Compression Yamaha g8 j38 engine

Tommybh · 05-10-2019, 03:07 PM

I just did a dry compression test and got 145 I’ve been did a wet compression test and got 170 does this mean I need just all new rings or does it need to be bored and honed out thanks

chrisr32 · 05-10-2019, 08:05 PM

Can't really answer that until you pull the head and inspect the cylinder wall. When you say wet, you added just a little oil? If you added to much that could explain the raise in compression.

CP241 · 05-10-2019, 08:51 PM

As long as the cylinder wall isnt scored, you might could run a hone through to deglaze it, install new rings, and be fine. If theres any scoring it is a different story

raydhd · 05-11-2019, 07:58 AM

You really should measure both the piston and the cylinder.Even if the cylinder isn't scored the piston/cylinder clearance can be way out of spec which is something like 0.03-0.05 or 0.0012-0.0020 inches.The max limit is .10mm or .0039 inches.My G14 cylinder has no scoring BUT mt piston to cylinder clearance. The net result is a LOT of piston slap.P.S. My compression #'s were very similar to yours

Tommybh · 05-11-2019, 09:08 AM

Okay thanks guy I’ll have to get it apart and take a Look do you think it’s fine to still run it at 145psi

raydhd · 05-11-2019, 10:20 AM

I am surprise at that # it doesn't smoke. If you take it apart and bring everything in spec your compression should be higher.

smallblock450sl · 05-11-2019, 10:40 AM

Several things come to mind. As I said in my PM's with OP. Was the test done at wide open throttle? Was there any "leak down" of compression after the test? Does it smoke? What did the spark plug look like (oil soaked ect)? Is it a held in (rubber tipped) or screwed in compression tester? Is the gauge known good? All these must be considered.

raydhd · 05-11-2019, 11:03 AM

I did not know there were other conversations with the OP. BUT the point here is before you start tearing an engine down the compression test must be done correctly. But I would also say when it is apart you measure ,measure,measure and not just hone and put on new rings.

Tommybh · 05-11-2019, 11:52 AM

I used the loaner tool from autozone
https://m.autozone.com/loan-a-tools/...cation=&review

Cranked engine until gauge stopped moving like 5 seconds in

Spark plug was oily well the threads where wet ...

And I had gas pedal pressed all the way down not sure if that’s considered fully open throttle

raydhd · 05-11-2019, 12:01 PM

Sounds like you did everything right. Is the engine smoking,using a lot of oil, have excessive blow by or not running properly ?

05-10-2019, 03:07 PM	#1
Tommybh Getting Wild Join Date: Apr 2019 Posts: 114	Compression Yamaha g8 j38 engine I just did a dry compression test and got 145 I’ve been did a wet compression test and got 170 does this mean I need just all new rings or does it need to be bored and honed out thanks
Tommybh Getting Wild Join Date: Apr 2019 Posts: 114

05-10-2019, 08:05 PM	#2
chrisr32 Gone Wild Join Date: Jan 2019 Posts: 190	Re: Compression Yamaha g8 j38 engine Can't really answer that until you pull the head and inspect the cylinder wall. When you say wet, you added just a little oil? If you added to much that could explain the raise in compression.
chrisr32 Gone Wild Join Date: Jan 2019 Posts: 190

05-10-2019, 08:51 PM	#3
CP241 Nincompoop village idiot Join Date: Apr 2018 Location: Michigan Posts: 9,673	Re: Compression Yamaha g8 j38 engine As long as the cylinder wall isnt scored, you might could run a hone through to deglaze it, install new rings, and be fine. If theres any scoring it is a different story

05-11-2019, 07:58 AM	#4
raydhd VC 460 CLONE GONE WILD Join Date: Oct 2014 Location: Ray BY The Bay NJ Posts: 10,633	Re: Compression Yamaha g8 j38 engine You really should measure both the piston and the cylinder.Even if the cylinder isn't scored the piston/cylinder clearance can be way out of spec which is something like 0.03-0.05 or 0.0012-0.0020 inches.The max limit is .10mm or .0039 inches.My G14 cylinder has no scoring BUT mt piston to cylinder clearance. The net result is a LOT of piston slap.P.S. My compression #'s were very similar to yours

05-11-2019, 09:08 AM	#5
Tommybh Getting Wild Join Date: Apr 2019 Posts: 114	Re: Compression Yamaha g8 j38 engine Okay thanks guy I’ll have to get it apart and take a Look do you think it’s fine to still run it at 145psi
Tommybh Getting Wild Join Date: Apr 2019 Posts: 114

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05-11-2019, 10:20 AM	#6
raydhd VC 460 CLONE GONE WILD Join Date: Oct 2014 Location: Ray BY The Bay NJ Posts: 10,633	Re: Compression Yamaha g8 j38 engine I am surprise at that # it doesn't smoke. If you take it apart and bring everything in spec your compression should be higher.

05-11-2019, 10:40 AM	#7
smallblock450sl Sometime's............ Join Date: Oct 2007 Location: Tacoma Washington Posts: 11,874	Re: Compression Yamaha g8 j38 engine Several things come to mind. As I said in my PM's with OP. Was the test done at wide open throttle? Was there any "leak down" of compression after the test? Does it smoke? What did the spark plug look like (oil soaked ect)? Is it a held in (rubber tipped) or screwed in compression tester? Is the gauge known good? All these must be considered.

05-11-2019, 11:03 AM	#8
raydhd VC 460 CLONE GONE WILD Join Date: Oct 2014 Location: Ray BY The Bay NJ Posts: 10,633	Re: Compression Yamaha g8 j38 engine I did not know there were other conversations with the OP. BUT the point here is before you start tearing an engine down the compression test must be done correctly. But I would also say when it is apart you measure ,measure,measure and not just hone and put on new rings.

05-11-2019, 11:52 AM	#9
Tommybh Getting Wild Join Date: Apr 2019 Posts: 114	Re: Compression Yamaha g8 j38 engine I used the loaner tool from autozone https://m.autozone.com/loan-a-tools/...cation=&review Cranked engine until gauge stopped moving like 5 seconds in Spark plug was oily well the threads where wet ... And I had gas pedal pressed all the way down not sure if that’s considered fully open throttle
Tommybh Getting Wild Join Date: Apr 2019 Posts: 114

05-11-2019, 12:01 PM	#10
raydhd VC 460 CLONE GONE WILD Join Date: Oct 2014 Location: Ray BY The Bay NJ Posts: 10,633	Re: Compression Yamaha g8 j38 engine Sounds like you did everything right. Is the engine smoking,using a lot of oil, have excessive blow by or not running properly ?

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