, it ought to be far more "streetable" than either the 550 or 750 Carbs, and has larger venturis than the 550 carbs.
I think the clymer article is rather dated and printed before either the 750 or 650 were available with accelerator pump carbs.

Anyway, you only benefit from larger venturis at WOT and RPMs near and above red line.  At those work loads, it is also expected to rebuild the engine frequently.  It is the Hot Rodder's bane.  ...Unless you really enjoy wrenching on engine internals.

Big venturis don't help midrange air speed through the carbs, resulting in poor atomization.  Most street riding is in the mid throttle range.  But, if you are going to do more cry-for-mercy drag racing than street riding, then go with bigger venturis.

[Jerry Rxman Griffin aka MuthaF'er]

They are not necessary, just a nice upgrade to clamp things together tighter and help prevent head leaks of oil and possibly compression too. The original studs have a tendency to stretch the more you twist your wrist and with more miles. More horsepower also causes this to happen. Different head gaskets also compress at different rates. If you don't replace them you may want to make a habit of retorqueing them occasionally.

[tvs avery]

Thanks for all the great information. So it looks like my stock studs and carbs will be just fine for my application. This next question is a bit off topic, but if I do mill the head, is there a known 'safe' amount to remove before I start having valve clearance issues? I have to true the head anyway so I thought I might just bump the CR a bit. Say just .010 or so?

[PeWe]

Invest tuning money in a CB750. Same money more power.
CB550 might be better as it was born in the Land of the rising sun.

[tvs avery]

Well I got the cb550 in trade, so its what I have. Believe me, I'd love a 750 but here in northern VA people are wanting absurd prices when you can even find them for sale. So anyhoo, how much to (safely) mill the head?

[dave500]

mill the head as little as is needed to flatten it,.have you cleaned the surface and placed a straight edge over it?

[tvs avery]

Yes, cleaned the surface, put an edge on it and its within spec. But there are a few marks from an errant valve spring compressor that I'm nervous about. I was just going to just have it resurfaced, but I thought milling it a bit would bump the power up. What are the down sides? Does milling the head slightly decrease reliability? Provided of course my fuel and timing is dialed in.

[mazingerzeca]

Whenever you need peace of mind

[bjbuchanan]

Milling will raise static compression by reducing the chamber size afterwards and like you said could cause interference. I don't know what number the 550 is safe to skim but I wouldn't do any more than necessary like Dave said. The power increase from the compression bump is usually not worth it when you compare the money spent if you crash valves or have to relieve the pistons so they don't smack

There is a bunch of other stuff you can do to bump power without having to mill the head for power. No decrease in reliability you just really gotta measure out stuff stuff, piston to deck height, clay on the top of the pistons to see if you have clearance and maybe afterwards tweak the timing because it will probably want something different with all the changes. The marks most likely won't matter and depending on how good it got knicked you would be hard pressed to mill it enough to make it go away anyway

[tvs avery]

Noted, I'll just resurface the head then. The only reason I thought to mill it was based on my machinist friend's suggestion, hes doing the work for me for free so I thought why not? I appreciate everyone's input.

[dave500]

post a photo of the head surface,if its flat and your only worried about some marks on we might be able to get a better idea,you dont really want to raise the compression if you dont have to,its mostly done to compensate for a hotter cam with more over lap and later closing of the valves etc.

[PeWe]

Smooth and flat surfaces on both sides, head vs cylinder open the door for an MLS head gasket.