Diagnosing a Blown Race Motor

#1

So I think I’ve blown up my recently rebuilt B25 for my track car. I strongly suspect it was the engine builder’s fault. He claimed he knew what he was doing, but I’m pretty sure he didn’t. I should mention that it was me that rebuilt the motor…

I tried to make this description as short as possible, but to be thorough I had to make it a little lengthy.

Here’s the story:

  1. Craigslist B25 motor that I bought to rebuild. I deglazed the cylinders, had the pistons hot tanked, put on new rings and swapped in new rod bearings. Head was checked out and got a valve job with new seals at the machine shop.

  2. I deglazed/assembled the motor with a friend in my back yard. Did our best to be as surgically clean and consistently accurate with everything during reassembly. Ya know, check and recheck how things fit, lots of assembly lube, used an angle gauge and decent torque wrench.

  3. B/C the old cam was toast, a new one went in. I broke in the motor by following cam break in directions (about 30 minutes between 2.5 and 3.2K RPM). No oil leaks, nice smooth idle, no oil consumption.

  4. Changed the oil and did about 100 miles of prescribed road driving. Got the motor up to temp, did 4 or 5 highway pulls for about 15 seconds at about 60% throttle in 4th. Let it cool off. 4 or 5 pulls at about 90%. Changed the oil and filter. Still no oil leaks, nice idle, no oil consumption

  5. HERE’S WHERE TROUBLE STARTS - Track day 1. Took car to track testing day. Ran for about 4 hours with no problems (no oil consumption, pulling strong). After the last 45 minute run, I noticed that it was low on oil. Did an oil filter change and put in new oil.

  6. Track Day 2, Race Day 1 - Car runs strong all day but still uses oil (1 quart an hour). Looks like it’s burning some, but the underside of the car is oily too. Car develops an intermittent stutter coming out of esses. Sometimes there’s a stutter, sometimes there isn’t. Think the high pressure pump is almost dead.

  7. Track Day 3, Race Day 2 - Fuel pressure is up to snuff in the morning. Car runs great for about 3 hours but is still going through a lot of oil. Fuel pressure problem finally takes us off the track for the day. Car is missing, plugs are oil fouled and have lean burn dust on them. Makes it on to trailer under its own power.

A few days later and not surprisingly, car won’t start. Haven’t done a leak down or compression test yet, but I’m not expecting to find anything good. I’m also thinking that I may not find anything conclusive either (multiple problems). Some potentially big mistakes I may have made or other causes for failure:

[ul]Time between finished rebuild and first startup was about 3 weeks due to unexpected and unavoidable delays with other “life” stuff.

Over-revved motor - Both myself and another driver put the motor just over 7K a few times for a few brief moments. No rev limiter (that I know of).

Cylinder 3 had some scuffing near the bottom that I didn’t see until I deglazed. Too late to turn back at that point, so I just went forward.

After motor got up to temp, oil pressure at idle was around 12 psi. Always above 30 with throttle though.
[/ul]

I’m hoping I don’t have to pull the block out of the car again, but if that’s what it’s gonna take, then that’s what it’s gonna take. I’m new to rebuilding motors, but not scared to do the whole thing again. It ran really strong at first, so I feel like I got sort of close to doing it correctly. Hopefully I’ll get it right this time :S

Any thoughts/guesses as to what went wrong or what I can do to get it right? It was running strong and then sort of nose dived after about 8 hours of run time (4 hours of hard running on a track). Aftermarket oil pressure and coolant temp gauges seemed to always show normal ranges. I’ll update with leakdown results when I have a chance.

#2

Not sure that you did anything wrong, up until #5:noticed it was low on oil.

Then it was too late…

Let others weigh-in.

RP

#3

What were the piston to cylinder clearances? If you deglazed and honed without boring and using oversize pistons, they could be out of tolerance. Excessive clearance will cause your piston rings to fail in a matter of a few hours. Ask me how I know? Do a leakdown to be sure, but I’m betting on rings…

#4

Did you install any type of baffle in the oil pan or a crank scraper? Did the low oil indicator light ever come on while on track?

#5

theironman9154 wrote:

I doubt oil starvation would cause excessive oil consumption and/or power loss in the first few hours of operation… Excessive bearing wear and knock definitely, but the oil consumption sounds like a valve or ring failure to me.

#6

I’m with Fishman… When the motor was torn down were the cylinders checked for wear out of tolerance? Were the cylinders honed before reassembly? Were the ring gaps checked/adjusted during assembly?

#7

I deglazed the cylinder walls with a ball hone, but I didn’t actually remove (hone) a significant amount of material, so I used a standard sized set of rings. I used a set of measuring calipers on the top of the block to measure cylinder bore and everything was within Bentley specs.
Note, I didn’t use a bore gauge, just a set of calipers with legs that are about an inch long, so I was unable to get a read on any ovalization

I wasn’t really able to get a good reading on the piston/cylinder clearance either. I just couldn’t get the feeler gauge in there the with the crank installed. From what I read, I thought that taking a feeler gauge reading from the top wouldn’t be accurate either. Since the pistons were from the block, and it ran before I bought it, I assumed things were OK and reinstalled the stock pistons with new rings.

I did see the low oil light come on twice during track time, but never had the oil pressure warning light go off. Both times the low level was caught as soon as it came on. Car pitted and was topped off. I’m running a crank scraper.

I didn’t want to bias any answers but since FishMan threw it out there, I’m also thinking that it will likely be bad rings and/or valves. More likely rings, since a reputable machine shop did the valves.

#8

lateracer wrote:

I would actually assume that it is the valves and the machine shop. While they may be reputable they may not have known what they were doing with your perticular engine.

#9

I just rebuilt my motor as well. I used a ball hone 240 grit that was within spec suggested by my piston ring manufactuer - what grit did you use?

I used bore gauges and all of my cylinders were within spec - no ovaling at all. In addition my ring gaps were right on the mark.

One thing I caught myself doing was not paying attention to ring orientation. On most rings there is a definite TOP & Bottom. Luckily I caught myself after doing two pistons and turned the rings right side up.

I would definitely do a leak down test before tearing into it. If it is a ring problem you’ll know in short order.

Good luck

#10

[quote]
King Tut wrote:
I would actually assume that it is the valves and the machine shop. While they may be reputable they may not have known what they were doing with your perticular engine.[/quote]

That’s definitely how I would describe the shop. Reputable, but perhaps not knowledgeable.

I forgot to mention in my earlier posts that once the oil consumption started to get out of hand, a big puff of white smoke came out of the motor whenever it started. Whether it sat for a minute, an hour, or overnight, it was just one big puff that immediately went away.

I’m assuming that’s a strong indicator of a problematic valve seal, but I’m concerned it might be both rings and valves. I would be really happy if the problem was limited to the valves, if only for not having to pull the motor out again.

I definitely need to invest in a bore gauge…any recommendations in the low to medium price range? I know HF sells them, but I don’t like the idea of using their tools for precision work (I’m fine with their hammers, screwdrivers and sockets). Sears?

#11

I have a Fowler 52-646-400 cylinder bore gauge. It seems to work well enough. You can pick it up online or from your local Cornwell tool truck for $120ish…

#12

lateracer wrote:

[quote]I deglazed the cylinder walls with a ball hone, but I didn’t actually remove (hone) a significant amount of material, so I used a standard sized set of rings. I used a set of measuring calipers on the top of the block to measure cylinder bore and everything was within Bentley specs.
Note, I didn’t use a bore gauge, just a set of calipers with legs that are about an inch long, so I was unable to get a read on any ovalization

I wasn’t really able to get a good reading on the piston/cylinder clearance either. I just couldn’t get the feeler gauge in there the with the crank installed. From what I read, I thought that taking a feeler gauge reading from the top wouldn’t be accurate either. Since the pistons were from the block, and it ran before I bought it, I assumed things were OK and reinstalled the stock pistons with new rings.[/quote]
Measuring the top of the cylinders with calipers tells you, essentially, nothing. First of all calipers can’t read tenths of thousands and second the wear is always further down the cylinder bores. Only a tenth reading bore gage will give usable data. And you don’t measure piston clearance with a feeler gage. That is done by measuring cylinder diameter with a bore gage and piston diameter with a micrometer and comparing the numbers.

Deglazing the cylinder walls with a ball hone seldom yields the same results one gets from a cross hatch hone machine. If the bores aren’t properly honed the rings may not seat and excessive oil consumption will be the result. It also matters that the ring gap, between the ends of the rings, be correct. And the rings must be installed with the gaps radially separated around the piston.

#13

Got around to doing a little more diagnosis work.

Dry compression test on a warm motor had cylinders 1, 2, 5 and 6 at about 160lbs. 3 and 4 were at 110 and 115 respectively.

Went back and did a wet compression test. 3 and 4 came in at 160 with oil in the cylinder.

Pulled the valve cover and found a broken intake rocker on cylinder 3. Leak down on 3 confirmed ring leakage. Same on 4.

So, it looks like I’m in for another rebuild…or something drastic. I haven’t spent much time thinking about it at this point, but I’m wondering whether the smartest thing to do would be to get my cylinders bored out .020 over and run larger rings, or to search for a used motor in OK condition and use my rebuilt head (assuming it’s OK after I replace the rocker arm and get it pressure tested)?

#14

Since a set of pistons and rings, which would be required if you re-bore the block, will cost nearly as much as a good used motor, the cheapest solution would be a used motor. But a used motor is always a bit of a roll of the dice so the answer isn’t clear cut. Dome properly, a rebuilt engine should be more reliable than a used engine with an unknown maintenance history .

#15

If you decide to get a motor, I know of one that came out of an E30 track car to allow an S52 to go in. The motor is in Cleveland and will be a very reasonable price. Email me if you want the contact info. Z3SpdDmn at aol

#16

Pistons are expensive. I’ve done .020 over and I suppose that there’s a hp or two there, but it might not be the best use of $$. Swapping out parts at the dyno, careful weight distribution and corner balancing, experimenting with suspension settings, and good tires are probably all worth more.

The two bottom ends I did at MM both use standard size pistons. And they weren’t new either, they were take-offs.

Pull the block and pull the pistons out. Carefully mic (.0001 dial bore gauge) the cylinders and figure out how big they are going to be once you have them round and straight. I bet that you are going to find that the condition of the cylinders is such that you can re-use your pistons.

Then take your block and pistons to a machine shop and have them get your cylinders round and straight, while keeping the cylinder to piston gaps around .0015" to .0028". The tighter ones in the middle and the looser ones at the ends because of heat expansion. The reason you’re bringing your pistons to the machine shop is that they are going to differ a little in size so you’ll need to match them up.

Have the machine shop also check out your rod bottoms for roundness (.0003" tolerance). Then they should put your new bearings into your crank’s rod journalas and match the ones together that provide a gap closest to .0017". Be emotionally prepared to have to buy some undersize or oversize bearings. Alternately the shop might have to lap a few thousandth off of the crank.

Now you should have each rod matched to a crank journal and each piston matched to a cylinder.

Then take the rods and get them balanced within 1g. Pistons too.

Assemble rods and pistons and put in new rings. Carefully inspect the pistons for any burrs of aluminum that might be sticking out. This sort of thing happens when the wrist pin clips are being removed or as the pistons are being moved from one box to another. These burrs will scratch the cylinder walls.

Put crank in motor and put pistons in.

#17

Z3SpdDmn wrote:

I have a shipper that will happily move engines around at a bargain price. I shipped an engine to Chuck Baader 7hrs away for $100. It was fast and convenient.

#18

You did not mention gaping the rings…if you did not, you probably broke rings in the two low cylinders. May take out block or not, but disassembly is definitely in order. Chuck

#19

FishMan wrote:

Responding to my previous post… I thought the Fowler bore gauge setup was a great value before my trip to Metric Mechanic, but found out the precision isn’t really high enough. The Fowler setup, and most other cheaper units, only reads to .0005"; you really need gradations down to .0001". This setup does however include two “heads”; one works for less than 2" and one for 2" to 6". I’m planning to find a better dial indicator to try with the bore gauge hardware.

#20

Thanks for the responses.

I have 3 concerns about rebuilding.

My first concern is whether or not the depth of the scuff marks I noticed in one of the bad cylinders exceeds how large the cylinders can be bored out and still use the stock pistons. That is, will overboring to a cylinder that can use 84.23mm rings with stock pistons result in a clean cylinder wall, or will there still be scuff marks. Seems like a question for the machine shop. I’m 99% sure I would rather rebuild another motor than use larger new pistons.

Second concern is about the sturdiness of a stock piston with oversized rings in a race motor. I’m not looking to pick up hp, but I’m concerned that oversized rings aren’t as good as stock rings. Are they weaker? I can see how they could be weaker because of their larger size resulting in greater amounts of downward force being applied during combustion. This increased amount of force could be so marginal as to not make a difference, but I’m not familiar enough with the engineering aspects of this motor to know.

Final concern is about throwing good money after bad. It’s not about being cheap, but I don’t want to spend extra money on this block if there’s no reason to do so. The cylinders and broken rocker arm seem to be the only things catastrophically wrong (haven’t pulled head yet). Everything else looks good and all the seals are new.