230 Compression Readings

[rickv100]

I took compression readings on my truck and I have 100 lbs across all 6 cylinders.

Is this in the normal range?

The engine has been rebuilt but not been run in to seat the parts.

Rick

[M-Thrax]

Run her first then take compression readings, Make sure throttle and choke are locked all the way open before taking readings

[MSeriesRebuild]

I took compression readings on my truck and I have 100 lbs across all 6 cylinders.

Is this in the normal range?

The engine has been rebuilt but not been run in to seat the parts.

Rick

100# is not a good reading at all. The engine not being run in will certainly have an effect and you should run another test after it is. The result should improve; if it doesn't, something is wrong with that picture as 100# compression will never produce a good performing engine. Actually that reading is quite low. With a correctly run in new engine, readings should be in the 125# range at least.

We have found in a number of cases that valves were not properly lapped to the seats and don't produce the best seal. We outsource the machine work, but do our own engine build up 100%; in all honesty it's the only way we can get a quality build anymore. Many builders just don't take the time to perform all the steps in such a way that things are the best they can be. I would rather spend extra time if required and put out a better performing engine. I can add this also, replacement valve seats are not what they used to be in the area of seat angle cuts these days. Used to be seats required only minimal cutting and grinding to achieve proper contact position on the valve face. The correct valve face contact width is 1/16", and that contact area should be near center or slightly inboard of the center of the new valve face. We just did an engine a few weeks ago; I had to relieve the new exhaust seats at the bottom and top of the face and re-grind the 45 degree seat angle in order to get centered contact area on the valve face before we could lap them in correctly. Lapping for a tight seal is also a pains taking process to get right, many builders simply don't spend the time. Another issue is valve springs; first off they can be installed upside down. If the tighter wound coils are not installed up, erratic performance at higher RPM will be the result. New springs will also take a heat set the first time the engine warms up to operating temp. This weakens spring pressure and requires that a .030" thick valve spring insert (shim) be inserted at the top of the new spring so it produces the intended pressure for a tight valve seal after the heat set takes place. In the event you are reusing the old valve springs, a .060" thick insert is required to help renew the correct pressure from the fatigued springs. We never do a valve job without installing new springs, but I've heard many folks say they did reuse springs. Without taking these extra steps, performance simply will not be the best it can be, especially at higher RPM's. It will be erratic and noticable. I've found many asking the question; "why is my new engine running like this after I spent $$$ to rebuild it?" Rebuilding engines is a complex issue to get the best job possible, it can't be rushed and short cut without the end result being less than satisfactory. Sometimes just reading and going by the book just won't produce the best result. Many small details that will adversely affect performance and longivity are not even mentioned in the manuals, thus are overlooked by the inexperienced builder or builders not familiar with a specific type of engine. Builders not used to working with older engine designs often overlook critical points in the older designs. Many builders these days are looking at taking any short cut they think will slip by all in the name of trying to make a buck in a competitive market place. Most know it won't be the best, but hey, in their eyes it's just an old truck.

My advice is this; always insist your builder give a detailed, itemized list of exactly what he intends to do. Do your homework, know what should be done, if it isn't on his list, ask questions. Insist that the job be done the best it can be. If you don't feel comfortable with the answers, move on to another builder and start over. There is no sicker feeling than to invest in a rebuild only to find the end result is something you will never be happy with. Finally, don't expect a top quality build at a bargain basement price, top quality work will take extra time for which your builder will have to be compensated. Parts cost have risen dramatically over the last year as well. Above all, remember this; the old saying "you get what you pay for" has never been more true than it is today.

Good luck, hoping for better readings after the run-in is complete.

[cuz]

Not to be overlooked are the seating of the piston rings. The use of the readings you have to justify any immediate repair work is not a good idea. They indicate that the run-in process needs to be completed before accurate compressions results can be obtained and used for serious evaluation of the quality of your rebuild. If you pay close attention to oil consumption and engine temps you will know when the run-in is complete. Then do a compression check to score the quality of the rebuild.

As Charles mentions above most modern engine shops just are not familiar enough with the oddities of these old engines to be able to complete a truly top notch rebuild. This doesn't mean the shops are not quality shops. It just means that century old engine designs are not their "cup of tea".

[rickv100]

Thanks for the advice. I was actually taking the readings to try to figure out why the truck was not starting.

I have a feeling I have not adjusted the valves correctly. Manually adjusting valves is before my time.

I did have the valve seats replaced and installed new springs and valves when I rebuilt the truck. I did the assembly myself following the engine rebuild manual.

Rick

[MSeriesRebuild]

Thanks for the advice. I was actually taking the readings to try to figure out why the truck was not starting.

I have a feeling I have not adjusted the valves correctly. Manually adjusting valves is before my time.

I did have the valve seats replaced and installed new springs and valves when I rebuilt the truck. I did the assembly myself following the engine rebuild manual.

Rick

If you will give me a call at the shop, I can give you some pointers and things to look out for that will help you. If I'm understanding you correctly, it is not starting, I think we can get to the bottom of the problems. We are here 8 - 5 Mon - Fri. 704-474-4683. Will be happy to help you out.