Hello,

I'm new to this site, will be writing an introduction post and updating my profile page soon but right now I have a pressing question...

I'm rebuilding a 1995 Ford 302, it's not from a truck but from my boat (a 1996 Four Winns Soundowner). I've only owned the boat around a month, The engine had a knock and was down to 90psi compression on cylinder2. I found that the starboard side (bank 1 cylinders) marine exhaust manifold was cracked (likely freeze damage) which was allowing river water to enter bank 1 cylinders via exhaust valves, did a compression test and most cylinders were reading around 150psi but cylinder 2 on bank1 was only reading 90psi. I suspected a bent rod due to hyrdo-lock but when I removed and stripped the engine all the rods were straight, however cyl 2 had up/down bore scoring. I now believe that the lack of compression and bore scoring on cyl 2 were due to water ingress diluting the oil film, due to heat from lack of oil and due to too tight ring gap...

The engine has obviously had a recent rebuild, it was fitted with aftermarket Federal Mogul H654CP (coated hypereutectic) pistons, no ring wear on bores, new looking main and rod bearings, The knock will have been due to the lateral grooves on cyl 2 and the wear on the piston that was in that cylinder.

I stripped it all down, honed all the bores (cyl 2 is +2 oversize compared to other cylinders after honing), bought new same spec H654CP coated aluminium hypereutectic pistons and E251k rings (moly coated top ring) from Rock Auto. No fun at all removing the rods from the old pistons and refitting them to the new pistons but I managed to do it all quite well DIY, the old pistons however were all ruined during the pressing out of the pins. I was expecting to receive ring gap instructions with the new pistons and with the new rings from Rock Auto, but I received no such info, internet searches are giving me various results but most results point to aiming for 4thou per inch to 4.5thou per inch, some results say I should allow a little more gap on the middle ring, all results are saying to aim for 15thou on oil control rings (and no need to check or adjust the corrugated oil ring)... I'd just like further confirmation and to know if I should allow some oversize of gaps for my marine application...

My question is - What ring gap (top, middle and oil control) should I aim for with the H654CP pistons and E251K rings? Further complicated because I'm doing this DIY, I didn't have a torque plate for the honing, the cylinders do measure slightly out of round toward the head area (perhaps due to lack of torque plate), the cylinders taper toward the bottom... When I install the rings supplied by Rock Auto in the bores the gaps measure 12thou at the top of the cylinders and 15thou at the bottom of cylinders. I have read on the internet that I should aim for 4thou clearance per inch of bore, which would be 16thou, but at the tightest area of bore (toward the bottom of bores) I only measure 12thou,.. Should I file all the rings so there's 16thou clearance near the bottom of bores? Heh, some sources say make clearances 4.5thou for every inch of bore with hypereutectic pistons, in which case that would point to 18thou clearance (but again near the top of bores or near the bottom, at the lowest clearance area in the bores or just towards the top)?

I'm really looking for guidance on what gap to use, and where to measure ring gap, for my specific combination of pistons, rings and bore taper... I already know the usual rules of thumb advice but the out of round readings due to not using a torque plate and the taper is causing me some confusion.

I don't know if the previous rebuilder made a mistake of assuming new rings supplied would be correctly pre-gapped but didn't account for bore taper narrowing toward the bottom of bores. That and/or water ingress could have accounted for the bore scoring I found on cylinder 2...

I would guess I should aim for 16thou ring clearance (4 thou per bore inch) at the narrowest part (lower end of bores) but instead of guessing I thought it a better idea to ask here. I would rather have slightly large ring gaps than not enough which could lead to the same problem of bore scoring again... That is if it was too tight rings that caused the bore scoring, it could have been due just to water ingress.

I have bought new marine exhaust manifolds ready to fit with the newly rebuilt engine, so water ingress shouldn't be a problem in future . But I don't want to mess up the new build by setting rings gaps too small leading to bore scoring or worse, or too large leading to blowby.

Not sure how relevant it is but I've also read raw water cooled marine engines (like mine) need more ring gap than usual because the block runs cooler than normal but the pistons and rings still run at same temperature.

Regards, Simon.

 

It seems weird to me that it would have run out of ring end gap on only one cylinder but anything is possible. Giving the engine a little bit extra ring end gap in this situation where you have low rpm, high loads(hot pistons and rings) in a block that's cool(does not expand very much) will be just fine. Typical rebuilder ring sets like this one usually use about .004 per inch of bore for end gaps so opening that up to .0045 or .005 per inch of bore will cause no problems with blow by.

On a lot of those coated pistons when I measure them I find them to be on size to the bore or nearly so if I measure where they are coated. The idea is that the coating is sacrificial and can't stick to the bore so the pistons can run very close and very quietly. Uncoated they would probably be running with .002 and having one cylinder .002 over the rest will probably make some noise. You will probably hear that piston especially on a cold start. What's weird about this is if all of the pistons were fitted with .002 extra clearance the noise would not be noticeable.

 

Thanks Dave, I'll aim for 18thou gaps at the tightest area of the tapered bores then.

Another question I have is how to set the ignition timing. The marine application uses an electronic distributor (as in no points, some sort of magnetic pickup, there's a positive feed from the coil to the distributor). I want to get the engine set so it starts and runs on all 8 at the first turn of the key, - I'll be priming the oil system by turning the oil pump drive using a 3/8 socket taped to a long extension and running it anticlockwise with an electric drill, will be trying to fill the 2barrel Holley with fuel etc, but I'm not sure how best to reinstall the distributor to get timing close enough for it to run first turn of the key.

I believe this marine spec 1995 302 runs the same firing order as the 351.

Simon

 

You're going to need to use a 1/4 inch socket to turn the pump on a 302. The marine cam does use the 351 firing order which is no problem. What I would do: Before you install the heads double check the timing pointer, make sure that it is correct. Then mark the damper with some paint at what is recommended for your initial timing. Then later after the engine is all together turn it so that cylinder number 1 is on the compression stroke and then line up the pointer with your initial timing paint mark. Then take the distributor and mark it in line with the terminal for the number 1 plug wire. Install the distributor and then turn it so that the rotor is aligned with your mark. This will take some fiddling to get the oil pump driveshaft to connect, the gears to engage and to have it in the right position so that all of the wires are in the right spot. Once this is done bolt the distributor down and you should be good. Once it is running again then set the timing using a timing light. It will probably be a little bit advanced or retarded but it should be close enough to run.

 

Thanks for the correction, I should have remembered 1/4 not 3/8 for the oil pump drive.

The marine engines, or at least this one (it's a Volvo Penta OMC Cobra setup), are a bit of a mish-mash of components, 96 Block, 95 heads, 72 cast iron timing cover, water pump and inlet manifold, 1 Piece rear main seal.

There's a timing pointer (which is bent) but I haven't seen any timing marks on the harmonic balancer or pulley, maybe they've been painted over. I did intend on doing as you've said, manually find true TDC and set some timing marks.

Still a bit unclear on how to set timing with the electronic ignition setup, If it were points I could use a bulb or multimeter wired to the points to check when the circuit was made/broken but with the electronic ignition I don't know how to check if I've got the distributor set in the correct position. I may have some photos that will help get it roughly in the right spot but I still won't know to within around 1/4 of a turn (90 degrees) if it's just about right or very wrong.

Today I reinstalled all the pistons. Last time I posted on forum I had only installed cyl#1 piston and I had to file (ahem grind) rings on #1 to get the ring gaps above 16thou. Today I was pleasantly surprised because none of the rings for any other cylinders needed any filing at all, in fact I was more worried about ring gaps being too big rather than too small. Ring gaps on #3 to #8 are all between 18 and 21thou, as big as 24thou on #2 (the cylinder I honed and honed to remove scratches).

If the supplied rings were pre-gapped (as I now know they were) I should have expected the gaps to be wider than 16thou anyway due to honing the bores a little oversize (a little over the +40 size) but I'm still thinking about why I had to file #1's rings, there was nothing much different between #1 and and all the other cylinders except for #2.

Got to hold my hand up now and say I messed up gapping the 2nd ring for one of the cylinders, ended up with a gap that was over 25thou. So I fitted a used 2nd ring from one of the old pistons I removed. But I am thinking about buying another set of rings or at least 2nd rings and replacing the used ring I refitted with a new one. I do intend on removing the piston from #1 cylinder and rechecking the gaps (#1 was the only cylinder with tight rings that I had to open the gaps on - I have a nagging feeling that I didn't open the gap on one of the oil rings on #1).

Simon

 

lace the cap on the distributor then make a mark in line with the terminal that's used by the number 1 wire. Then when you install the distributor with the engine in the correct position just line up the rotor with your mark and you should be pretty close. At least close enough to start up one way or the other. You will be dealing in distributor degrees so 1 degree there = 2 degrees at the crankshaft.

 

Thanks Dave,

Sorry the late reply.

I didn't know if just aligning the distributor like that would get timing close enough for it to run but you've cleared that up thanks.

If it had points I'd rig a bulb to check dwell and see when points open/close in relation to TDC and try to make sure timing was close to correct using the static timing method before even starting the engine... but with the electronic ignition I don't think it's possible to do the same(?). I expect the electronic ignition will control dwell itself and it probably won't even earth the negative coil wire unless it's spinning above (say) 50 rpm.

Just bought a new timing light and magnetic dial gauge - If after cleaning up the balancer I still can't see timing marks or if (as expected at the very least) the timing pointer needs bending to be accurate I have the means to make sure the timing marks are accurate now, even if none exist I can mark up the balancer myself.

Another thing I don't quite get is why marine engines don't have vacuum advance. When this is discussed on boat forums people usually explain that it's because marine engines run at close to high load (they must mean near atmospheric pressure manifold pressure, i.e. little vacuum) all the time so vacuum advance is unnecessary... But to me it seems obvious that they don't always run with near atmospheric manifold pressure and the engine could be made a bit more efficient at low engine loads when there is vacuum if they had vacuum advance. I wonder if none EFI boat engines don't have vacuum advance to intentionally cause timing to be a bit retarded at low engine loads to help keep exhaust pressure up to prevent water in the exhaust flowing the wrong way and into cylinders. Or maybe the concern is that a vacuum line between manifold and distributor would be another potential explosion risk in case the pipe failed and a backfire introduced a spark to an engine bay containing fumes (there'd be no spark arrester on the failed pipe but there is a spark arrester on the carb air intake instead of the usual type air filter).

I checked all the ring gaps again on #1, all good. Just need to buy and fit another 2nd ring for #2 cylinder, refit the cam thrust plate, timing cover, balancer, pulley, etc, sort my timing marks and the bottom end will be done.

 

I've had customers run a vacuum advance on a jet boat engine before and it might help economy a bit when just running along at say 3500 rpm with a big engine. I wouldn't buy another ring if you've got a little extra end gap especially on the 2nd ring. It won't hurt anything and in fact it'll probably help.

You always hear that sucking in water story on boats especially when it comes to cams and a lot of boat engines get built with cams ground on lobe separations that are far from optimum for that reason. The only time I've ever seen that happen on a boat was with a Ford 460 that had a flat intake lobe on the camshaft.

 

The reason I was going to buy another ring for 2nd ring position on cyl2 is because I fitted a used ring that came off one of the old pistons after messing up the new ring by increasing the gap too much using the side face of an angle grinder cutting disk (hehe)... I know that's not as bad as fitting a used top ring but I still haven't been comfortable with fitting the used ring. In light of me fitting the used ring would you still advise leaving it in or fit a new ring? I read about having a wider gap on the 2nd ring being a good idea but it's not the gap I'm concerned about, it's the 'used' aspect I'm concerned about.

Wondering if it'd be a good idea to buy a standard distributor with points and vacuum advance, then I could static time it.

I convert vehicles to run on LPG (propane), I've had no plans to convert this boat but I've thought about at least temporarily running it on LPG during the run-in period. On LPG there's never any fuel wash on bores even if there's a misfire, that could be a benefit if it turned out there was a problem like (say) too rich mixture from the carb or a bad plug lead.

Sucking in water due to what they call reversion, I suppose the biggest risk of reversion would be when cranking to start the engine, not much chance of reversion once the engine is running if the cam is just a bit hotter than standard? Not that I have any plans to change the cam.

 

Look carefully at the face of the ring. The 2nd ring is not really a compression ring it is for oil control. The ring is designed to be a scraper to remove oil from the cylinder wall. It is cut with a bevel on the bottom inner edge to give it a reverse twist and this causes it to load the bottom edge of the face against the cylinder wall. As it wears that contact point gets wider and wider. After initial running the wear won't extend all the way across the face from top to bottom it'll only be near the bottom. So if the ring looks that way it is just about as good as new.

I don't think that I'd sweat washing down the bores too much as long as the carburetor is in good condition.

 

Thanks Dave.

I'm on holiday at the moment, will be early September before I'm back.

Heh, I'm still thinking about maybe changing that ring... I spent a lot of time honing the cylinders, taking the rods off old pistons and fitting them on new pistons, all the other rings are new. When I first asked about re-using the old ring I expected to have time to finish building the engine before my holiday, waiting for new rings would've caused a delay. I could wait until the last days of my holiday, order another set of rings then, they'd arrive at around the same time as my return. The only real downside to replacing that single ring for me now is that I'll probably have to buy a complete set of rings.

 

One good thing about it is that you should be able to easily get a pouch of rings for a single cylinder and if you have to get a whole set of rings they are about the least expensive set there is.

 

Great, thanks.