Reunited with an old friend. (1992 851 content)

PTwo

Hello all,

I was recently reunited with my first love... my 1992 851! It's been sitting for 20 years (long story, driven by relocation from the USA to Sweden, growing family, etc.), but now it's time to get it going again. The bike was recently shipped to me here in Sweden and I've started pulling it apart.

Specs:
1992 851 Strada
888 high compression kit
FBF slipons
Nichols lightened flywheel
Pankl rods

My plans are to go through it to make sure there isn't any internal damage from storage (none found, yet) and also to improve key features while following Brad Blacks research (thanks for the generous info, Brad!) so I got an ST4 exhaust system and a PCIII. Will set the squish, mill the heads and time the cams. Also have Nichols engine bolts, RaceTech gold valves and springs, rebuild kits for the brakes, new clutch slave, etc. Will put in relays for the lights, new wiring for battery and stator, lithium battery, mosfet recitfier, along with whatever I can think of to make it as reliable as possible.

So, first question (of many...):
I've pulled off the vertical cylinder and found this piston. From what I was told, "it has an 888 high compression kit in it...". I haven't pulled the piston out to confirm, but this looks like a Pistal. If that's the case, can anyone tell me what the compression would be? It would've been bought between 1990 and 1994. The Pistal website just says "HC" next to a number of their pistons but no hard numbers. I am considering milling the head but maybe this piston, along with the squish setting will put me in the right compression ratio range.

Thanks for any help,
Peter

griff851

Lay solder, and check the squish while all you have to do is just back track a bit. Then you'll know how much you have to play with.

brad black

pistal didn't exist back then. looks like an sp piston to me. the dome is offset by the deep cutouts for the old sp cams, so the net result is not much.

PTwo

brad black wrotepistal didn't exist back then. looks like an sp piston to me. the dome is offset by the deep cutouts for the old sp cams, so the net result is not much.

Interesting. Thanks for that.

So, in that case I don't have to worry about stock cam and valve clearance if I take some material from the head.

The question is... how much?

Mr.R

You can reliably skim 1.00mm off the heads to bump up the comp' ratio to around 12.5-1 to 13-1.
BUT you will then need oversize belt tensioner pullies to overcome the resulting slackness of the timing belts.
Then you will need to re time the cams, as the lower height of the heads will alter the original cam timing.
If you know what you're doing? This isn't a very hard job to do.
IMO it would be easier to fit some OEM 888 flat top pistons before you start playing with the motor.

Steve R

PTwo

Mr.R wroteYou can reliably skim 1.00mm off the heads to bump up the comp' ratio to around 12.5-1 to 13-1. BUT you will then need oversize belt tensioner pullies to overcome the resulting slackness of the timing belts. Then you will need to re time the cams, as the lower height of the heads will alter the original cam timing. If you know what you're doing? This isn't a very hard job to do. IMO it would be easier to fit some OEM 888 flat top pistons before you start playing with the motor. Steve R

Thanks for the response, Steve.

If I don't know the current compression ratio, I'm hesitant to mill the heads. Unfortunately I don't know what numbers "high compression" would be.

The belts are hard to get on/off with the tension pulleys loose. If be amazed if 1mm would have a huge effect. Do they always become to loose with 1mm?

I assume you suggest getting flat top pistons so I'm starting with a known quantity?

Peter

brad black

not sure how heavy those sp pistons are, but the std 888 pistons are the heaviest 4v pistons i've ever weighed.

flat top is the "best" piston top to have. anything else compromises flame travel to some extent.

i'd go flat top and 748 heads. the 748 head will give it enough comp and if you're not going further power wise they seem to work better than the 888/916 head at this power level.

brad black

1mm does have quite an impact on belt tension. takes 2mm off the required length.

griff851

Two of these work well.

PTwo

griff851 wroteTwo of these work well.

Nice. But my heads are fresh with low miles. I don't think I want to budget replacing pistons and heads.

I checked my squish in the vertical cylinder. The problem, as I'm sure you guys know, is the dome position along the wrist pin. The clearance front and back is 1.35mm. the clearance on the edge of the dome is 0.9 and 1.0. see pic for position of solder.

This is how the piston looked from above, after opening it up. Lighter, with no major carbon buildup on the outer edges.

Peter

brad black

you could take that edge off pretty easily in a lathe.

PTwo

These pistons, made by Mondial, weigh 531g with wrist pins and rings

brad black

that's the sp piston in it? they are heavy.

PTwo

brad black wrotethat's the sp piston in it? they are heavy.

I guess so, luckily I have a lightened flywheel to offset some but of that.

As the parts are balanced already I'll stick to what I have. The honing marks are still in the cylinder so this stuff has a lot of life in it.

My current plan is to machine the pistons so the outer ~5mm of the dome is flat. Then taper up at around the same angle as now. Remove 1mm from the heads and either machine the barrels or play the base gasket game.

Would you guys suggest rebuilding the brake calipers? They aren't leaking and the system feels fine. I figured I would as it's been sitting for ~20 years.

Peter

PTwo

I just talked to the machine shop and have some questions:

1. Is it possible to remove the intake valves without removing the camshaft?
2. Can the head be disassembled and reassembled with the same parts in the same places without worry of affecting valve lash?
3. Is it possible to safely machine the surface of the head without removing valves? If I was to plug the oil return holes and rinse everything afterwards?
4. How long does it take to disassemble a head?

Thanks for any help here.
Peter

brad black

1/ if you want to make it really hard for yourself, yes. just pull the cam.

2/ if the valve seats and faces haven't been machined, yes. but you really wouldn't not do that if the heads off.

3/ no, the valve heads may be proud of the head face. or very close to.

4/ strip is pretty quick, half hour easy. more like 15 minutes really. reassembly is the time consuming bit.

do it right or don't bother.

PTwo

brad black wrote1/ if you want to make it really hard for yourself, yes. just pull the cam. 2/ if the valve seats and faces haven't been machined, yes. but you really wouldn't not do that if the heads off. 3/ no, the valve heads may be proud of the head face. or very close to. 4/ strip is pretty quick, half hour easy. more like 15 minutes really. reassembly is the time consuming bit. do it right or don't bother.

Great, thanks for the help Brad.

On point #2, the heads were just refreshed before it was put on the box due to a broken cambelt. New valves on one and seats done on both.

seven4nineR

...

PTwo

So this is odd. I'm pulling my heads apart and find that one set of retainers doesn't stick to my magnet! I get them out, and they are shinier and smaller in diameter. The valve they were on is also completely stainless. I know these were replaced at European Cycle Specialties due to a broken can belt when the bike was stored.

The valve has C. MENON stamped on it. Any ideas? Did I need to be concerned?

In the picture you can see the thickness difference between the two sets of retainers.

Thanks,
Peter

Mr.R

Menon were pretty good quality after market valves.
The differences between the retainers is the early bikes up to '91 had 1.5mm items, from '92 onwards these changed to 1.75mm items. This was because the thinner earlier ones were prone to breaking.
The early style thinner retainers were made from stainless steel, making them a real pain in the arse if you dropped one inside your head, Brancato Engineering in the UK used to sell magnetic replacements for them.
If it were my motor I'd get the 1.5mm grooves opened out to 1.75mm and then either fit the later retainers or better still fit MBP collets to all of the valves.

Steve R