50 cc compression

[redonkulousruntsracing]

Well, I finished rebuilding my motor again. I had to bore the cylinder .050mm over to get all the scratches out of it. I just got it running tonite, I haven't checked the compression yet, but as I was putting it together and it has a .022" thick base gasket. The piston to cylinder clearence is 1.5 thousandths. I checked the squish with a piece of solder and my micrometer and its about .055". I dont know where my compression is at yet, cuz I still have to put a couple more heat cycles on it, but I think I am loosing a lot from having that much squish. I could probably take off at least .020" off the cylinder. I would like to hear what some of you engine builders think. I will check my compression after I get rings set.

I always thought 24-28 thousandths in the squish area. .055 is alot.

[hotquads1]

.055 sounds excessive , but what was the compression before the rebuild ? I assume it had .055 squish before the rebuild , and all you have done is an overbore ? What was the reason for rebuilding, what caused the cylinder damage ? You must be sure thaT DETONATION DOES NOT OCCUR , several other factors at play here other than just squish . A more desirable squish would be .025 -.030 but the combustion chamber has be designed for such in order to work properly. A properly blueprintd engine can run a squish as low as .018 but ugly things can happen if its not correct.

[redonkulousruntsracing]

It has the stock Apex dome on it and the cylinder hasn't been cut yet. I will check the compression after it get a couple heat cycles on it. The engine was "rebuilt" just before this happened, it only made it thru 1-3 lap moto before it overheated so bad that it barely made off the track. I wasn't able to do a compression check on it because I was waiting for a little more run time on it. I broke it in at home, Seth practiced on it for about and hour then we went racing, it had about an hour and a half run time on it before this happened. When I sent it out to be rebuilt it had about 120 lbs. of compression. After that 1st moto it had 90lbs, a ring was seized into the piston and the cylinder had a nice gouge in it, the top ring gap was pointing staight down toward the exhaust port, and it was seized/jammed under the alignment pin. The piston was cocked in the cylinder and had some nice wear marks on it. Before we bored it last night the piston to cylinder clearence was over 10 thousandths.....on a "new" motor!!!!

So I had to order a .050mm piston and I had it bored last night, we set the piston to cylinder clearence to 1.5 thousandths inch.
The base gasket it had on it was .038". The new one is .020" and that is what I used to measure the squish and after 3 attempts, I got a consistant .055" of squish. So before it must have been like .073" with the old base gasket. I will check the compression when I get the rings set then will have to figure out how much to shave off the head if I need to. I am shooting for 140-150lbs. Anything has to be better than 90lbs........

[hotquads1]

I am assuming you mean a 40.5 mm piston , or are you saying you bored it .050" ? is this cylinder cast iron ? is this the same cylinder that has been on it since new , if so , i suppose they could have intended for it to be a .070"+ squish , although it would be odd. If it had 120 psi at .073 sqish then a dropping to a squish of .055 as you mentioned should net about 135 psi after break-in.

[redonkulousruntsracing]

Yea Marc, that is what I meant, it's a 40.5mm piston, I get my metric and SAE numbers messed up sometimes, you know what I meant. Its the stock cast iron cylinder that it came with last year. It was untouched until I sent it out to be rebuilt. I never checked the squish on it before, when it was new last year it had 140 lbs on my gauge so I didn't bother checking any thing else out. It could have had the thinner base gasket on it, IDK, all I know is what I have now because I am doing it, not someone else.

[hotquads1]

sounds like your base gasket change will get you close to where you want to be . Have you figured out what caused the overheating ?

[redonkulousruntsracing]

The only thing I found wrong with it was the piston all messed up.
The plug looked suprisingly good. We managed to piece it back together with used parts, a cereal box and Yamabond to get 110psi of compression with the cylinder as messed up as it was, and Seth ran both motos on Sunday bringing home a 1-1. After the race and it had cooled down, it was back down to 90psi, it sounded pretty sick the last lap, but he made the best of what he had. I cant wait to get a good running motor under him and see how he does at 'Dilla this Sunday:)