Vintage Mustang Forums banner

Wet compression test in bad cylinders

850 Views 7 Replies 4 Participants Last post by  Guest
One thing I failed to mention yesterday was I only cranked the engine three-four times (don't remember - but I was consistent across all cylinders) to get my 170lb average across the six good cylinders. Today, I acccidently squirted oil in #6 instead of #7, and cranked it until the compression stopped boosting. I think the reading went to 220 lbs or something. I don't know if this means anything.

Anyway, I just did a wet compression test on my bad #2/7 cylinders. These were the results from the dry test yesterday: (37k mile boss 351): (1) 180: (2)95: (3) 175: (4) 175: (5) 170: (6) 175: (7) 100: (8) 160

I liberally squirted oil in the sparkplug holes (that's a tough one...had to use that 1/2" clear tubing from my gear oil lube pump), so I don't know if I got too much oil in there or not. Anyway, both #2/7 pumped up to around 130 lbs (that is with me cranking away until she stopped). Whenever I would clear the guage and retry, the poundage would always drop a few lbs. So, after a few tries, I was already below 120 lbs.

I was hoping my readings were going to be identical, or only slightly higher. I guess the leakdown test is in order. What do you guys think? Could it be bad rings with only 30-35lb jump in the bad cylinders after a shot of oil? Thanks again.
1 - 8 of 8 Posts
Bad rings.
Bad rings.
... Bad PISTON rings.

Bad rings and/or scored cylinder walls is my guess...

I'll assume this is a rebuilt with 37K on it, yes?

If so, start considering your options, considering the difficulty of finding a 4 bolt block....

Good luck!
OUCH! Pat, the car has had 3k miles put on it in 20 years, so I would think it's the original bore. But, of course, there's no way of knowing until I get in there. But you're probably right. Looks like I've got a challenge ahead...that's ok. I wasn't expecting it, but I will definitely accept the challenge. Thanks guys for the imput. When I tear into the cleve (a couple months), I'll definitely repost my findings. Or, if I do a leakdown first. Take care.
Pat, the car has had 3k miles put on it in 20 years

Ah, that would explain a lot if this is an OEM shortblock...that's good news...

Maybe just some rings went bad or seized...I don't recall if the Boss engines float the pins but, if they do, a snap ring could gotten out and scraped the cylinder wall...

If the former, going .030 will likely do it...if the latter...hmmm...I've seen blocks junked because of rattling pins/spirolox/snap rings...

No funny noises??
No. All quiet in deep, dark depths. No exhaust smoke either. Thanks again for your help. Best regards.
1 - 8 of 8 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.