Advice about rebuilding a 3.9 engine

[Vulture]

Hi. This is my first posting on this board, though I have read a thread or other. I am not the best english speaker, but I hope I can make myself understood.

I have recently rebuilt a 3.9 litre engine for my SD1 as follows:

- Balanced the crankshaft with con rods, pistons, crank pulley and (lightened) flywheel
- New piston rings
- RPi Engineering Stage 3 Heads
- Piper 285 camshaft
- Weber/Edelbrock 500 carb

I have done about 1200 miles with this setup now. A few days ago, after a 150-ish mile ride on a german Autobahn (I was going at a constant 75 mph - honestly!), the engine started backfiring and didn't pull as it should. I stopped immediatly and my brother towed me home. First guess was a problem with the ignition system, but after carrying out a compression test, we found out that cylinders 5 and 8 have no compression left at all. Additionally, there's a rattling sound coming from the engine. I guess it's the piston rings, because the valves open and close alright. My guess is that the machine shop where I had the engine rebuilt and honed should have used oversize pistons but didn't, somehow causing the piston rings to break.

As I have to rip the engine apart again , I was thinking about putting in a Stroker kit. I had in mind the 5-Liter crank I saw at http://boostperformance.co.uk. The questions that arise are:

- Can I use any pistons on it, e.g. oversize pistons my machine shop will supply?
- Can I re-use my OEM 3.9 con rods?
- Is it possible that parts from the broken piston rings are causing the rattling sounds I mentioned earlier? If that is the case, can they have caused some more damage inside the engine?

Regards,

Fritz

[Coops]

Mike the owner of boostperformance is a regular member on this forum,
he will hopefully see this post and be able to advise you regarding the stroker crank.

[ian.stewart]

2 guesses here, either you have melted or detonated a piston or 2, or the pistons are far too looes in the bore and as you said the rings are breaking up.
Yoy will only find out when you lift the heads
Ian

[katanaman]

stroker kits usually use first oversize pistons but they are most likely a different crown hight.
Conrods most likely wont be standard dimensions so no you probably cant use your old ones.

If you have broken rings then you most likely have scored bores and possibly damaged heads depending on how bad the breakup was. If you have no compression at all though I would say you have holed pistons for some reason.

I guess the only way to find out is to take it apart unless you have a scope you can stick in the plug hole.

[Vulture]

We will take the engine apart in due time; until then I can't be certain what happened exactly and how bad the damage really is. I hope the heads are whole, there were pretty expensive after all.

But, just for the record: how can I actually melt a hole into a piston, let alone 2 at the same time?? Shouldn't the engine run hot long before it comes to punctured pistons?

[sidecar]

We will take the engine apart in due time; until then I can't be certain what happened exactly and how bad the damage really is. I hope the heads are whole, there were pretty expensive after all.

But, just for the record: how can I actually melt a hole into a piston, let alone 2 at the same time?? Shouldn't the engine run hot long before it comes to punctured pistons?

If your motor was suffering from bad detonation then you could but a hole in a piston without seing hardly any rise in the water temperature.

You have a good point about putting a hole in two pistons at the same time.

At the end of the day you can guess all you like, but you'll only know what the problem is once you've pulled the heads off.

MinorV8 had similar problems recently, unfortunatley when he pulled the heads of his motor it was distroyed. I hope that you are luckier than he was!

Regards,

Pete

[JP.]

I have recently rebuilt a 3.9 litre engine for my SD1 as follows:

- Balanced the crankshaft with con rods, pistons, crank pulley and (lightened) flywheel
- RPi Engineering Stage 3 Heads
- Piper 285 camshaft

A few questions about this:
Was your engine hi speed balanced or low speed. Some machine shops can only do low speed balancing till 3000 rpm. A few thou of a gram @ 3000 rpm can be a lot of weight @ 6000 rpm. You can see this information on your balancing tech sheet.

You replaced your heads and cam, what lifters did you use and how about lifter preload. Wrong lifter preload can resuld in a wrong mixture and in a worst case scenario even damaged pistons and valves in a pump up action.

[Vulture]

I wasn't supplied with a balancing protocol, so I don't know at which rpm it was done. I gave the short engine to my machine shop, and they gave the crank to another shop specialised in balancing stuff. The bill I got from them says "measured and polished crank".

The lifters are standard, as recommended and supplied by RPi.

[JP.]

The lifters are standard, as recommended and supplied by RPi.

Even with standard lifters you'll have to check lifterpreload.
Lifterpreload should be set at 0.060" (about 1.5mm).
Standart lifters also suffer from pump up action above 5000 rpm that in basic means that the'll keep the valves slighly open above these revs. With to much preload this hapens earlyer and can resuld in valves hitting pistons.

Atleast use Crane Hi Rev or Rhoads lifters next time with your build up.

[Vulture]

The lifters are standard, as recommended and supplied by RPi.

Even with standard lifters you'll have to check lifterpreload.
Lifterpreload should be set at 0.060" (about 1.5mm).
Standart lifters also suffer from pump up action above 5000 rpm that in basic means that the'll keep the valves slighly open above these revs. With to much preload this hapens earlyer and can resuld in valves hitting pistons.

Atleast use Crane Hi Rev or Rhoads lifters next time with your build up.

The preload was set correctly on my engine (just asked my brother who installed the cam). I asked Holly, RPi's engine builder, about whether to use those high-revving lifters, and he told me something similar to "What good is an engine revving to 7000 rpm if then the valves hit the pistons?", which I took as an indication not to use them.

Anyways, tonight we'll rip the engine apart to see how severe the damage really is. I'll post some pictures here later!

[Boosted LS1]

The lifters are standard, as recommended and supplied by RPi.

Even with standard lifters you'll have to check lifterpreload.
Lifterpreload should be set at 0.060" (about 1.5mm).
Standart lifters also suffer from pump up action above 5000 rpm that in basic means that the'll keep the valves slighly open above these revs. With to much preload this hapens earlyer and can resuld in valves hitting pistons.

Atleast use Crane Hi Rev or Rhoads lifters next time with your build up.

The preload was set correctly on my engine (just asked my brother who installed the cam). I asked Holly, RPi's engine builder, about whether to use those high-revving lifters, and he told me something similar to "What good is an engine revving to 7000 rpm if then the valves hit the pistons?", which I took as an indication not to use them.

Anyways, tonight we'll rip the engine apart to see how severe the damage really is. I'll post some pictures here later!

If you plan to rev you have to use better lifters. RPI like to sell stuff that's safe if you know what I mean. They get their really 'good' engines built elsewhere. Rovers are like buicks and all revving buicks had aftermarket lifters. Or, solid valve trains and rev kits. I found that the stock lifters acted as a safe rev limiter, ie if they pump up and the inlet valve stays open the cylinder can't seal and generate any pressure ;( so revs don't increase.

Boosted.

[Vulture]

If you plan to rev you have to use better lifters.

I will, now that the engine's ripped apart anyway. It was never my plan to go up to 7000, I only quoted his approximate answer when I asked whether I should use aftermarket lifters.

RPI like to sell stuff that's safe if you know what I mean. They get their really 'good' engines built elsewhere.
Boosted.

Yes, that's true. They like to sell stuff. But is it really safe? Look at these pictures:



This is cylinder number 5, one of those that had no compression. Have a good look at the inlet valve.



This is a shot of the upper side of the cylinder head of cylinder 5. Note the broken inlet valve spring. And do remember that this happened at way below 2000 rpm! I don't think this is supposed to happen on a "Stage 3" cylinder head with

Hi-rev Valve springs to match your camshaft choice

I haven't come round to calling RPi yet, it's a bit late now, but I sure will do so tomorrow to clarify matters.

Now at least I've been able to solve the mistery why cylinder 5 had no compression, but cylinder 8 looks pretty healthy except on the comp tester. You can see no damage with your bare eyes on the head. Maybe the piston rings on that one are broken; if so, how can I find out whether they are? We poured some fuel into it to see if it would stay inside the cylinder, but the level stayed much the same while we were draining our beer. Is that a reliable method of checking for ring damage?

[Boosted LS1]

Inlet valve isn't centred on it's seat? That could be because of the broken valve spring and some guide play which may be within tolerance. I'd check the guide clearance.

Sounds like the rings on 8 are ok. Double check all the head stuff and gaskets etc.

What spec is the engine?

Boosted.

[Vulture]

The out-of-centre inlet valve would explain the loss of compression.

What do you mean by "spec"? It's a 36D from a Range Rover, rebuilt with those heads, a Piper 285 cam and some balancing done. It's fed by an Edelbrock 500 with jets supplied by RPi for a Stage 3 3.9.

[Boosted LS1]

Hi,

"spec" is short for specification, the parts used and way it's been built. I also thought the inlet valve could be bent but it's not common for inlet valves to hit pistons. That's an event for exhaust valves as they open towards a rising piston. If the inlet valve is bent I'd double check the cam timing and how much lift you have etc.

Boosted.