Diagnosing a Blown Race Motor

#21

If you overbore, you must use larger pistons and rings. We are allowed up to .020" (.50 mm) over. With minimal scoring, you’ll never be able to hone them out and stay stock on pistons. You might want to give Metric Mechanic a call and see if they have a good block that would work well with stock pistons for sale. They seemed to have a few…

#22

It’s a mistake to swan off into all sorts of scenarios. Pull your engine out and remove the pistons. Carefully measure the cylinders. That will tell you if the block can be re-used. Chances are whatever scuffs are on the cylinder walls can be dealt with by taking off a little cylinder wall material. Piston to cylinder wall tolerances tend to be a little larger then DD tolerances anyhow.

If you find out that the cylinders are too messed up, one way or another, then figure out how to get another block. There’s always engines around available for sale. Heck, there’s a guy here in Savannah that has 2 he’s looking to sell. DD engine blocks are usually fine. It’s the heads and cams that are often thrashed.

#23

Think I’ve just had a moment of clarity after a quick chat with Z3spdDmn and I’ve accepted the hand I’ve been dealt.

Strip the block, replace with a different block (or bottom end) that’s in good shape. Build again!

More than anything else, I’m most bummed about the $100+ in stupid TTY bolts that don’t have much time on them, but will have to be thrown away.

An expensive way to remind myself that I’m neither a mechanic nor an engineer. One day I’ll learn.

#24

lateracer wrote:

Keep the bolts. You’ll want them for measuring your rods.

Bimmerparts has the bolts for pretty cheap. They’re a MidAtl SpecE30 sponsor.

#25

lateracer wrote:

[quote]An expensive way to remind myself that I’m neither a mechanic nor an engineer. One day I’ll learn.[/quote]If only that sentiment would rub off on a few more of these bastards we could get back to talking about racing Spec E30s instead of tinkering with 'em!! :laugh:

I am sorry your motor took a crap. That sucks. Find a junkyard motor from a car with a little front end damage. That way you know at least the car was running… :woohoo:

#26

Where the best value on .020 over pistons? My brother has a block waiting for me to get built when I can afford it. He had a machine shop that owed him enough money for all the work. He says the cheapest he has found is around 700 at the dealer with his employee discount, however he is going to work for an independent import shop soon.

#27

$530 from Bimmerparts.com. For the best price, call and tell them you are a NASA racer.

#28

call ziggy or chris@bimmerparts. they’ll hook you up.

#29

I sort of have an update on my motor situation that’s more of a question actually.

I’m going to check out a few blocks I’ve found around here. One is supposed to be in really good shape, but the guy wants money for it. A few others are free and supposed to be in decent shape. Old motors pulled for swaps.

I got an OK quality digital bore gauge and I’m planning to take it with me to do a few measurements on the blocks. I’ve never used one before so I’ve spent some time reading the instructions, watching some youtube videos on how to use them and calibrating them with multiple dial calipers that have also been checked using a feeler gauge. Long story short, I thought I knew how to use the gauge and I calibrated it with multiple calipers and then double checked the accuracy of the calipers. Now, I’m not so sure.

When I measure the cylinder bores in my current block, I’m getting a measurement of around 83.220 mm. According to Bentley, it should be around 84.00 I think. Clearly, it looks like I’m doing something wrong. Actually, I would be surprised if I wasn’t doing something wrong, but I can’t find any errors in how I’m checking my measurements.

Any ideas on where I should start or tips for figuring this out?

#30

It would be easier to get relative measurements instead of absolute measurements. Unless you suspect a block of being bored out, you don’t really need absolute measurements at this stage.

Measure a cylinder’s dia. top, middle, and bottom. Do this both longitudinally (parallel to the motor’s major axis, and laterally, for 6 total measurements. The top measurement should be about 1cm from the top of the cylinder. Take good notes and then figure out how much the cylinders are deviating from being “perfect cylinders”.

Chances are the blocks you’re finding are ok. Our blocks are pretty rugged and you can hone out most problems you might find with the cylinders. Race engines tend to have higher cylinder to piston gaps. This gives you some freedom to remove a little cylinder material and still be ok.

Note that your desired cylinder dimensions depend on piston dimensions and expansion rate. You can’t just get a spec out of the Bentley and say “ok, that’s perfect”. The best you could say is “ok, that’s in the ballpark”. Measured with sufficient precision, pistons are not all the same size, nor do all pistons expand at the same rate. So if you’re going to do something non-OEM like get forged pistons (I think they are legal now as long as shape and weight are the same) then you’d have to talk to the manuf about their expansion and how much piston to cyl wall gap to give them.

#31

To get a relative measurement, you’ll need to measure your piston diamater. Using a quality micrometer (not caliper), measure the piston diameter perpindicular to the wrist pin just above the highest point on the piston skirt). Lock the micrometer in position and set the cylinder bore gauge to zero using the micrometer. You simply place the bore gauge inside the micrometer and zero at the point where the gauge has the smallest reading; it’s easier if you have a helper for this. Then measure your cylinders in the six places described by Ranger. A block with .020" clearances in each cylinder will work well for your rebuild.

#32

I just refreshed my motor this past(?) winter… Although I did not get to Ranger’s & MM precision my results when measuring the bores were either right on the money at 84mm or very close - within a thou or so. My notes are not with me.

I used the old school method of using a spring loaded bore gauge and measurement with a caliper. I was suprised to see that I had almost imperceptible ovaling and the crosshatch was clearly visible.

I then used a ball hone whose grit matched my piston ring manufactuere’s reccomendation and introduced a new surface to the bores.

Afer running the motor and putting a good load on it I checked my compression and all looks great.

This is not rocket science. Take your time & measure everything three or four times!

My motor was weak due to piston ring failure. When I tore down the motor the rings were carboned up and the relief holes in the pistons were completely coked up and closed. This was motor that I purchased used. Surprisingly, the bearings - rod & crank were in great shape - although I did change them out. I would have thought that the rings were in bad shape because of oil change neglect but with everything else in good shape I just don’t know.

#33

Thanks for the feedback. Relative measurements definitely seem to make more sense at this point, just wanted to make sure I’m not (completely) crazy. Thanks for the micrometer method, I think that’s what I was missing in this equation.

I’m planning to do this rebuild on the cheap, so I’m either going to use the pistons that are in a “retired” block that I pick up, or drop my pistons in a good condition block. Not planning on buying oversized pistons.

Some things I didn’t do, which seem to be pretty big mistakes in hindsight, were to check ring gap or piston/cylinder clearance. I’m going to pull the pistons to measure them with a micrometer, compare with bore gauge readings in the block and see if I might have just messed up the ring gap and can keep my current block.

Fingers crossed!

#34

Hang in there! I used Deeves rings - I heard good things about them - but are a little expensive. My piston ring lands measured fine and my ring gaps were right on. No fitting necessary.

Also, the danger with piston to bore clearance is if they are too tight - very unlikely with your motor. A little additional clearance is not necessarily a bad thing in our motors. We don’t expect 100k miles in race conditions and a “loose” motor often equals a fast motor. If it seals well (leak down & compression) and has good oil pressure & good bearing surfaces you’ll have a good motor.

Concentrate on getting a good cylinder surface, acceptable piston to bore clearance, good rings, and rod bearing clearance. Check a few main bearings for wear. Then it’s up to the head to make power.

#35

Be sure to get mics and gauges that can do a ten-thousandth. The ones I bought could only do 5 ten-thousandth, and were therefore inadequate.

Measuring things is surprisingly difficult when you’re trying to be accurate within a ten-thou. Just keep at it until you find a technique that allows you to get the same result over and over again. As usual, I’m forced to substitute tenacity for skill. I just keep measuring it over and over again until I start getting the same #'s.

I simply reused old pistons in my MM blocks and it went fine. Well, except for me scratching the cylinder walls. MM expected to find that a lot of the old pistons I came up with had hammered out ring grooves but that wasn’t the case. We were planning to have to recut the grooves on the pistons and MM made a fixture for this before our visit. But the pistons turned out ok. Perhaps MM is used to M20s that have been running higher than OEM hp and the cast pistons weren’t quite up to that, but maybe cast pistons are durable enough for our 150hp.

Re. PDS’s carboned up pistons. I agree, prob not enough oil changes.

#36

Any idea what kind of piston to cylinder clearance can I live with?

Since this is my first motor build, I want to get as much learning/life span with this as possible.

I think Bentley lists maximum clearance at .12mm (.0047"), but I’m curious as to how loose I can realistically run.

#37

With cast pistons, I believe .0017" to .0020" is ideal with larger clearances at the outer pistons and smaller in the center. The Bentley spec (.0047") is too big, but .0030" would likely be okay. According to my engine builder, it’s best to stay below .0030" if you can’t get the ideal clearances.

#38

Here’s your goal for each piston.
1 .0020
2 .0017
3 .0015
4 .0015
5 .0017
6 .0020

I’d call .0025 as max gap unless there was some desperation.

#39

I disagree. All he’s trying to do is to establish the state of wear of the cylinders. First one has to figure out how egg shaped the cylinders are, and by extension, much material would have to be removed in order to make the cylinders round and straight again. If the cylinders are eggshaped, that can be established without bothering with the pistons.

#40

I disagree. All he’s trying to do is to establish the state of wear of the cylinders. First one has to figure out how egg shaped the cylinders are, and by extension, much material would have to be removed in order to make the cylinders round and straight again. If the cylinders are eggshaped, that can be established without bothering with the pistons.[/quote]

How can you disagree? To get a relative measurement [to the piston] you have to measure the piston. Point taken about measuring for straightness… Bottom line, he really needs to know both measurements.