compression out of the ski

[chriselmore_1]

i just built a 61x and the compression is 150/160 the motor is out of the ski and i just plugged in the ebox and ss switch. will i get an accurate compression reading like that? intake is off and so is exhaust. i also have a billet triton head on so shouldnt comp be alot higher?

 

[Matt_E]

What compression is the head built for?

Taking readings without reeds may change things a bit

 

[chriselmore_1]

ohh ok. not sure the dome size but i was told the head produces 185 psi

 

[chriselmore_1]

i put the reeds on and rechecked compression. now i have 155/150 now the cylinder that was originally low was the one with the higher 155 psi its supposed to a high comp head . should i reseal the cases? or get a fresh bore and new piston kits

 

[Matt_E]

I wouldn't fret this too much.
Lots of factors influence the compression reading:
The shape of the starter
Condition of the battery
Compression gauge


If you used a starter that was spinning too fast, you might get a reading that's 220psi instead of 180psi, for example.
If you use a starter that's old and spinning slowly, you might only get 150psi instead of 180.

 

[chriselmore_1]

thanks matt your the man. but what about the psi difference in each cylinder? whats acceptable?

 

[Matt_E]

A 5 psi difference is nothing to worry about.
Is the cylinder ported? That will lower your compression reading.

 

[norcal ex]

chris i wouldnt worry. there are a lot of factors reguarding comprression in a newly assembled motor. Did you use any type of lube on the rings and bore? if it is dry the compression could be lower.

Put it in and run it and the numbers should even out.

did the motor have a fresh hone and rings?

 

[chriselmore_1]

no, old rings. should i get new ones? they looked decent so reused the rings. i coated the cylinders in yamalube oil so i could get em in the holes. yea i honed it too.and no, its not a ported cylinder. i just cleaned up the sleeves that stuck out in the exhaust ports on the bottom

 

[Matt_E]

You should get new rings. They're not overly expensive.

 

[Waternut]

As I found out when I recently rebuilt my engine... the oil you use when assembling the engine can seriously affect your compression numbers. The 5psi isn't major but it will probably balance back out once you've run it. As MattE also stated, there are quite a few things that can affect your compression reading but the most important thing you're looking for is consistency. Since very few of us have quality compression gauges, we're just looking to see if the cylinders are close to each other or to see what the difference is after porting or installing a high compression head.

As stated before new rings are relatively cheap and help ensure the engine will last. Go rent a cylinder hone at your local autozone and put the new rings in.

 

[norcal ex]

yea i would do new rings