Shop Products
Houzz Logo Print
edhap

Briggs Opposed Horizontal Twin Compression

edhap
9 years ago

Briggs 402437-0666-01.
I have had this engine re-bored and piston .020 over. Valve job was done. Compression has been weak since rebuild. It was only run briefly. I checked compression and got numbers in the low to mid 50lb and thought the rebuild was done poorly.
After much investigation and testing: does this engine have an intake valve compression release built into the cam lobe? I have a briggs manual for this engine...not a word.

I checked the valve action with head off and find that the intake valve stays open a few thousanths until very near TDC. This seems to tell me the compression release cam lobe "system" is at work. (no spring, arms, etc. on camshaft.).

How does this not effect compression/power at operating speeds?

I put a bolt in the PTO end of crankshaft and spun the engine backwards a few turns with a variable speed electric drill with a socket on the end. the compression read 120lbs. (cold) in both cylinders. In normal operating direction, checking compression with engine starter, I only got around 55lbs. Does this confirm an "intact compression chamber", and that I should put the engine back in the tractor and cut grass?

any briggs bulletins or explanations of this system? any list of which engines do or do not carry this compression feature?

Many thanks for help!

Comments (13)

  • hippy
    9 years ago

    Who did the rebuild?

    Second. Do they know that these engines have two different ways to set the Cam timing?

    One for Engines with Ball Bearings and another way for engines that have Pain Bearings.

  • walt2002
    9 years ago

    The IPL does show that this engine has ball bearing mains and they do require a different method of setting the valve timing because the usual timing marks are hidden behind the ball bearing. It is a possibility that the valve timing is not correct. I am getting ready to do one in a day or so.

    These engines do utilize the, "Easy Spin" camshaft grind to relieve compression on the Intake Valves. B&S says this has minimum affect at operating speeds. It does hold the Intake Valve slightly open well into the compression stroke. For this reason, Intake Valve adjustment is critical and it must be done properly.

    My explanation of why holding the Intake Valve open late does not affect engine operation is that as engine rpm's increase, the "lag" in air coming in increases so that the air only "catches up" late into the compression stroke. By the way, this method is used today on the V twin Inteks.

    Walt Conner

  • bluemower
    9 years ago

    The Briggs opposed twin has "easy spin" compression release. The intake lobe on the camshaft is designed to hold the intake valve slightly open (1/100 of an inch) during a portion of the compression stroke. This prevents excessive compression during engine starting. After the engine starts, events occur so quickly, the engine does not experience loss of power. This description is from an older Briggs repair manual.

    Briggs does not publish any compression figures. Compression testing is recommended to compare readings between cylinders. The pressures should be within 25%. I have tested many of these and normally see about 95 psi and I am at 7,250 feet.

    My thoughts are:
    1. evaluate cranking rpm. Make sure the battery if fully charged.
    2. Check the valve clearance. Make sure the pistons are positioned 1/4" past top dead center. Setting the clearance on the higher side will increase compression.
    3. Consider performing a blow down compression test to determine if there is excessive leakage at valves, head gaskets, or piston rings.

    The engine manual has a couple of pages devoted to compression testing and performance issues. The part number is 271172. This was last published in April 1999.

  • edhap
    Original Author
    9 years ago

    I really appreciate the feedback. It confirms what I sort of stumbled onto myself.

    One issue I can address mentioned above is the dual timing instructions for ball bearing vs. plain bearing set ups.

    Early on, I thought I had found why my compression was low after the rebuild when I saw the manual describe the two methods. I thought perhaps the machine shop did not follow those instructions.

    I took the sump cover off and checked the situation out.

    My PTO end ball bearing, while supposedly a press fit, moves a bit forward on the crankshaft. Enough to see the crank gear behind the bearing.

    I found that while the plain bearing instructions are to align the cam gear marking with the crank gear marking, and the ball bearing engine instructions are to align the cam gear marking with the marking on the #2 crankpin, these timing procedures result in the SAME timing.

    When I set the marks up aligning the cam gear mark with the #2 crankpin, and then slowly turned the engine over, (with my "loose" ball bearing slid forward so I could peek behind,) I saw the crank gear marking come into perfect alignment with the cam gear marking, just like the plain bearing instructions demand. So whichever method you use, your timing ends up in the same place!

  • walt2002
    9 years ago

    "So whichever method you use, your timing ends up in the same place!"

    We knew that, it is only logical. So that apparently is not your problem.

    While B&S does not publish compression specs. as said above, 50 lbs is too low for newly rebuilt engine. Since you got much higher when spinning backwards, suspect insufficient Intake Valve clearance.

    Incidentally, I am about to turn another vertical shaft twin into a horizontal shaft.

    Walt Conner

  • edhap
    Original Author
    9 years ago

    "Only logical" ? Thats what Einstein said to the rest of the theoretical physicists after he wrote E=MC 2 on the blackboard! I took it as if carbed engines require 8deg BTDC and 10 deg with fuel injection!!! It only became logical AFTER the light bulb lit up!

    That being said, my new problem after reading the above post (by walt) is this:
    I thought spinning backwards, with the tappet riding the back side of the cam lobe, without the intake valve being held open during most of compression was why compression was dramatically higher. I thought this proved the valves were sealing ?????

  • edhap
    Original Author
    9 years ago

    Convert vert to horizontal? See there you go again, Albert...
    er...Walter.

    So if intake valve clearance is set to spec, and set to wide side of spec, is this enough, or should i cheat a few thousanths wider?j

  • edhap
    Original Author
    9 years ago

    Mownie, thanks for the dissertation, er, explanation!

    That is how I understand the process too. I am a little confused about Walter saying "50# too low". My reverse spin compression was 120#, does 50# in the normal direction, using starter, still too low with this type of engine using this compression release system? Maybe valves are not sealing well enough? I did do a leak down test and could hear exhaust valve leak, even with valves closed, but do not know how much leak is too much.

  • mownie
    9 years ago

    Well, you really shouldn't be able to hear any leakage at all if the valves and seats were properly ground and lapped.
    It is necessary that the engine be properly positioned to the precise spot after TDC of the compression stroke and the engine crankshaft be securely locked against rotation while the cylinder is pressurized with compressed air.
    If the crankshaft is not locked down when the air is applied, the piston will move the crankshaft and rotate the crank, along with the camshaft......to Bottom Dead Center of the power stroke position and at that point is where the exhaust valve is beginning to open for the exhaust stroke.
    So, if you have not been holding the crankshaft securely against rotation, that could explain leakage out the exhaust.
    Double check that there is clearance on the exhaust valve stem while you are in position for the leakdown test.

  • edhap
    Original Author
    9 years ago

    yes, I have held the crank shaft in position at TDC and at 1/4 past TDC. also have checked that there is valve/tappet clearance, and I still hear exhaust valve sound, on one of the cylinders. the other cylinder seems better.

    I did take the head off, I put a new exhaust valve in and lapped it once, and it seemed to leak less. I have not done it again, yet.

    I just went back to the shop and did compression in reverse again on both cylinders, using variable speed drill. I got about 120# each again. I then put a teaspoon or so of oil in each cyl and re-did compression test again and now get #150, with reverse direction and variable speed drill. I know this says rings are leaking. they have not been seated (engine only run 20 minutes or so after re-build). also, cold engine. does this test seem in line for fresh rebuild, with not yet seated rings, or am I looking at a problem with the machine shop boring, honing, ring work?

  • mownie
    9 years ago

    It was mentioned that the cranking speed should be evaluated.
    That could have a bearing on how much compression is being lost at cranking RPM. If the cranking RPM is somewhat slow, that means there is more time for the combustion chamber to decompress. So, what you see as 50# may not be an accurate assessment of the compression at all.
    Rings do need some run time to seat and polish themselves into place. 20 minutes is hardly enough time for that.
    The thing I would be most concerned with on a rebuilt, bored out cylinder is whether it was bored and honed too large.
    The best way to double check on that is to measure ring gap by placing the rings into the cylinder (not on pistons) and measuring the end gap with feeler gauges.
    Since the engine is already put together, that is impossible without a teardown.
    When you ran the engine after being built, did it seem to lack in power? Because you have not actually stated exactly why you came to be checking compression values. Are you simply checking to satisfy a curiosity? Or does the engine seem not to be as powerful as you think it should be?
    That the compression rises to 150# from 120# with the addition of some oil to the cylinder sort of tells me this engine needs to be run and worked. If it runs and works well, you can come back later tinker with getting some fresh compression readings.

  • edhap
    Original Author
    9 years ago

    Originally, I put the rebuilt engine back in the tractor, it did not run smoothly at all. you could feel the vibration. I took it back to the machine shop on a trailer, and let them hear it run in the tractor. at that time they decided it was a valve problem. (a valve job had been done). they found a slightly bent valve, replaced it. it did run smoother, but the engine did not feel peppy and compression was low, and one cyl was still 30% lower than the other one was. since then, I have replaced all valves, all springs, tore the engine done once. I tried measuring the bore myself. it seemed right. this has been a learning process for me. even using an inside micrometer on the cylinder is/was a challenge. it was hard to measure twice and get the same result.

    the good news is that I have another engine, that I bought from a guy as a back up. I have used it to cut grass the last year. I now have another tractor to cut grass (as soon as I get the head back from the machine shop, (a different one), I have a thread about the valve guide moving.

    So, I am going to get the valves as tight as I can with lapping. I will put the engine back in its tractor and run it some hours. if it seems good, great. if it still seems weak, I will tear it down when the weather changes, and start at the cyl/rings etc. Thanks for all the advice.

Sponsored