Cylinder Compression Incongruences.-

[Faztbullet]

Without a t-stat 2 scenarios can happen...1) Motor can run too cold no matter the incoming water temp. The cylinders cannot expand in relationship to piston heat and the rings/piston skirt scuff as expansion rate is off due to over cooling . That the purpose of t-stat to hold water till block warms up. then its a restrictor at WOT. Number 2) The motor runs hot as water passes thru block to quickly and never removes heat from cylinders that is transferred from the rings , block will fill good to touch or I/R gun but hot internally. You can see this with a EGT probe and all this can happen no matter the oil ratio

 

[Sea Rider]

I'm plainly in agreement that those scenarios can and will happen, have installed a new thermo and now can hand feel the discharged water temp difference when the thermo opens and closes, down here the pseudo mechanics give a crap about re installing thermos on previous shot ones thinking mindlessly that a motor will cool much better and avoid salting scenarios on them.

Happy Boating​

 

[Sea Rider]

Updates :

The crankcase was re opened for a second time, having 130 PSi cylinder compression was too much, although the motor started right up the rope was very hard to pull. The machine shop polished both cylinders and now with 120 PSI on both, much softer to rope pull than before.

After only one month of daily motor use since full restoration the following scenarios were found :

-Lower Leg's screens vertical water passage with reduced diameter. This leads for the new installed impeller to suck less amount of water seen when the motor was peeing at idle/neutral.



-Salted thermo housing and thermo.

As cylinder and exhaust cover mating surfaces were highly corroded all new installed motor's gaskets were sealed with Permatex Aviation Gasket Former and torqued to factory specs with a special aviation torque wrench, after just one month of daily use were found out of torque specs resulting in a visible white line salt formation around the cylinder head gasket and to a much lesser degree around the exhaust cover. If this situation is not corrected in a timely water will pass and seize the bolt's threads which will inevitably break the bolt's heads when removed, that's why everybody is afraid of removing them, not me LOL!!​

Both situations led for both spark plugs to rust/oxide their tips due to small amounts of water intrusion and make the already warmed motor to hesitate a bit when opening the throttle from idle to say mid wot rpm range.​

The crankcase had to be removed to re torque all cylinder and exhaust cover's bolts, both lower bolts of such parts are impossible to reach if the crankcase sits bolted on the pan. A new pan gasket which is an alum sheet was previously greased with thick marine grease at both sides, installed and torque to specs. Will be able to remove it any time without the need to buy a new one.


Find that 25 NM for the cylinder head gasket and 10 NM for the exhaust cover bolts are too darn soft as such gaskets will cede due to heat compression way before the break in periods ends, which is bad music. So have torqued both gasket's bolts with 20% more torque to sit firm in place.

As this motor powers a small tender dinghy will be taken out of water each 20 days of service, have the lower leg's screens and water passages, thermo housing and thermo cleaned to impeccable condition and the crankcase flushed with high water pressure for 15 minutes through the dedicated exhaust cover flush port. it's the only way to have such a nice motor operational all year round running strong and avoid salt formations going on and overheating the motor needlessly.



Have modified the Exciter Coil's cable bending angle, now bends much less than when factory delivered. Used to break the wires located inside the black sleeve and motor shutting itself with no reason....



Let's hope all goes well till next service...

Happy Boating​

 

[Faztbullet]

What did machine shop "polish" ??? Polishing cylinder walls will not reduce compression but extra honing will. Also increases cylinder wall to piston clearance and ring blow down thru excessive ring gap.

 

[Sea Rider]

Polishing cylinder walls will not reduce compression but extra honing will.

Wondered exact same, definitely the moron machine shop techie forgot to hone both cylinders assuming were bored and honed to factory specs after installing 2 new cylinder sleeves. it's the last straw that one has problems associated with work not done correctly by others who are supposed to perform such specialized work. You always learn something new....

Happy Boating​

 

[Faztbullet]

The only way honing would effect cylinder compression is if ring gap clearance is excessive, as compression would blow by. If this is the case the cylinders are past the MFG wear specs. To tight clearance would not increase compression as it is created on top off piston and the squish radius distance between head and piston at 0-2° BTDC. Only real way to change compression 10 psi is to use a thicker head gasket

 

[Sea Rider]

The only way honing would effect cylinder compression is if ring gap clearance is excessive, as compression would blow by. If this is the case the cylinders are past the MFG wear specs. To tight clearance would not increase compression as it is created on top off piston and the squish radius distance between head and piston at 0-2° BTDC. Only real way to change compression 10 psi is to use a thicker head gasket

What can I say, seems the cylinder sleeves internal gap were bored too darn tight to work adequately with 2 new OEM standard piston and piston/rings, keep in mind that the mating surface of the cylinder head was shaved even as was darn pitted and corroded and a new gasket wasn't going to sit and seal well, both situations must have raised the compression way higher, now with well honed cylinders and BTDC set to 0º starts and runs like never before...

Happy Boating​

 

[Faztbullet]

If piston clearance was to tight shirt would scuff on pistons....shaving head would increase compression. Cylinder wall clearance has no effect on piston only the ring end gaps. if they honed it enough to drop compression 10psi that engine wont be living long.

 

[Sea Rider]

Yep didn't mentioned it before, the shirt was beginning to lightly scuff on pistons and rings as well, was seen when turning the cranckcase upside down. That's why was sent back to the machine shop to have it properly honed as the moron techie didn't do his work right probably due to having mental problems derived from the C19 pandemic. BTW those 18 HP motors are being factory delivered achieving 120 PSI, mine with 1K run hours achieves 118 PSI on both cylinders. On next service will check compression numbers to be sure all went OK and Long Live the Tohatsu 18.

Happy Boating​

 

[Faztbullet]

The problem is you didn't check the machine work before reassembly... the blame falls on you not machinist. Did you give him cylinder specs? Still honing wouldnt drop compression 10psi unless ring gap is now excessive.

 

[Sea Rider]

Did you give him cylinder specs? Still honing wouldnt drop compression 10psi unless ring gap is now excessive.

Wrong conclusions, gave him the Service Manual's factory specs along a new piston and ring in his hand, how could I know the moron didn't do its homewrork right ?

How can rings gap be excessive ? both pistons and rings are standard ones, now well matched to new cylinder sleeves achieving adequate 120 PSI compression on both cylinders. The motor is starting fast and running superb now, so I'll leave it working as is for now ...

Happy Boating​

 

[Faztbullet]

You always check any machining work . I checked my own when I used to bore blocks and any others I had sent to have done. How do you know its at spec as you didnt check it the first time and state no where where you checked it when you got it back. Do you understand the purpose of skirt to cylinder wall clearance? Do know a piston is tapered? Did you put ring in cylinder and check end gap? Anytime you you increase the ID of cylinder the ring gap gets bigger. Anyway as stated the only way honing will cause compression to drop is by making ring end gap excessive. Also compression test does not tell you anything about a motor other than cylinder has enough compression to support combustion, I have seen scuffed cylinders on motors with good compression.

 

[Sea Rider]

Gee what's the purpose of continuing indefinitely with your high tech explanation. The techie moron finally confirmed not having honed the cylinders, the picky Club's staff working with this combo daily confirmed yesterday that runs as never has run before, so I'm very pleased with all the long and tedious work done so far. End of the Story...

Happy Boating​

 

[Faztbullet]

The techie moron who reassembled engine doesn't understand engine mechanics...honing will not drop compression 10psi...End of story.

 

[Sea Rider]

The techie moron who reassembled engine doesn't understand engine mechanics...honing will not drop compression 10psi...End of story.

Amen, Persecula Seculorum...

Happy Boating​

 

[Sea Rider]

Before reaching the 3-4 week scheduled preventive service the motor was found having hesitation issues when idling long and opening the throttle to plane the tender fast, clear indication of water intrusion inside the combustion chamber. ..​

Spark plugs were founded again highly rusted which accounts to achieve hesitation issues. Although both exhaust and cylinder head new OEM gaskets were torqued bit more than factory specs calls for the never ending issue continues. ​

The entire cylinder's head contour has developed a notorious white salt line formation while the exhaust cover gaskets none. Seems that will need to tear the cylinder head to inspect the crankcase's mating surface to check if the pitting/corrosion continues, file, smooth that out more than before and apply an overdose of Permatex Aviation Form a Gasket Sealer on the entire gasket and then torque it back to 30 NM...​

Happy Boating

 

[MattFL]

Are you sure the head bolt torque is 30nm? Maybe double check that somewhere, 30nm seems pretty light for a head bolt.

 

[racerone]

Well ---30 NM sounds correct for a head bolt in an aluminum block.

 

[Sea Rider]

Are you sure the head bolt torque is 30nm? Maybe double check that somewhere, 30nm seems pretty light for a head bolt.

Although 25 NM torque value stamped in front of the cylinder head seems light, that's why torquing all 10-M8 bolts to 30 NM. My boating club has several 5-8-18-30 HP Tohatsu motors and all with such issues, the ones that have not been retorqued since new way worse, our water is extremely salted don't know if it accounts for that, it's the quality/material of the head gaskets or a combo of both situations ?

Question : if the head gasket has been sealed with Permatex Aviation Sealer, will the head be easy to be removed from the crankcase once all 10 bolts are removed ?

Happy Boating​

 

[Faztbullet]

Head needs to be surfaced.you can torque all day on a warped head and still wont pull it flat