Low compression cont'd

so I have the engine sitting in my spare bedroom and I have taken it apart down to the crankcase. I have ordered a cylinder bore gauge (actually got a great deal on ebay) and I'm ordering a new set of gaskets today as well.
I am going to be cleaning the parts as well as possible, as there was quite a bit of carbon build up on the valves as well has the piston heads. Does anyone have a recommendation for a solvent to aid in this? Is it just good ole' elbow grease?
From just a visual inspection the cylinders don't have any major pitting.... won't know for sure until I measure the bore.
There hasn't been any glaring foul-ups so far. Would a fouled gasket be obvious? Or could it look fine but not be? I would assume that a cracked head would be obvious as well.
Also: can anyone recommend something to stuff inside my crankcase to keep foreign matter from getting inside? I don't want to stick regular towels with lint inside.
I will post pics as soon as they are available.
Thank you for all the help so far, guys.... let's keep it going. I'll be riding this damn thing if it's the last thing I do.

you can buy some red shop towels at the parts store, they dont have lint

I am in the process of installing new rings in two bikes. I just placed a plastic trash bag over the cylinder studs and held it tight against the block with a bungie cord. My cylinders were ok so I just broke the glaze with a hone. I should have one bike back up this weekend. I have a spart head so it is all ready to go. I just need to finish up removing the old gaskets.....good luck....chris3

Bicycle Lee wrote:

Also: can anyone recommend something to stuff inside my crankcase to keep foreign matter from getting inside? I don't want to stick regular towels with lint inside.

I used soem blue cotton shop towels. wash them to make sure you get the lint off. Scraping the residue of the base gasket off the crankcase top edge is the most dangerous part. Roll a towel up and make sure it is against the edge as you scrape the gasket material off, use a vacuum to suck the debris out before you move on.

Bicycle Lee wrote:

Does anyone have a recommendation for a solvent to aid in this? .

Acetone spray. carb cleaner or brake cleaner in spray cans.

Another important thing to measure is the ring gap.

With the rings off the piston, insert them, one at a time, into the cylinder. I ususally use a piston to push the ring down into the cylinder about 20mm. This way, the ring stays "level" in the hole.

Next, use a thickness gauge to measure the ring end gap. Your manual should provide the service limits for this measurement, but I think the general rule of thumb for the ideal gap is .003" to .004" per inch of bore. So if you have 70mm bore, that equals 2.755" x .004" = .0011" of gap.

Regarding measuring your cylinders, I would go with the recommendation to have a professional look at them. I ordered a set of t-bore gauges online, and tried measuring the bores myself. I could never get consistent measurements. And the tolerances for these measurments are so tight that I think the cheap gauges aren't good enough for the job. I would recommend you have someone who knows what they are doing, and who has good quality instruments take a look at them.

In my case with my B2, I put new rings on old pistons and just did a light hone on the cyls. My ring gap was good, but after I got them back together, my compression numbers were low. It runs okay, so I haven't done anything about it yet. But I think the problem is that my cyls were slightly ovaled. And so although my ring gap was okay, I think I'm getting some compression leakage past the rings. Leakdown test confirmed leakage into the crankcase.

So I wish I had had a pro take a look at the cyls before putting all back together. Could have saved me some hassle and disappointment.

1 more option. buy a used 900 block and have it bored to match youre 1015 pistons and re ring.

Skyman wrote:

Another important thing to measure is the ring gap.

With the rings off the piston, insert them, one at a time, into the cylinder. I ususally use a piston to push the ring down into the cylinder about 20mm. This way, the ring stays "level" in the hole.

Next, use a thickness gauge to measure the ring end gap. Your manual should provide the service limits for this measurement, but I think the general rule of thumb for the ideal gap is .003" to .004" per inch of bore. So if you have 70mm bore, that equals 2.755" x .004" = .0011" of gap.

That's exactly what I read is right for nominal ring end gap, but the surprising thing is that the service limit (in my manual) is about DOUBLE the nominal. There is not a lot of compression loss at the end gap, although that seems odd, so the max end gap on my 750 twin was specd as about .028".




Skyman wrote:

In my case with my B2, I put new rings on old pistons and just did a light hone on the cyls. My ring gap was good, but after I got them back together, my compression numbers were low. It runs okay, so I haven't done anything about it yet. But I think the problem is that my cyls were slightly ovaled.

Could be, but you could be getting blow by the rings even if they are new. According to the references I read, most blow by does NOT occur at the ring-wall interface, it's in the grooves of the pistons. The pistons are aluminum and the ring gaps wear due to the motion of the rings as the pistons go up and down. As the space between the rings and piston increase, blow by increases. New rings in old pots is better than nothing, but may not seal really well.

mark1122 wrote:

1 more option. buy a used 900 block and have it bored to match youre 1015 pistons and re ring.

Good point. The prices on used parts from dismantlers is cheap. I priced a low mileage used head complete with cams, valves and covers for my 750T for $100 from a bike dismantler online.

I vote for getting a set of 900 jugs and having them bored. They are cheap on ebay or mabey even your local bike boneyard.

well, I am getting new gaskets and I will do my best to measure the bore when I get my bore gauge to determine whether they are off. If there's even any question I will take them to a shop to have them measured professionally.
I mean, I don't know what else to do at this point. I'll measure everything and see what happens once I put the motor back together.
I've never tried starting an engine outside of the bike so this should be interesting.

another random question:
What is this bolt called? In the upper left corner of this picture. My brother was explaining how a torque wrench worked and he broke this damn thing off:laugh: