Topic: CB 750 pistons ???

[LTD]

Good Day to all I hope read this! And yep I am a newbie here!  OK I've got a couple of old 750 engines.My !977 F2 that I broke in at the local dragstrip and took home the trophies every time in et bracket[brag /brag lols].Was a youngster then .Put 55,000 on it at a hundred a day going to work.Then decided to put new cam, rockers, and rings.After another 10,000 Job dictated I had to move. So sadly to say it sat for about 25 years. OK Here, is my question.What Piston will work in any 750 engine from 69-77  without concern about a little less compression, without damage?   F2 piston stamped 410 K7 piston stamped 392 Thanks a Bunch! Later

[nancy]

LTD
I bought these a year ago: http://www.cb750supply.com/products/4/engine/75/pistons-ring-sets
to replace 392 stamped pistons - installed in my K1 generation motor. Check that F2 motor is OK with these in terms of head/valve clearance first though...I'm not sure about the F2..but vaguely remember reading here about head/cylinder clearance differences. 392 piston sets are now unobtanium,.
Mark

[Jerry Rxman Griffin aka MuthaF'er]

Are you saying you already have 410's and 392's? The 392's F/F1/K7/K8 9.2:1 engines will work in all but will provide less than 9.2:1 in the F2/F3. The 410's need the F2/F3 head since they have higher domes.

[Don R]

Welcome aboard. A bunch of us tell a similar CB story, you will get a lot of good info here. The 77-78 F has a hemi style combustion chamber and if it hasn't yet, will probably need valve guides.

[LTD]

Now I'll try this again. My replies seem to disappear. Picked up a k engine at salvage yard,serial # says 1977 ,but as I took apart found one new valve and a hole in a piston.Pistons are stamped 392.Head does not seem to match as it is stamped X in a O and a 3 beside. Want to put the k head and cylinder on the f2 bottom.I have 3 of each pistons [392&410]. Both of these seem to be hard to locate . Seen a few for crazy $$$.Spigot on the f2 is cracked.So what I want to know if the old flat top pistons of 1970-73 will fit and function ,even if has less compression would be ok for now.Just piddling with f2 anyway while I recouperate from surgery. Just ant to hear her run again!!! And thanks much to all for info.

[Jerry Rxman Griffin aka MuthaF'er]

Where are you reading the stampings on the head? The 77 K also used a 392 head.

I'd suggest to put a Piston Wanted post up and state you need a 392 piston. Keep in mind there were 2 distinct 392 pistons so, I'd put up a picture of the top of the piston as well as the 392 casting.





 

[LTD]

 Thanks Jerry Rx Man for info and pics of pistons.As I expected two differant pistons.I have the 392 with the slight platform,and the 410 domed of course for f2 head,which not going to use at this time. X n O with a 3 stamp on head  top fin near # 3 intake. I read where k7  head should have 392 there.F2 head has 410 there same as it's pistons. That is why I think somebody had changed, heads on the k motor, but I am not that familiar with the early heads.Thinking maybe head and pistons stamped 300 go together, which are the old flat top pistons of 69 -73. Probably will eventually rebuild F2 back to original.Even though many had problems I only did proper maintenance for the first 55,000 miles  which I thought wasn't so bad .Every day to work,drag 1/4 mile on weekends& tour the east mountains and coast. That is why I m looking for a piston that would work for all heads 69-77 [just piddling around] Have seen good many sets of 300 pistons ,but not many single 392 with platform pistons.Thanks a Bunch!Later LTD

[Jerry Rxman Griffin aka MuthaF'er]

The piston on the right was from my 75 750F and the left piston came from a 78 750K engine.

Can you post a picture of the combustion chamber of your head? Your markings location is the correct place to look. A 392 head is easily ID'ed. My two 392 heads are 'marked' differently but they both have 392 on them.

[LTD]

Took Photos of husbands pistons, etc, and have uploaded them to PC... Tried to attach them into a reply and am getting the message they are to large to post.

Issue is , I have just gotten windows 8 and have NO IDEA how to make the photos smaller so they will post.

Any help would be kindly received as to the solving of  this issue.

Thank you ever so much for your time

wife of  : LTD

[flybox1]

quickest way..
open them with PAINT, and use the tab IMAGE/Stretch & Skew, to reduce length and width by 50, and then save.

[LTD]

O K Here we go!
 First there has to be a better way  to reply in here. It is taking me many times every time . It is saying error then next time says I have already posted message . Please advise me.

 MR. RXman hopefully here comes the pics  you needed to see .Two of the good piston,one of the hole in piston ,head stamp mark,& one  compression chamber. Take a look at and let me know what ya think.Would like your opinion on the cause of the hole also. I'm thinking was the valve that was replaced. Thanks alot ... T'll next time L.T.D.

[LTD]

hole in piston

[LTD]

head stamp

[LTD]

compression chamber

[Jerry Rxman Griffin aka MuthaF'er]

Took Photos of husbands pistons, etc, and have uploaded them to PC... Tried to attach them into a reply and am getting the message they are to large to post.

Issue is , I have just gotten windows 8 and have NO IDEA how to make the photos smaller so they will post.

Any help would be kindly received as to the solving of  this issue.

Thank you ever so much for your time

wife of  : LTD

Photobucket. Upload. Copy "IMG Code". Paste into the message. That easy and no resizing needed.

[Jerry Rxman Griffin aka MuthaF'er]

compression chamber

Earlier head. Note the "shrouding" below the spark plug hole and the lack of 392 on the top fin.

[Jerry Rxman Griffin aka MuthaF'er]

hole in piston

NOT caused by a valve. Detonation? Lean? How was that cylinder running?

Get in touch with Bill Benton for a piston possibly.

Check all 4 carbs and ensure the jetting is identical on all 4.

[Xnavylfr]

I'll take a look at the spare pistons I have and get back to you!!!  I know I have some spares I'm not using!


Xnavylfr(CHUCK)

[LTD]

Thanks Jerry for info! The k engine was not running or in frame.Was sitting on ground in someones yard with Dirt Dobber nest in the intakes and exhaust. But hopefully was worth the effort of lifting into truck. Thanks again !


Sounds good Chuck! Thanks

[LTD]

1976 750 f pistons ? will they work in my k 7 engine with the early head?

[Xnavylfr]

Was able to check out pistons I have. It is a SET , all 4 are labeled 300 on them. I'm not sure if they will work for you, hopefully someone else will drop in to say yes or no.
I have also been told F model pistons have to be matched to F model head, but that may only apply to 77/78 models because of higher comp.



Xnavylfr(CHUCK)

[Jerry Rxman Griffin aka MuthaF'er]

1976 750 f pistons ? will they work in my k 7 engine with the early head?

Should be fine.

Did you PM Bill Benton? His bat cave basement is a virtual salvage yard. I'd bet he can find you one matching 392 piston. Bill/BentOn racing is his handle here.

[LTD]

Ahhh!!! That set of 300's sounds like what I am looking for now.They have flat top right,no dome or platform? As I am going to use that old k head first,Until I can figure out what to do about the f2 head # 4 intake spigot...Yep these 77-78 are domed to fit only black motor head.
And I will be contacting Bill Benton Racing now also.Was trying to figure who he was and where I had seen.My mind gets minced sometimes....Hope I am not to annoying for you fellows .Just trying to relearn the 750;s a bit .
Jerry the 300 flat pistons should run with either head don't you think,minus compression I realize.
OK Off to check on 392w/platform piston at BBR SEE YA IN AWHILE. Thanks

[Jerry Rxman Griffin aka MuthaF'er]

I've never had a 300 piston to compare to a 392 HOWEVER you'll notice in my picture that there are 2 different 392 pistons!!?? I've seen pictures of both styles in a 300 also??!! WTF, right?! I'd like to think that some of the advertised compression ratios were not correct based on the piston tops. The full dome in theory should perhaps have that 0.2 compression bump.

Buying 1 piston may be less expensive than 4.

[LTD]

In my couple months search of used pistons and top ends,I've found many strange combinations.As we know many of the k7's inside parts came from the f's.So many of the early f parts should and will interchange,but also found that in both the early f's and the k7's that they kept the same stamp but changed piston styles in the middle of production according on exactly where they were manufactured. Most confusing to an old mind !!!
P.S. Better check serial # for sure!!!!!!!