Topic: Diagnosing newly-rebuilt top end / rings / cylinders

[wohali]

About $100 more than the Dynoman kit, eye.

My plan:

I'm waiting to hear back from another shop here in Toronto that has experience with CB750s. I'll run pistons + cyls to either them or Gord and get measurements done / honing if appropriate. If that looks good I'll buy the factory original $80 rings and stop there for now. (I lost the information to contact whomever bought the Total Seal rings from me.)

If not I'll start evaluating these three options: new cyls+836 kit to MReick, these cyls+836 kit to Gord, and new 61mm cyl sleeves in my current cyl head.

[wohali]

OK, quick update.

I took the cylinders out to Willy at WASP Performance (like the insect, not the kind of person ) to have them properly dial bore gauged. He was able to eyeball them and say that the wear pattern indicated they were worn asymmetrically, and that if it was up to him, he would overbore and get new pistons. That pretty much clinched it for me.

Now I need to find the right pistons to go to. He was aware of the F2/F3 valve guides but not that the pistons were different; he also warned me away from the crappy Chinese piston kits on eBay these days. (Especially funny coming from an older, greying Chinese man!) He seemed surprised that there was nothing smaller than the 836 kit available, but given that the F2/F3 engines have weird requirements, he wasn't completely shocked.

I contacted CycleX and they are not presently carrying the F2/F3 pistons, but if they did, they'd have the same 836 kit.
I noticed also that Z1 Enterprises no longer carries the Wiseco 836 kit.

That pretty much leaves me with the Dynoman 836 kit, the more expensive Scheuerlein-Motorentechnik ones, or attempting to source NOS 0.25/0.50 pistons. I'm quite sure Honda has deleted those part #s at this point.

Anyone know of any other piston kit options before I just dump the money into the Dynoman 836 kit and be done with it?

[brandEn]

I think you can use k pistons you just sacrifice some compression ratio. Don't take my word though it's just something to consider researching...

[grcamna2]

wohali,
Try to PM HondaMan and ask him if you can use 2 pairs of CB/CL350 twin pistons in your F engine,if so I know that bore-tech.com has a good deal on quality pairs of pistons that're less expensive(65mm); I think it would be good to know if the high dome on the CB/CL350 twin pistons will give you the correct squish area and compression ratio for your CB750F 

[HondaMan]

Well, I tend to be a cheapskate, if I can while still getting good performance. I've done this with some F2/3 engines lately, and it worked out well: I used the "cheap" Thai-made 836cc "low-compression" pistons, and also milled the head (both were F2 engines) by 1.05mm. This brings the compression (with a stock cam) up to the low 9.0-9.1 range. Since it is adding displacement, the extra lift of the F2/3 cams get along well with this change and no further engine mods (besides HD studs) are required.

The use of Honda's 350 twin pistons is a possibility, but they are heavy and the engines don't come out very smooth in the end. This was the first "big bore" kits, way back when. The crank weights are not quite matched to those piston weights, that's where the buzziness comes from, and you'll probably find that you have to tool down the crowns a little to clear valves.

Wiseco and CycleX both offer a high-compression 836cc kit in forged pistons, which have their own issues if you are planning on touring with the bike. They also empty a wallet pretty fast, so they are not my favorites. (See opening statement...)   

About 2 months ago I was surfing the 'net one night and came across someone who is supplying some pistons that may work: they are the CB900 pistons, in Honda's oversizes. If I can find that site again, I could possibly put the link here? The pistons are the same deck height and piston pin size, and they have domes similar to the K7/8 tops. If the head were also milled .5 to 1.0mm with those tops, the CR would be up around 9.4:1, I think.

[wohali]

I already have the APE HD studs installed.

Hm, without a milling setup the cost differential between the "cheap" low compression pistons and what the shop wants to mill the head is a wash - actually cheaper to get the Dynoman pistons by a small bit.

What do you mean about the forged pistons having "issues if you are planning on touring with the bike?" Touring and weekend puttering around is about all I do, so I am certainly concerned. Is it just overheating - in which case would adding an oil cooler be sufficient? Or does the increased hardness mean increased cylinder wear, meaning I need new sleeves or similar? Do I need to replace the camshaft at the same time?

Do let me know if you find that link to the CB900 pistons in Honda's oversizes, though again I'd have the milling problem...or if you know any way to get the correct original factory pistons for the bike.

[grcamna2]

Who manufactures a nice set of 4 oversize pistons for your CB750 that are available now ?

[wohali]

There are 5 sets available that I can find:

• $500: Dynoman 836cc (65mm): http://dynoman.net/engine/pistons/DP836-2010.html
• $500: Dynoman 915cc (68mm): http://dynoman.net/engine/pistons/dp-915-f2.html
• ~$638: Scheuerlein-Motorentechnik (62.5mm): http://www.scheuerlein-motorentechnik.de/motorenersatzteile/motorrad/Honda/700-799-ccm-h7/CB750-F2--OHC-/kolbenkit-cb750f2-1-uem.html
• ~$638: Scheuerlein-Motorentechnik (63mm): http://www.scheuerlein-motorentechnik.de/motorenersatzteile/motorrad/Honda/700-799-ccm-h7/CB750-F2--OHC-/kolbenkit-cb750f2-2-uem.html
• CycleX emailed me this week to say they are out of stock and aren't carrying F pistons at this time but they used to carry a Wiseco 836cc set.
• $640 ($160 x4): Wiseco 836cc (65mm): http://www.wiseco.com/ProductDetail.aspx?ItemID=4016M06500&ModelID=116&ModelYear=1978&AppID=6417 - note, would have to add rings/pins/clips to this set
  

Frustratingly, there is an eBay hit for a NOS 0.50mm oversize piston (part number 13103-410-003). Just a single piston. http://www.ebay.com/itm/171279195078

Original part numbers for Honda F2/F3 pistons (that are all discontinued and no longer available):

• 13101-410-003 stock piston
• 13102-410-003 OS 0.25
• 13103-410-003 OS 0.50
• 13104-410-003 OS 0.75
• 13105-410-003 OS 1.00

[wohali]

Hm! NOS set of 4 pistons, 0.25mm OS:

http://www.ebay.com/itm/Honda-CB750F-piston-SET-OF-FOUR-4-13102-410-003-O-S-0-25mm-new-NOS-/111149662253?pt=Motorcycles_Parts_Accessories&hash=item19e109402d&vxp=mtr

4 sets of matching set of rings + pins + clips would be an additional $113 from cb750supply.com, bringing the total to about $450.

[wohali]

What do you mean about the forged pistons having "issues if you are planning on touring with the bike?"

So I did a little web searching and turned up this post over at the American Rider Magazine forum:

Wiseco's are forged pistons and unless you are going to run some sort of boost in your motor or are racing, there is little need to run forged pistons. The main reason, as I understand it, to run a forged piston is for strength. A forged piston requires more initial setup clearance, usually .002-.005 more depending on application because it expands more when hot. Harley's by comparison have large bores and forged pistons require more clearance as the bore gets larger. This causes a noisier motor when cold (piston slap).

Forged pistons also require a more controlled break-in procedure, as the metal heat cycles it changes before taking its final shape. Motors with forged pistons should also be warmed up thoroughly before riding (most of us don't do this) or they can "cold seize", less likely if properly broken in.

Cast vs. Forged .... A cast piston will hold its tolerances better from the initial setup. It expands less when heated and maintains its original shape over the life of the piston.

Now with all that being said, I run forged pistons in my bike. But I originally was going to run Nitrous on it. I took great care in selecting my pistons and cylinders. I made sure that my forged piston measured at the tightest allowable tolerance for forged but fell within the maximum allowable tolerance for a cast piston. I then went through many heat cycles before I actually rode the bike. I used great care at the very beginning of the break procedure, which included riding it until hot and the cooling it off several times. I have had no problems with the pistons but if I had it to do over again, I would run cast pistons.

In the build, in the original post, they use a flat top and very little increased compression and suggest that it is a build for everyday riding. I see no need to run a forged piston in that situation.

So, compromised durability and reliability, you need more of a piston-to-cylinder gap to tolerate the dissimilar metal expansion rates, and you probably should move to a different cam as well.

Oof.

[grcamna2]

wohali,
I imagine you might have already thought of this but did you try either checking the site w/ your piston,rings,etc. OE part #'s(I'm sure w/ OE price will be up)or calling them w/ their USA phone # at davidsilverspares.com ? I wondered if you would prefer OE ? it could be that those parts are in short demand everywhere?....

[wohali]

I've tried Honda direct and none of the warehouses around here have it. I've not tried calling David Silver direct but the website shows nothing. I suspect since Honda deleted the part numbers a few years ago, the supply has grown very thin, you're right

I'll sleep on it but probably buy the eBay NOS 0.25 OS pistons tomorrow.

[grcamna2]

Are you sure going only .25mm will resolve your out of round cylinders and give you perfectly round bores ? That's the question I'd ask the machine shop.

[wohali]

Willy actually suggested 0.25 or 0.50 if necessary.

My measurements with feeler gauges show that, at most, I can get a 0.004" == 0.1 mm in on the worst worn cylinder. Shouldn't 0.25 over be sufficient in that case? I guess you can't be 100% sure without doing the full round measurement.

Now we're back to the money problem: I own no car and don't want to strap the cylinders to my motorcycle. That means renting a car to get the cylinders to the shop, which is a $50 proposition. At that point I might as well just bring the new pistons over and have it go.

I'm thinking I buy the 0.25mm pistons and if it turns out I need to go up higher, I can sell the pistons again on eBay to someone else, or hang onto them for another basket case F if one every crosses my path

[seanbarney41]

get a back pack

[bwaller]

Some forged pistons require larger clearances than other forgings, depends on the silicon content. Lower the silicon higher the rate of thermal expansion, but are also very strong. Some forged pistons run fairly tight and you would never hear any piston slap when cold.  It has nothing to do with reliability.

I believe Honda only ever used cast pistons.

[wohali]

Thanks for the advice everyone! I really, really do appreciate it.

The plan: I bought those Honda original NOS 0.25 OS pistons off of eBay. I'm going to bring them and the cylinders into Gord Bush for reboring and fitting. While that's in progress (2 weeks delivery on the pistons, then 2-4 weeks wait for the shop) I'll pick up new gaskets, factory valve seals, and I might as well get new piston pins and clips.

If it turns out these pistons won't fit, I'll make them available for sale here on the forum and go with one of the commercial offerings I listed above - probably either the German set (less of an overbore) or the Dynoman pistons. If I go Dynoman, I will probably also switch out the camshaft.

[HondaMan]

Thanks for the advice everyone! I really, really do appreciate it.

The plan: I bought those Honda original NOS 0.25 OS pistons off of eBay. I'm going to bring them and the cylinders into Gord Bush for reboring and fitting. While that's in progress (2 weeks delivery on the pistons, then 2-4 weeks wait for the shop) I'll pick up new gaskets, factory valve seals, and I might as well get new piston pins and clips.

If it turns out these pistons won't fit, I'll make them available for sale here on the forum and go with one of the commercial offerings I listed above - probably either the German set (less of an overbore) or the Dynoman pistons. If I go Dynoman, I will probably also switch out the camshaft.

That will be a fine choice!
Talk to the machine shop, MAKE SURE THEY KNOW that you want .0008" to .0012" clearance, NO MORE. Too many machine shops over-clearance for these pistons. It took me 3 engines to "break in" my machine shop guys, but now they at least ask me every time I bring them another one.

[wohali]

Thanks Hondaman! Gord used to build and race CB750s himself in the 70s. When I brought my head in, from across the room he said "Wow, an F2 head." So I think he knows what he's doing, but I'll state it just in case.

[welard]

Hello wohali

i only came across your thread in the last couple of days. Like a great thriller, I haven't been able to put it down and have read both threads about Bessie over the last day or so. I appreciate its now been 3 years and your tenacity is to be commended.

I've got a 76 F1 which I am in the planning and budgeting stages of a rebuild and enjoyed your thread as an informative and cautionary tale.

Hoping there has been a fairy tale ending and would love to hear an update.

[wohali]

Hey there,

I'll keep updating the main thread: http://forums.sohc4.net/index.php?topic=80554

Quick update: top end problem was definitely the cylinders/rings, had them overbored to 0.25, found the first oversize pistons and ring combo, and will be reassembling them soon. Bottom end is now back together, at least!

[PeWe]

Another possibility to use the low CR 836 pistons together with CB900 Dohc rods that are 1.5mm longer and beefier than old CB750 sohc rods. CR will be good enough?