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I may be wrong on this, but it reminds me of scuffing that can happen when a fresh bore is run a bit too hard before being fully broken in, and the friction level and the resulting temperature is still too high. the front side is the thrust side because of the direction of rotation. I wouldn't think it was because it's too sloppy, I'd suspect the other way around
 

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Bore is probably tapered and just a bit tight at the bottom? Normal reason is hone got things hot and the thin part expanded way before the main block.
Did they use an automatic hone? When I used to teach this stuff 'we' used to leave block at least 20 mins then re-measure and adjust if needed. If bore is round and parallel, piston has gotten too hot during break in (or more likely during first start) Problem is, you need to hold around 2,000rpm to make sure small ends and rocker pivots get oiled. You didn't go out in freezing weather or hit any rain? That could chill the front of block enough to pick up slightly (much more common on dirt bikes)
How do piston skirts look? Should be possible to polish things up with some 4~600 grit 'wetordry' with kerosene to wet it
BTW, the REAR of block is were the thrust happens, con rod is 'leaning' backwards as crank is turning 'forwards' The thrust on front of cylinder is minimal in comparison to the pressure developed by combustion. If bore was far enough oversize for it to be piston slap it would also be on back of cylinder and piston wear would be obvious. It's tight,
 
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Discussion Starter · #5 · (Edited)
Thanks.
No idea how the shop does the work I thought I was being gentle with it during break in? Always been told...don't labour it and don't rev it, and keep rolling it up and down rather than cruising for extended periods? :D
Piston top ring gap is 0.4mm.
I'm using those Wiseco 71mm pistons. There is a small amount? of scratching on the piston
Auto part Metal
 

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I have a cheap set of micrometers and telescoping bore guages. Good enough to judge and check a machinists work. Tapered bore or whatever could allow some blow by that would take out any oil film.
 

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Possibly, the Wiseco pistons need a bit more clearance than the stockers.
 

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Thanks.
No idea how the shop does the work I thought I was being gentle with it during break in? Always been told...don't labour it and don't rev it, and keep rolling it up and down rather than cruising for extended periods? :D
Piston top ring gap is 0.4mm.
I'm using those Wiseco 71mm pistons. There is a small amount? of scratching on the piston
Piston doesn't look bad... I agree with you on the break-in thoughts - keep it in an unlabored rev range, not too low or high. I notice you're using the 219H chain. Any issues with cam timing lining up correctly during assembly?
 

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If they are the Wiseco Higher performance/compression Pistons they do need more clearance then Stock.

I believe the spec sheet that comes with them indicated .004-.005 VS .002 for Stock
 

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Someone on DTT says he uses stock clearances with Wiseco pistons in CB750?
Of course, I had to disagree and pointed out Wisco 'pink slip' says to use at least 0.003" and as much as 0.004+ for race applications.
Anyway, you need clearance checked and also need to check with Wisco for correct clearance.
There are an awful lot of racers using them without issues
I'm more familiar with CB360's nowadays and use 0.0017"~0.0018" clearance with cast pistons (Suzuki GS850) Haven't had any 'pick up' issues so far (neither have any of te other people who have done conversion)
You did get 'instruction sheet' with the pistons?
After looking at piston picture on larger screen, you didn't ave any 'foreign objects' stuck to pistons wen you assembled things? Maybe something on underneath front of cylinder got caught in the oil on piston skirts? The tops are too black for piston to have overheated althoug a 'cold seizure' is a possibility?
 

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Discussion Starter · #11 ·
I notice you're using the 219H chain. Any issues with cam timing lining up correctly during assembly?
Don't think so...seems to be Ok? Tell me more :D
I did take the paper work in to the shop that did the boring work...just in case. They seemed to know all about what was required though.
Had another thought on this last night..
Does the cylinder lubricate from crank splashing in the sump, or does it run back down the cam tunnel and spray from there? Reason being at first start up some bozo had been polishing the oil spinner cover and had replaced it without the guts in it. The result of that was 20-30 seconds of running with no oil being pumped...luckily said bozo had the cam cover off waiting for oil flow to appear. :(
 

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Don't think so...seems to be Ok? Tell me more :D
I did take the paper work in to the shop that did the boring work...just in case. They seemed to know all about what was required though.
Had another thought on this last night..
Does the cylinder lubricate from crank splashing in the sump, or does it run back down the cam tunnel and spray from there? Reason being at first start up some bozo had been polishing the oil spinner cover and had replaced it without the guts in it. The result of that was 20-30 seconds of running with no oil being pumped...luckily said bozo had the cam cover off waiting for oil flow to appear. :(
"Said bozo" caused a brief interruption of oil flow to your camshafts :D - hopefully he had pre-lubed them well *grin*. I just re-read the FSM with respect to oiling and there is no direct mention of piston lubrication, but it's always been my understanding that they get splash from the crank and rods. FSM states the camshafts' rotation splashes lube on the cam chain rollers and tensioner down the tunnel. As for the 219H chain... I'm surprised you hadn't read some of the many discussions on here about the heavy duty version of the 219 chain being more difficult to get cam timing correct, and the differences in master links as well (can't use one with the other and vice-versa). After some research, I chose to use the 219T chain from DID which is the OEM chain - I'm familiar with the chain, have never had any trouble with them in any way, and the master link is easily staked as design-intended. Just personal preference in some ways but there are a few people here who have tried to use the 219H and had difficulties getting the marks to line up as they expected (I haven't ever tried one so no personal experience here). Some feel it's the diameter of the rollers in the chain, others have suggested it's the side plates' size causing the chain to lay on the rubber damper rings on either side of the sprockets and therefore ride higher in the sprockets (I don't use the rubber damper rings, they just deteriorate and come apart after a while anyway and the slight additional noise supposedly generated without them isn't an issue to me)
 

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Oiling is almost exactly the same as CB 350 and other 'early' designs. (125, 160, 250, et.al.)
Oil is fed from pump through drilling's in crankcase and clutch cover to centifugal filter on crank end then back into cover and eventually to oil gallery behind cylinders where it feeds main bearings and transmission (and through drilling's/jets to cam/top end)
From main bearings, oil 'leaks' onto side of flywheel where it's centrifuged to a lip which directs oil to big end bearings. From big ends, oil is thrown around the motor, including the cylinder walls. Even then, the rear of block should have less oil than the front because of crank rotation so back of block normally shows greater wear.
Oil draining from head only lubricated cam chain and tensioners
The only thing I can think of, you had some distortion from exhaust heat?
 

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Discussion Starter · #15 ·
"Said bozo" caused a brief interruption of oil flow to your camshafts :D - hopefully he had pre-lubed them well *grin*. I just re-read the FSM with respect to oiling and there is no direct mention of piston lubrication, but it's always been my understanding that they get splash from the crank and rods. FSM states the camshafts' rotation splashes lube on the cam chain rollers and tensioner down the tunnel. As for the 219H chain... I'm surprised you hadn't read some of the many discussions on here about the heavy duty version of the 219 chain being more difficult to get cam timing correct, and the differences in master links as well (can't use one with the other and vice-versa). After some research, I chose to use the 219T chain from DID which is the OEM chain - I'm familiar with the chain, have never had any trouble with them in any way, and the master link is easily staked as design-intended. Just personal preference in some ways but there are a few people here who have tried to use the 219H and had difficulties getting the marks to line up as they expected (I haven't ever tried one so no personal experience here). Some feel it's the diameter of the rollers in the chain, others have suggested it's the side plates' size causing the chain to lay on the rubber damper rings on either side of the sprockets and therefore ride higher in the sprockets (I don't use the rubber damper rings, they just deteriorate and come apart after a while anyway and the slight additional noise supposedly generated without them isn't an issue to me)
Yes, plenty of assembly lube thank goodness :p
I can tell you it creates one hell of a sinking feeling when you realise your mistake.....don't think I've ever reached for a kill switch so fast. lol
Regarding the 219H chain, I bought it from our local bike shop, who looked up the chain from his book at the time. As far as I know it hasn't caused any issues for me? My cams lost the damper ring when they were reground so that may be a factor.
 
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