gpz750 pistons in a cb350 and other engine mods

Q

quach

New Member
I'm in the process of rebuilding my cl350 engine and have run into a few blanks in my research. My plans are to up performance from the motor slightly, not looking to build an all out race engine, just something a bit more peppy than the stock 325cc mill.

Ive been doing alot of research on pistons, and I know I don't want to spend 400+ dollars on THR or Bore Tech big bore sets, so I've been looking at alternatives. One that really appealed to me was the GPZ750 pistons, I know they are 66mm and will up the compression to 11 or 11.5:1. I havn't found any substantial information on all that needs to be done to integrate these pistons into the 350 mill. My main questions are about the milling that needs to be done to the gpz pistons for clearance, how much needs to be done and where. Also, what do you do in terms of a headgasket?

I've also been seeing NOS "Becks" pistons on ebay here and there that are 3mm oversized but they mention nothing about whether or not the stock compression ratio is maintained. Does anyone have any information on these and how much I should spend on a set?

One last thing I've been researching is carburetors. From what Ive heard, mikuni vm32's don't offer up much extra performance on a stock mill. At what point in an engine build would the stock carbs become obsolete? I have a set of cb360 carbs at my disposal, does anyone have the specs on these? would they offer any performance gains over the stock cb350 carbs?
 
ive gone over all this multiple times for myself... i looked into the gpz pistons, and they are a nice boost in performance, but they dont have deep enough valve cutouts for a high lift cam. in the end i ended up deciding bore tech is the way to go; cheaper then THR, stock compression (not a race motor), and deep valve cutouts. best street piston there is for this motor IMHO, and much lighter then the gpz pistons.

if you still want to go with the gpz pistons heres a couple things to look out for...

the stock size will fit without anything but an overbore. compression will be uped to 11.25-1 if im not mistaken, so high grade fuel will be necessary, and you may need to retard the timing a bit if you dont have a high enough grade available to you. bore tech head gasket should work fine. Good luck finding gpz NOS pistons... i havent been able to find any, and the ones youll find for sale may be in cruddy condition.

Stock carbs work fine for the motor... its just that tuning them is a bitch. VM30's are nice on a stock bore motor, even nicer on an overbore. with a 1.5mm overbore you can run vm32s nicely without having to run very small jets.

cb360 over cb350? i think they are a bit larger in size... Crazypj would know for sure.

just double checked on bore-techs website... 1 or 1.5mm over pistons are only 130 for a pair... about the same as good condition gpz pistons... plus you dont have to go looking for them. the 400 dollar pistons are their racing pistons which you cant run easily on the street ;)


i also highly reccomend a performance cam chain slipper... the ones from bore-tech are nice and cheaper then thr.

dynatek coils and an electronic ignition and you got a strong reliable motor without touching the top end at all.


if you want to go all out on the motor, it will cost you about 1400 dollars plus machine work if you get everything through bore-tech (Oh, and that cost includes a sweet cam from megacycle).


try using the search function a bit btw ;) gobs of information on both google and here.


http://www.bore-tech.com/CB.html
 
Do you have weight of Boretech pistons?
I would be interested to see how they compare to Wiseco

PJ
 
hhmm there is a listing for wiseco pistons :o,,other than todd henning??? please tell us!!! gpz pistons are estimated at, or about 10:5 to 1...the outer diameter has to be relieved as not to contact the cylinder head,,,stock head gasket will work no problem.If you can find a mild lift cam(I have 2 of them with the#94 stamped into them)and have no farking idea who ground them!! that cam will work with said pistons and 32mm mikunis with stacks and copied Hindle one off shemgaphones ::),,,,,,timing was never altered,,more octane was added(90-94) Bore Tech offers a CHEAP overbore piston(s),,,and this is a good value considering the alternatives.,.choose a stock cam or a 68- 69 cam and rockers,,,chuck the cv's for 30mm mikuni's.......and don't forget$$$$$$$$$$$$$$$$$$$$$$$$$$,,,,,,I highly doubt it will be a commuter......make sure it has compression ,good electrical,,,clean fuel system,,,,,make it run 8) and spend the money on good tyres and suspension.......then sort out the engine performance.........
 
Wee Todd said:
hhmm there is a listing for wiseco pistons :o,
Click to expand...

Nope, but I know what fits CB350 with minimum modification, got a pair with sleeves
These are only 65.5mm but I've had up to 67mm
PJ
CB350Wiseco.jpg
 
crazypj said:
Nope, but I know what fits CB350 with minimum modification, got a pair with sleeves
These are only 65.5mm but I've had up to 67mm
PJ
CB350Wiseco.jpg
Click to expand...

no weight on the bore-techs... ill shoot then an email this weekend.

what wisecos and that mods did you do pj?

why dont you just write us all a book...

i would honestly spend 100 dollars on a book written by you on the 350 if you were to make one.
 
This is great info guys, thanks!

From the looks of it, gpz pistons sound like a bigger hassle than I want to deal with for this motor. Also the highest grade gas we get in California is 91 octane... unless i go all out on 110 race fuel which is $$$ for a street driven bike.

The Boretech 1.5mm overbore pistons sound like a great option, I didn't know they were lighter than oem. This may be the way to for me. Hopefully I can get my hands on a 68-69 cam and some vm30/32's and i'll be quite happy.

Anyone heard of Beck's cb350 pistons? The only info I have on them is that they are 67.5mm. last time i came across them they were going for a steal on ebay, but unfortunately i snoozed on them because i couldn't find any real info or opinions on them. Here is the set that I saw recently:

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=300394870396&viewitem=

I have most of what I need for suspension, brakes, and electronics sorted out for this bike, but the engine has been the part of the puzzle that I've had the most questions about and least experience with. I appreciate the input guys, keep it coming ;D
 
quach said:
The Boretech 1.5mm overbore pistons sound like a great option, I didn't know they were lighter than oem. This may be the way to for me. Hopefully I can get my hands on a 68-69 cam and some vm30/32's and i'll be quite happy.
Click to expand...

FYI the early cam has smaller lobes than the later cams. It allowed for higher redline revs. Combining lower a lower lift cam with larger pistons may not provide the results you're looking for.
 
NOTE: You can only use the early cam with the early header internals... not sure exactly what it was... but the splining on something is different between the early bikes and the later ones. also as drj said... the early cam will give you a rev happy, large bore motor, but you lose some in the low and mid range. either keep the cam stock or splurge on a megacycle.

those pistons look good, but they will be comprable if not worse then the boretechs. im not sure if the boretechs are lighter... my thinking is that they will be, but ill double check with them this weekend.
 
Early cam and rockers generate about .020 more lift and the lobe centers are further apart(higher rpm hp) but the rockers are prone to wear faster than post 70-73 motors
 
ooooopppss the cam sprocket has to be the early type as well,,later sprocket have a larger internal diameter,and won't bear on the machined boss and flange properly
 
spec for my slugs,bore tech 67.5mm ,12:5 to 1 are 189 grams...(hand written and circled on the build sheet)....these are forged pistons and NOT oversize cast pistons.....cast items should be much lighter
 
For anyone interested, I just heard back from Bill over at Boretech and the weights for the cast 1.5 o/s pistons are 192 grams. For comparison, most stock pistons are around 202 grams he says.

A bit off topic, but what is a good price to have my 350 block bored and honed? Anyone know a good place in the LA/pasadena area? Should i have a shop do the deassembly/reassembly it for me? My dream for this bike was to learn how to do everything myself, which includes the full engine rebuild (minus machine work) but if that sounds like a bad idea for any reason please let me know.
 
quach said:
For anyone interested, I just heard back from Bill over at Boretech and the weights for the cast 1.5 o/s pistons are 192 grams. For comparison, most stock pistons are around 202 grams he says.

A bit off topic, but what is a good price to have my 350 block bored and honed? Anyone know a good place in the LA/pasadena area? Should i have a shop do the deassembly/reassembly it for me? My dream for this bike was to learn how to do everything myself, which includes the full engine rebuild (minus machine work) but if that sounds like a bad idea for any reason please let me know.
Click to expand...

build the motor yourself if you know how. if not, learn. depends where you live... boring+hone can be seen as cheap as 40 a cylinder, around my house a shop quoted me 100 each cylinder. since i plan on getting over 1000 dollars worth of engine supplies from bore-tech (yes, i will buy there entire catalog ;D) im going to see if i can work out a deal with them such as having them bore my cylinders for free. either way id like to send them to someone like bore-tech seeing as how thats what they specialize in.
 
$40.00 a hole aint too bad.
If your going first over, wouldn't bother to bore, just hone.
Second over, I would think about rough honing with 80 grit then finish with 280~400.
Rocan,
I used to do a lot of boring/honing. (hell, I used to teach it)
It takes time to set up properly and more time to hone correctly to size.
I'm usually taking 2mm or more from my cylinders so 'normal' bore times don't apply but it's usually around hour a hole to be completely finished
Whichever way its done, there is a bunch of expensive equipment to pay for.
Getting it 'free' is doubtful unless they are doing the full engine work (in which case it's part of total cost for job)

PJ
 
I lucked out and found a set of NOS Rocky S12 over bore pistons (67mm) on eBay for 9$. Found the S12 head gasket as well. Just got them loaded up this past weekend. Search around and see if you can find some. Rocky was an OEM Honda supplier I believe. I had trouble finding S12 rings until the lovely folks at Bore Tech informed me that CB360 rings will work, and they do.
 
djelliott said:
I lucked out and found a set of NOS Rocky S12 over bore pistons (67mm) on eBay for 9$. Found the S12 head gasket as well. Just got them loaded up this past weekend. Search around and see if you can find some. Rocky was an OEM Honda supplier I believe. I had trouble finding S12 rings until the lovely folks at Bore Tech informed me that CB360 rings will work, and they do.
Click to expand...


Damn! nice deal! Thanks for the tip, have you gotten to test out the bike yet? How do you like them?
 
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