Author Topic: Need help with ID of RC piston application (Read 7971 times)

Tews19 · « **on:** August 17, 2013, 02:43:56 PM »

Picks to follow after this type up... I was able to score me a set of NOS RC HP6450 pistons with rings... Of course never used. Still in the original box... With the help of 754 I decided to purchase them....

I am trying to determine if these can be used for everyday use in a 750... I read the post of PeWee on here and there isn't much info on these pistons..

This is my first post in the Hypo section and if I am in the wrong area please tell me...

Thanks in advance for the insight!

Tews19 · « **Reply #1 on:** August 17, 2013, 02:47:53 PM »

Here are some pics... If additional measurements need to be done please tell me what to measure and where...

I figured if I were to do a performance engine I should buy parts slowly as budget is limited and to be done correctly quality parts are needed...

Thanks again for any insight

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BPellerine · « **Reply #2 on:** August 17, 2013, 05:33:57 PM »

did you buy the ones on ebay tews mine don't look like that they made cast and forged as I understand it yours have the grooves on the side .they say you were supposed to remove some material from the chamber to use them.the ones I bought might be in completed listings on ebay.pewe had big inlet valves so he had clearance issues.bill

Tews19 · « **Reply #3 on:** August 17, 2013, 05:40:25 PM »

Yes I bought on eBay but they weren't listed as RC.. I will take a look at yours.. Happen to have any pics?

BPellerine · « **Reply #4 on:** August 17, 2013, 05:42:34 PM »

pistons should be 65 mm across the top the box says cb 750 std? that would be 61 mm.

Tews19 · « **Reply #5 on:** August 17, 2013, 06:01:04 PM »

Maybe I measured wrong but these are these do not even measure 61mm...

754 · « **Reply #6 on:** August 17, 2013, 06:10:14 PM »

Measure the skirt not the blouse....
If they are high compression stock size, they are probably pretty desirable for roadracing in certain classes.

Tews19 · « **Reply #7 on:** August 17, 2013, 06:15:30 PM »

Thanks Frank! You are the man... pics ti come in a few minutes

Tews19 · « **Reply #8 on:** August 17, 2013, 06:49:00 PM »

Well the piston rings say 350 1.00 on the box.....

Inside the piston it also has 77 marked and what looks to be 12... this is in all pistons..

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754 · « **Reply #9 on:** August 17, 2013, 06:51:18 PM »

Measure in millimeters..y

scottly · « **Reply #10 on:** August 17, 2013, 07:41:39 PM »

Put the edge of the ruler across the center of the piston skirt, regardless of which scale you use. It also helps to measure from an inside marking, such as the 1 inch mark, rather than the end of the rule, and then subtracting that from the total. This is called "burning an inch". As near as I can tell from the pics, those are 65mm pistons.

scottly · « **Reply #11 on:** August 17, 2013, 08:13:03 PM »

Sorry for the crude pics, but this is just to give you an idea of what you are doing wrong. Using a cheap plastic rule will only be close to 1/2 mm at best.

Tews19 · « **Reply #12 on:** August 17, 2013, 08:19:07 PM »

I think I got it.... I measure roughly 2.5 inches..

Tews19 · « **Reply #13 on:** August 17, 2013, 08:22:46 PM »

I have 1inch at the very farthest.... it measures right around to 3.5 inches on the opposite side....

Once again I really appreciate the insight guys

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scottly · « **Reply #14 on:** August 17, 2013, 08:34:17 PM »

You need to slide the rule down in the first pic. Center the edge of the rule over the centerline of the piston pin, and look straight down. Try to get the maximum reading.
BTW, it's a hell of a lot easier, and much more accurate to simply use a cheap Harbor Freight caliper.

Tews19 · « **Reply #15 on:** August 17, 2013, 08:37:25 PM »

Quote from: scottly on August 17, 2013, 08:34:17 PM

You need to slide the rule down in the first pic. Center the edge of the rule over the centerline of the piston pin, and look straight down. Try to get the maximum reading.
BTW, it's a hell of a lot easier, and much more accurate to simply use a cheap Harbor Freight caliper.

LOL I will be heading to my mom's house tomorrow for a BBQ... Her husband is a CNC guy and I will take the pistons there to measure more accurately.. I will post a pic..

Scottly, I have read these dome pistons need head work if used... Are they able to be used for everyday use bike?

scottly · « **Reply #16 on:** August 17, 2013, 08:53:44 PM »

Quote from: Tews19 on August 17, 2013, 08:37:25 PM

Scottly, I have read these dome pistons need head work if used... Are they able to be used for everyday use bike?

I'm not sure what you mean by "head work"? Any time a non-stock piston is used, piston-to-valve clearance should be checked. Any motor will benefit from porting, but it's not absolutely necessary for a daily rider.

Tews19 · « **Reply #17 on:** August 17, 2013, 09:28:05 PM »

This was the thread I read..

http://forums.sohc4.net/index.php?topic=104925.0

Pewee mentions head work such as grind the head down??/

and the cylinder block?

Oddly 754 and Scottly are contributing to the thread! Besides hands on experience, do you guys recommend any reading material to learn about compression ratio's the proper ways to do a build? I don't know, basically a Book for dummies when attempting to build a vintage cb750 supped up?

PeWees thread was like reading Spanish..

754 · « **Reply #18 on:** August 17, 2013, 09:33:28 PM »

Center the numbered part of ruler over widest point.
Use millimeters.. Even if you are out closer to 61 or closer to 65 will get us where we are going.

Stop reading what you just mentioned, will just confuse/ scare you at this point.

Terry in Australia · « **Reply #19 on:** August 18, 2013, 12:26:25 AM »

I've got those old RC pistons in my bike mate, no-one told me that I had to modify the combustion chambers (you can't machine any off the domes, they're too thin) and my engine goes 'tick tick tick" when it's hot. I'm gonna pull them out and send them to Bill when I rebuild my engine. Cheers, Terry.

BPellerine · « **Reply #20 on:** August 18, 2013, 12:26:01 PM »

the rings are 4th oversize 350 twin rings that came with these sets.they fit 65mm pistons.bill

BPellerine · « **Reply #21 on:** August 18, 2013, 04:15:26 PM »

my pistons do look like yours tews just a bad pic on ebay.bill

Terry in Australia · « **Reply #22 on:** August 19, 2013, 02:31:58 AM »

Here's a pic of mine when I was building my engine.

PeWe · « **Reply #23 on:** August 19, 2013, 03:30:04 AM »

I have not started my engine yet so I cannot write any review about these pistons. (New parts must be ordered all the time. I'll order new oil lines so oil can be filled. After that some electrical stuff.)

I had to grind a little bit to clear the wider inlet valves. I clayed the pistons to verify clearance AND tightened the valve adj screws at max lift to measure how much left before hitting the pistons, OK.

My head has opened chambers for an old set of forged RC pistons. The chambers in head is almost 65mm where it face the cylinder compared with original 61 so more material on the piston domes is welcome.

Tews19 · « **Reply #24 on:** August 19, 2013, 12:37:44 PM »

Thanks for all the help! I think they are for sure 65mm...... I measured with a more accurate tool?

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Author Topic: Need help with ID of RC piston application (Read 7971 times)