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CJ/CB360T Head porting

RustiePyles

Member
Joined
Dec 3, 2020
Total Posts
45
Total likes
1
Location
Mission, Ks
I'm getting ready to tear the head off my CJ360, its got some boogered up plug threads on the left side and I'm putting an insert in. I figured while I have the top end apart it would be a good idea to freshen everything up. So here's my plan: three angle valve job, cut seats and grind valves. Clean up casting slop in intake and exhaust runners. Match intake ports to intakes and surface intake runners to 80g. Polish exhaust runners. I'm not trying to change the profiles or sizes of any of the ports, just a clean up. I'm not looking for recommendations on labor or someone to do the labor. I'm more looking for information on how responsive these 360 twins are to a 3angle valve job and some port clean up. If its not worth my time and resources I'll just lap the valves and call it good. I'm not really looking to build a fire breathing motor just to realize the motor's full potential and increase efficiency. But maybe I'm barking up the wrong tree. This is the formula I've followed with most all other parallel twins with success so I'm betting that this one is no different. What says the brain trust?
 
Other than removing casting mating ridges you're best to leave the ports alone. You want the rough finish there to create turbulence which keeps the fuel atomized in the air stream. The exhaust doesn't really matter but polishing usually causes flow problems, unlikely you'll notice any and if done it's unlikely you'll see any difference.
A 3 angle valve seat cut is the minimal work to perform on the seats. Haven't checked the FSM but most of the time Honda seems to call for a .040"(1mm) intake seat and .060"(1.5mm) exhaust seat. I prefer to use .020" intake and .040" exhaust. If done properly there's no need to lap the valves when done. These valves should be the one time use type, thin hard face, so you can't grind the valves. Replacement only.
With replacement valves be sure to check the actual seat face where it meets the head of the valve, sometimes there's a ridge there which impedes flow and should be removed.
 
Ok, Didn't realize these valves weren't fully hardened, that makes grinding a 3 angle profile on em difficult. I guess I'll pull the valves and clean em, check em for leaks and put em back together with new valve stem seals.
 
The 3 angle valve seat cut is done to the seats not the valves.
Before making decisions you need to inspect the seats and valve faces for any damage like pits, scorch/burn marks, etc. Be sure the valve face isn't worn in concave also.
IF they are good and clean then clean everything up and you should be able to get away with a very light lapping of the old valves. The hard facing on the valves is only .0002" so it takes very little to cut thru that.
 
RustiePyles said:
I'm getting ready to tear the head off my CJ360, its got some boogered up plug threads on the left side and I'm putting an insert in. I figured while I have the top end apart it would be a good idea to freshen everything up. So here's my plan: three angle valve job, cut seats and grind valves. Clean up casting slop in intake and exhaust runners. Match intake ports to intakes and surface intake runners to 80g. Polish exhaust runners. I'm not trying to change the profiles or sizes of any of the ports, just a clean up. I'm not looking for recommendations on labor or someone to do the labor. I'm more looking for information on how responsive these 360 twins are to a 3angle valve job and some port clean up. If its not worth my time and resources I'll just lap the valves and call it good. I'm not really looking to build a fire breathing motor just to realize the motor's full potential and increase efficiency. But maybe I'm barking up the wrong tree. This is the formula I've followed with most all other parallel twins with success so I'm betting that this one is no different. What says the brain trust?
Click to expand...


I had one of my cylinder heads "cleaned up" and the ports were opened-up in size but cut to match the original port shape by a porting expert and it made a Big difference on a CB350 I used to own,along with the exhaust having better breathing(making sure my OEM mufflers were cleared-out inside and not clogged,lightly sand blasting a little,then blowing it all back out:OEM muffler is #HM344,they do fit and work good on a CB360 but are unobtainium now..)and the replacement OEM style high flow air filters.This was using the stock camshaft.The changes made my bike rev much more freely.The valve seats were ground and new valves installed(these valves all seem to have a thin hard-coating that isn't very deep) plus I made sure my ignition(points & advancer:I also installed a HondaMan ignition box in conjunction with my OEM points and condensers)and carb jetting was all set 'on the mark' and functioning 100 percent.A pleasing balance was achieved and enjoyed.I had plenty of time to devote to these improvements,it did take a while to do but I enjoyed it.
I think the CB/CJ360's don't have many people doing high performance work to them compared to the CB350 twins,but that doesn't mean there isn't potential waiting there to be hooked-up :)
 
LongDistanceRider said:
The 3 angle valve seat cut is done to the seats not the valves.
Before making decisions you need to inspect the seats and valve faces for any damage like pits, scorch/burn marks, etc. Be sure the valve face isn't worn in concave also.
IF they are good and clean then clean everything up and you should be able to get away with a very light lapping of the old valves. The hard facing on the valves is only .0002" so it takes very little to cut thru that.
Click to expand...
Right, but when doing a 3 angle seat cut you also have to grind an additional back cut on the valve. I understand that the valves do not get ground to 3 angles. Grinding the additional back cut @30d would grind through the .0002" hard facing.
 
bilbikek411 said:
I think the CB/CJ360's don't have many people doing high performance work to them compared to the CB350 twins,but that doesn't mean there isn't potential waiting there to be hooked-up :)
Click to expand...

Why are the 350's so much more popular for tuning than the 360's, just more produced?
 
Last edited by a moderator:
I think that's mostly the reason, kinda like the SOHC 400/450 series didn't get much aftermarket attention
 
360 was only a 3 year ('74-'76) model, 350 was '68 to '73?. So yeah, a LOT more 350s produced I imagine. And yeah, PJ has posted some good info about 360s over the years. At Do the ton and the old Twins site.
 
350 is more popular as it's eligible for vintage racing. 360 isn't.
The ports are already too big on 360 for the stock motor, going 378cc makes them work a lot better (they are sized for a 400~420)
1mm oversize intake is good if you don't mind the cost, (around $100.00 for 2 valves plus matching them to seats)
Just fitting oversize valves without re-machining seats is a waste of time and opening things up properly can get pretty expensive.
The exhaust port can be opened up quite a bit as it is the major restriction in 360 head
 
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