Compression & Dome Size on Ported Cylinder

[#ZERO]

Stupid question but how accurate is your compression gauge and are you doing this test with a fully charged battery, with one plug in and WOT during cranking?

Your big bore LA sleeve 61x cylinder should have 200lbs with 35cc domes, 215lbs with 33cc domes and 230lbs with 31cc domes.

 

[Matt_E]

I checked the gauge by measuring compression on a stock 650 SJ - blows 150psi, so I am fairly certain the gauge is good.
Battery is fully charged, starter spins fast.
Always one plug in, always WOT.
I do not know what sleeves these are.

Link to the cyl in question:
http://www.x-h2o.com/threads/22573

 

[#ZERO]

Did you check the compression when the engine was first built and after the break-in period? :scratchchin:

Also did those Wiseco pistons have a brown colored tint in the aluminum on the top?

 

[Kennay]

Are you asking him that to see if it was lean? I genuinely do not know and am wondering.

 

[#ZERO]

No, I think he may have the older style Wiseco pistons from the picture in the link above and they had super thin rings.

 

[Matt_E]

Did you check the compression when the engine was first built and after the break-in period? :scratchchin:

Also did those Wiseco pistons have a brown colored tint in the aluminum on the top?

I checked it both fresh before running it and after running three tanks through it.
About a 5 psi increase.
The Wisecos do have a slightly brown tint on the crowns, and a nice wash pattern.
However, I had a pipe problem yesterday that really fouled up the crowns a bit.
What are you getting at with the crowns?
Why do the thin rings matter? They are about 1mm thick. Same as every other ring I've ever seen (on watercraft)

 

[#ZERO]

Did you check the ring end gap clearance?

Those thin rings can be overheated real easy and didn't seal as well as the thicker newer ones.

 

[Matt_E]

Remind me how to check the gap.
Simply installed in the bore as it sits? (With a feeler gauge?)
What should the gap be?

I have absolutely no reason to believe they've been overheated. The motor had adequate cooling the whole three tanks. 48 degree water, plenty of warmup.

 

[waxhead]

You need to do what super tune is saying
For some reason you have a larger amount of cc in there than you expect

Or you have the wrong rings in there and i have seen this before as well
1mm over
.75 rings

 

[Matt_E]

I'll check ring gap and volume tomorrow.
As I said, I tried checking volume tonight, but my syringe was too small.

 

[#ZERO]

I'm pretty sure those are LA big bore sleeves in your 61x cylinder and they have 62T port timing.

Raising the exhaust ports 2mm shouldn't lower the compression ratio that much.

How does the cylinder hone cross hatch pattern appear to be wearing?

The cylinder could be out of round slightly causing a lower compression reading and the engine may still have good power.

Just install the ring in the cylinder and push it down evenly with the piston top about 15mm and check the end gap with a feeler gauge.

 

[Matt_E]

The bore very slightly out of round, about 0.001 (per machinist's measurements).
Cross hatch is wearing fine. A few lines here and there, but no depth to them.
I will post some pics tomorrow.

 

[#ZERO]

I hate to say this but one thousands of an inch is quite a bit out of round for a freshly bored cylinder, the wear limit is only .002" on a well used cylinder.

If your piston clearance is at .006" your ring end gap will probably be around .022" to .024"

If your taking the cylinder off, try holding a bright light under the pistons install in the cylinder and see if you any light coming through around the edges.

 

[Matt_E]

Could someone tell me where to measure bore size?

I just checked at the deck level, and it comes to 84.38mm. A bit big for 84mm.
Also, where to measure the piston? I measured it at the crown, but I am 99% sure that's wrong.

Ring gap is 0.09 inches.
I did not detect anything out of round in the cylinder (deck height anyways)

 

[keefer]

I have always measured pistons at the skirts as they are a bit tapered for expansion when hot.

 

[sfab]

Could someone tell me where to measure bore size?

I just checked at the deck level, and it comes to 84.38mm. A bit big for 84mm.
Also, where to measure the piston? I measured it at the crown, but I am 99% sure that's wrong.

Ring gap is 0.09 inches.
I did not detect anything out of round in the cylinder (deck height anyways)

do you mean .009" ring gap or really .09" cause it definitely make a difference.

 

[Matt_E]

I mean that there is 0.09" between the ends of the ring.

 

[sfab]

I mean that there is 0.09" between the ends of the ring.

then its way out of spec. you should have at max 0.02", usually between .01" - .015"

 

[Kennay]

if it was blowing by the rings, if you pulled the cylinder off, wouldn't there be some dark areas where it had been blowing by on the pistons?

 

[Kennay]

Look at it this way though Matt, with a new bore, you will have alot more power, and you won't blow it up as quickly