1989 Blaster Engine Rebuild - Part 4

Part 1 - Part 2 - Part 3 - Part 4 - Part 5 - Part 6 - Part 7 - Part 8 - Part 9 - Part 10 - Part 11 - Part 12 - Part 13 - Part 14






Cases & Assembly


Two-Stroke Software Review

Part 1

Cases & Assembly

After spending time making a few surfaces flat I took the time to port the cases. Though there is little to gain from doing this, as far as I know it is never a bad thing to do. In this case it helped clean up the damage to the case made by the spent connecting rod. After completing that, it was time to start assembling things - it goes together quickly once everything is flat, clean and ready.


You have to go back to the last photograph in Part 2 to compare this to the way it looked in the beginning. The camera doesn't really do these kind of photo's justice because lighting becomes a problem - it either wants to shine off the surface (and you have to move it) or it gets too dark from being too far from the source. I like this texture for engine cases though.


In general though I am able to get the point across with these pictures. The surface is roughed up and it has a grain to it. The grain is applied properly when the direction of the crankshaft runs against it - oppositely, you can feel it with your finger. I'm sure it would be fine to let the grain go the other way but since there is a choice I choose to do it that way.


When I took this engine apart I bagged and labeled the parts so I could keep them together more easily. I ran a couple of tie-wraps around the gear clusters to keep them together. This is cheap insurance against kids who go discovering occasionally.


I heated the cases in the oven at 275 degrees for 20 minutes to get it to expand enough so I could easily drop the bearings in place. In the mean time I placed the bearings and crankshaft in the freezer to get them to shrink a bit from the cold.

Hot & Cold

This operation is fairly delicate. The idea is to make the cases get a little larger from the heat. The bearings need to be a little smaller - hence the cold. It's called an interference fit. Simply put, it works exactly as it should. Before I installed the crankshaft I installed all the bearings first. It is necessary to heat the case again after the bearing installation to be able to put the crankshaft in - only this time the bearings are getting heated too. After letting the case and bearings reach a comfortable temperature I put the case that will receive the crankshaft back in the oven. It is best to not roll the bearings at all when doing this procedure. When everything is cooled down I put a little oil on them. There's nothing worse than the thought of a scratch on a new bearing caused by it rolling dry - without any lubrication. I removed the cold crankshaft from the freezer and positioned it inside the bearing - it went in place without incident. I let it rest in this position until it cooled off completely. This procedure works well for engines that split vertically. Clamshell cases - ones that split horizontally like a Banshee do not have so many procedures to install their cranks.



The bearings have been replaced and the crankshaft has been installed. It stays attached to the flywheel side of the engine. The transmission assembles in the other case, then they are joined with a thin layer of Yamabond #4 as a sealer.


As a preference I replaced all the original Phillips head screw/bolts with hex head bolts. I can't believe they still use that kind of hardware on motorcycle engines. It is easier to use an 8 mm socket to get even torque on all the case bolts.


Once joined and torqued together it looks like this - again. Those are new cylinder locating dowels too - what's the sense of using the old oxidized ones. I like the look of ported cases - it even looks faster!


One of the last things I did today was check the volume of the head while the piston was at TDC so I could calculate the full stroke uncorrected compression ratio.

What Is Known

With a light barrier of waterproof grease preventing leakage past the piston I filled the combustion chamber with Marvel Mystery Oil from a graduated burette. The burette drained 22.4 cc's of fluid into it. I added this volume to the actual displacement of the engine (22.4 cc + 198 cc = 220.4). Then I divided the volume above the piston at TDC into the volume above the piston at BDC (220.4 / 22.4 = 9.839). To round it off I could say that the uncorrected compression ratio (UCCR) is 9.84 to 1. I don't know what the stock static compression was supposed to be but an online friend told me his Blaster produced about 125 to 130 PSI. Given the low UCCR, I don't doubt it.

Additionally I found out that this engine has a deck height of -.635 mm, the head gasket is .80 mm thick. The cylinder head has additional squish area cut into it. When I measured it I found out that on one half of the head that depth was 1.2 mm and on the other side it was .45 mm. Adding all these figures together reveals a squish depth of 2.645 mm where the squish is the thickest and 1.895 mm where it is the most shallow.



This engines port timing has been determined as being:

Exhaust Port opens at 91.5 degrees (33 mm) ATDC for a duration of 177 degrees.

Transfer Port opens at 119.5 degrees ATDC for a duration of 121 degrees.

Boost Port opens at 125 degrees ATDC for a duration of 110 degrees.

The Blowdown is 28 degrees of crankshaft rotation (figured from Exhaust port opening until first transfer port opens).


1989 Blaster Engine Rebuild - Part 4

Part 1 - Part 2 - Part 3 - Part 4 - Part 5 - Part 6 - Part 7 - Part 8 - Part 9 - Part 10 - Part 11 - Part 12 - Part 13 - Part 14

Cases & Assembly







Two-Stroke Software Review

Part 1

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