brad black
two strokes can go much closer than 4 strokes because they don't have the piston at tdc without gas pressure on it. it's tdc overlap when the rod is getting stretched trying to stop the piston that squish becomes a too tight thing.
my 888 had 1.0mm without base gaskets and the old fibre head gaslets(approx 1.3mm). sounds odd that you have that much without any base gaskets, like you're a mm short. you haven't got 916 pistons have you? that'd do it.
griff851
I brought the barrels and pistons which came out of an 888, but I guess that doesn't mean the pistons had to be 888's. Who ever owned it before was a bastard I know that much, he'd stuck down the head gasket on one pot with silicone. The pistons weigh 384 g each. I thought I read some where here that that was an 888 piston and 916 pistons were lighter.
I have 2 choices now S4 head gaskets, or the lathe. Or maybe both depending on the thickness of the S4 gasket. I did order a set of head gaskets from Cometic in anticipation of this, they are 0.45mm (0.018"), but unfortunately they don't quite fit the droop cam head. The little joining rivets don't quite sit in the water jacket. They fit fine on the barrels.
Griff
Mr.R
Hi Griff,
I bought my stock of 96mm Cometic gaskets from 851Neil, (later 955Neil) after his 955 Special was stolen.
He forwarded me a Word Document from Comectic that stated it's was OK to remove the rivets before fitting the gaskets.
Steve R
hauge
I have copper gaskets made up for 94 and 96mm 851/888/916 engines and 98 and 100mm for 996 engines. They are 1mm thick and requiers a slot inte the barrel for 1mm copper wire (1mm wide, 0,6mm deep).
They need to be heatet to be soft though.
A friend of mine (Tomtom) har allready tried them on his 100mm 996.
Cheers
Thomas
griff851
I have just done a quick comp ratio calculation using a web page calculator.
If I use the comectic 0.45mm head gasket and Threebond on the barrel base I get 12.17:1. The squish hight is at about 1.18. If I bring down the squish to 1 mm I have 12.55:1. Is it worth shaving off 0.06mm (0.002") off the barrel to get to 12.3:1? That is todays burning question. I'm swaying towards not worth the effort. I found a little comp ratio HP calculator, it told me piss off and not bother as it was less than 1%.
Griff
brad black
the next issue you're coming into is belt tension - once you start dropping the heads you have issues there. i haven't had any issues with older motors and no base gaskets, but they've had thick head gaskets. i don't recall if we had issues with drop head motors. actually, now i think about it i'm sure we replaced s4 or st4 belts under warranty when they wouldn't tension to spec at 10,000km services.
if you were going to bother to machine the barrels i'd take 0.23mm off at least to take it down to 0.95mm. no point going to the effort for 0.06mm. comp is good.
otherwise just run 1.18. plenty of motors from the factory have been happy with that.
haven't got a 916 or 900 crank sitting around have you? that'd fix it.
griff851
brad black wrotehaven't got a 916 or 900 crank sitting around have you? that'd fix it.
Ah, I was so close to that a while back. 😁
Griff
griff851
I have sort of been aware of the belt tension issue for a while, since talking to Bevan ages ago actually. I have 2 different sizes of belt tensioner pulley's (black original ones and gold ones off something), that combined with a range of belt sizes I hope it will be OK. Unfortunately its one of those things to be dealt with when the time comes.
Griff
griff851
Well today the cases are finally together. An interesting observation. Threebond does not take up 0.03mm. Its more like 0.01, which of coarse means all my gearbox end floats are no longer in the middle of the tolerance range. And my preload calculation was way out. I didn't get a chance to check the end floats today apart from a quick grab to make sure there was some end float, and there is,so next week I'll see how they ended up. They should still be with in tolerance. The rotation and selection feels great. The crank in theory should have now 0.08-0.1mm preload. I called in the feel expert, he whacked the crank primary shaft end with a soft faced hammer, rotated the crank, made a snuffle noise, and he seemed to give it a good pass by mumbling "nice". The young boy came in next and did the same thing. As the Scott approached I pleaded with him not to whack the crank as it had got the message.😄
Interesting to note that while I was going through the manuals, a kit and 851 gives a defined 0.1mm end float for gearbox shafts, an 888 gives it as 0.15 and 748 and others st4 etc gives a range.
griff
griff851
Finally got some more time on the engine this week. I checked the new end float figures and got about 0.09 on the primary and secondary shafts (Range is 0.05-0.15mm) so thats Ok, the drum I got about 0.13 (range 0.10-0.40mm) so thats a little tight, but still OK. Next it was fit the timing gears. No woodruff key for the crank shaft gear, so i went and got the genuine part. Yuk. the slop was...well.. not really acceptable. So I cam home and rummaged around and found a bit of key stock.Hack file and Perfecto!
I attached my new gear selector drum stopper, new selector arm (from the S4 mon, so newer than my old ones).
Then it was find a shim for the sprag clutch side of things. then on with the brand new second hand starter gear and.... bummer. Need to clean up the surface on the which the sprag grabs. There are nice new shiny ones at the shop, but has anybody priced one up? About AUS$350. I hoped my old one would be OK, but it had worn a 0.25mm step in itself after 64000km. Which does go to show how much wear they can have and still work well.
So I fitted the starter gear on and and did a trial fit of the sprag gear and fly wheel and my SP5 alt rotor. Then contemplated the SP5 rotor and the standard rotor. The SP5 rotor won in the name of rotating mass reduction.
Next its clean up the face of the sprag gear, change the bearings and inner race for the sprag gear. Change the bearings in my large primary gear, oil pump on, fit primary's and alternator shaft stuff and the bottom end is finished.
So no Paul the engine is not in the frame yet.
Griff
PS. I heard today a nice condition 851 bit the dust, rider Ok. Bike destroyed by the rear tray of a Ute. Must be only 10 or 11 in Brisbane now. As the numbers go down the price goes up.
griff851
Once again deciding that family life can wait, I got on with the project again today. The 748 throttle bodies have now finally had the second taper cut. A nice 4° from the 50mm section just under the plates. A quick clean and reassembled.
Large primary drive has new bearings in, and fitted there waiting for a new spacer to turn up from Italy. The old one had groves worn it from the seal. Still waiting for starter gear bearing and inner. The inner then has to have the oil holes ground out with the dremel, so the edges of the hole don't eat the needle roller bearing.
The rings went on to pistons and are now sitting in the barrels (long strong thumb nails a must). Tomorrow morning oil pump, and small primary will go on. I'm starting to anxious, as the cold dry mornings are here and I'm missing the engine sucking in all that lovely dense air. Not missing the frozen hands but.
Griff
griff851
The battle continues. After setting up the heads mit feeler gauges, the young fella mechanic presented me with the all singing and dancing dial gauge and little arms kit. This niffty little device also lets you find center maximum lift on your cams. However it quite clearly pointed out correct closer clearances as well. Faced with a little needle telling me I was with in tolerance, but not all the same by as much as 0.03mm, I spent some joyous time grinding closing shims. Now all exhaust are 0.09 and Inlets are 0.13mm. The openers required a touching up and I got them to 0.15 inlets and 0.2 exh as per the drop cam head spec. sheet. When you guys were saying MBP collets are a bit of mucking around you weren't joking. The young fella couldn't work out why it was taking so long, he came in, had a shot at one, got half way through it and walked away.😁 He has however volunteered to do the 10,000km check just to see how good they are.
Below are nice pictures of where I am at. Nice bike in the back ground for the enthusiasts to ponder.
As you can see it appears to becoming very close to almost nearly being completed. However if you look at the left had side shot you will notice only one belt (and maybe the little bit rusty tab washer, and possibly the el cheepo mirrors on the 95 monster 600 shop bike. It has the latest in Italian design variable noise constance speed engines.😁 ). Any way this the point I have reached that I have known about for zonks and wondered if I could cure the issue with a variety of different size guide and tensioner pulleys and belts. Well of cause the answer is I couldn't. If I fitted a barrel base gasket I would have been in, but nooo. I want compression and smallest squish band I could get. So it leaves me now with no alternative. PLAN B. Plan B is cunning and still cost effective if consider I got 4 pulleys for $50, new the are $60 something each. PLASMA SPRAYING. Yes plasma spray, the amazing technique of adding stuff when you've fucked up. Just throw on a couple of mm on the pulleys an a bit of machining of the lip that holds the belts in track afterward. Plan B I hope should work, if not PLAN C........
Just another small issue I came across today worth noting. Cylinder head bolts. The 851 ones are just a tad to long for the drop came head. Very annoying if you want to torque them up with the U beaut tool, you can't.The ring spanner bit would need to be ground down to far to remain strong enough. How ever I have a special torque wrench which takes standard spanners.
And as I was engaged with belts today, young fella the mechanic, proved to me that the twist the belt 45° and get the right tension was a crock of shit. It might work on cars, but sure doesn't on bikes.
Griff
marvin
Geez Griff! You're worried about .03mm!!😯 They'll change that much the first time you fire it up!
Looks great!
griff851
Yeah I had an anal retentive attack big time, but while I was there I thought what the hell.
I can't wait until tomorrow to find out how much Plasma spraying is. I need about a 1-2mm on each pulley to get plenty of adjustment. I had thought about using the cam grinding place, but the welding they use will distort the pulley too much. I was spewing when I went through the cam belts. A 1098 belt I could have used, but the belts are wider. Ah... the best laid plans of mice and men!
Griff
marvin
What diameter pulleys do you need? there are several OEM sizes. I might have one or two laying around.
I needed one larger pulley so I just made it.
griff851
The pulleys are at the shop. I'll get the sizes tomorrow. Darn it, you nailed my plan C.
Griff
Mr.R
Hi Griff,
That's looking pretty good chap, I'm sure that your efforts will be worthwhile.
All this fucking around over pullies when dropping in a longer stroke crank would have sorted it and given more power throughout the rev range...
Ah but with the benefit of hindsight we'd all be millionaires wouldn't we?
I dropped a 916 crank in my motor and had more adjustment than you could shake a very large stick at even with the squish set at 0.9mm.
I've really enjoyed this thread of yours so far, you may be on the other side of the World, but I feel that you're a kindred spirit mate.
I'm also fairly sure (and I've no doubt here) that there's a few others on here that feel the same.
Keep the info (saga) coming mate I wait with baited breath for the next instalment, it's a far better read than any of the mag's over here, even the Classic ones which are my favourite read!
Nothing beats getting in there, getting your hands oily/dirty and learning more about your bikes motor when tuning your own bike IMO.
Steve R
griff851
Ah Steve, in a couple of years I'll be as old as yourself, I'll also have scored another set of cases and a gearbox and a 900ss crank by then. So I'll give that a whirl. Why buy a new bike when you can up grade ad nausium for next to nothing. I do know where there is a decent 916 crank now, but its a little bit too expensive.
I could have a line of cases and the bike on a rolling frame. Then I could play Thunderbird 2. 😁
Griff
Thanks for enjoying the thread. It's brought about some great technical info for everybody I hope.
got2dv8
Hey Griff,
Just checked out your pix of the progress. Man that mill is clean. My cases look like crap! While she's out don't neglect the swingarm bearings and seals... good time for replacement/inspection. Plus I know you'll like running your finger in and out of there with some good lube. Surely you can't be gettin' any with all the time you are spending with your bike😁 .
Robert
griff851
Camera flash is an amazing thing for making things look shiny and clean. The cases are a bit stained, but you can't see that. The heads and barrels are all new and clean looking.
The reason I spend time on the bike is to get away from the wife. They steal your Yang don't you know. Leaving you with too much Yin, which ups sets your riding and beer drinking ability, and makes you want to stay home and bake cookies and cakes for competition at the church fete.
As for the swing arm bearings, they are new. (No slop now) and I use a very sexual Mobil 28 Lithium high pressure grease. Which stops the swing arm pin from becoming depressed as well.😁
Griff
