After suffering a debilitating back injury, I was unable to work on my Speedster all summer. That along with an absolutely demanding work schedule that included a ton of travel throughout the Northeast, got in the way of my recreational life. I did get a nice summer Motorcycle trip in, going through Kentucky's Bourbon Trail and then across TN for our BMW bike rally.  Other than that, I have just been working like a mad man. 

     Anyway, I am in the process of completing my engine build and renovation of my old and tired Speedster.  I have the short block together and after taking the motor out again and breaking it down due to poor compression, I am at the point of re-installing the Panchito heads. I am heading over to my shop this PM to check the measurements as I believe they are the issue.  I believe CB Perf MAY have shipped the wrong bore size heads!  I'll provide more details later today or tomorrow....

    Sorry for not being communicative but I had to give up this forum as time would not allow for the past 7 months and, I really miss you guys!!!

   Happy New Year....

Jim

 

Speedster Jim, Buffalo NY

Original Post

Glad to hear from you Jim. Please don't miss Carlisle! I also look forward to the forensics on your engine, as you're doing about what I was going to do on my Spyder build. 

Here's to every micrometer reading within spec and no crossed threads.

Caution, thread drift ahead! 

     I pulled the heads of my new motor and did a measurement of the cylinder spigot going into the head. (97.77mm)  Then, the bore into the head. (98.14 & 98.17mm)  I believe, .40 gap is the reason for rapid leakdown of very low compression after only 20 total minutes of run time. 

   Any thoughts from the motor experts.  This is the only mild performance build I have done. I normally go with stock 1600 motors.... This one has been a bug in my butt since the get go! 

Safety Jim Buffalo NY. posted:

Caution, thread drift ahead! 

     I pulled the heads of my new motor and did a measurement of the cylinder spigot going into the head. (97.77mm)  Then, the bore into the head. (98.14 & 98.17mm)  I believe, .40 gap is the reason for rapid leakdown of very low compression after only 20 total minutes of run time. 

   Any thoughts from the motor experts.  This is the only mild performance build I have done. I normally go with stock 1600 motors.... This one has been a bug in my butt since the get go! 

Are you sure there's not something else going on? Head bores are always cut slightly larger (any where from .010- .020") than the cylinders; yours do seem a little excessive, but as long as the barrel tops are on the same plane and and the head surfaces are flat I wouldn't think the head bore being a little large would cause what you're experiencing. 

Thanks Alb. I know the compression seal is formed at the surface of the barrel to head. It just seems odd that there is that much gap in the cylinder OD to head ID.  Even Removing the rocker shafts did not improve the low compression. The barrels are seated nicely in the case. It is a real problem on both sides of the motor.

Alb. I will pull the cylinders off the pistons and check the rings. I staggered them during assembly.  There is not much more it could be.... right?  The .020 - .040 falls in line with the gap I measured. I assume that gap is for heat expansion of the cast iron barrels.  I just never noticed it being this much slop.  Anyway, the mystery will get solved in due time.... What kind of sealant do you like for the barrel to case and barrel to head interfaces?

  Thanks again for your input. 

 

There is no sealant from head to cylinder.  For the case spigot, hi-temp rtv.

Heads and cases are cut a little over size because the machinist doesn’t usually have the exact cylinders on hand that the engine builder is using on a particular build.  The cast iron cylinder will expand less than an aluminum head. 

 

I am am more inclined to believe you didn’t get a good ring break-in.  When you assembled the cylinders, did you take the pistons out, remove the rings and clean off all the residue from everything.  A lot of people just lower the piston, put the rod on and reassemble.  That is a recipe for disaster.  When you get it apart, measure the ring gap.  I don’t have a Bentley manual handy, but should be around .020-.025, but check on that spec.

Last edited by LI-Rick

Jim, clean the bores of the cylinders again and again and again with brake cleaner and/or rubbing alcohol and CLEAN lint-free rags or wipes. It is entirely possible that the hone oil is still in the hone marks/grooves. The rags must come up clean with absolutely no dirt on them. I used Total-Seal #2 ring and their Quick-Seat compound on the totally clean and dry cylinder walls.

Did you use valve lapping compound on the cylinder/head seating surface? That is a step I do to ensure a good head seal.

Do you know combustion chamber volume? If the chamber is too big, not enough compression.

Did you check the deck height? If it's too large compression will suffer.

You can use either hi-temp rtv or permatex case sealer for the barrel to case junction.

Great info gents. Thanks. I’ll be back at it tomorrow night, Friday for sure. I am nuts about the cleaning, but I’ll go over it again. I know the ring gap is good too. However, I’ll check it again. Now I’m guessing the issue may be due to the permatex aircraft sealer at the head barrel interface. 

Thanks so much for the help!

 

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